Chapter 8

Quantitative elemental analysis

1. Introduction. Methods and equipment

Earlier, when new organic compounds of unknown composition were isolated from plants or animals or obtained in syntheses, quantitative elemental analysis was the most important branch of organic chemical analysis. The molecular formula of the compound was established from the elemental analytical data with a knowledge of the molecular weight, and this determination was followed by structural analysis, at that time almost exclusively by chemical methods.

In recent decades, instrumental analysis, primarily spectrophotometry, has developed greatly, and has reduced the importance of chemical analytical methods. It is now rare for total quantitative analysis, that is, the determination of each elemental component of an unknown sample, to be effected. Mainly carbon and hydrogen and some heteroelements are determined quantitatively today. With nitrogen compounds, the determination of the percentage of nitrogen is usually sufficient. When the data found for these two or three elements are near the calculated values, the identity of the compound is usually regarded as proved. The direct determination of oxygen is also rarely required. In industrial laboratories the situation is similar, here the identity of the sample is rarely questioned and the purity control can be based on the determination of carbon, hydrogen and nitrogen when the contaminant is present in larger amounts, while trace contaminants are detected by qualitative analysis and physical methods.

However, as quantitative elemental micro-analytical methods are applied in many laboratories, it seems justified to deal with these in a separate chapter.

As it has been mentioned in the preface, apparatus, equipment and chemicals are commercially available today for organic micro-analytical work.

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Theoretical problems and fundamental principles of the determination methods, operation of the apparatuses and elimination and prevention of errors are also discussed here, as the method of combustion and other treatments and the rate of operation may be modified when the properties of the sample differ from those of the standard substances. Additives are often employed to facilitate combustion or eliminate interferences, and the need for a sufficient number of reliably pure standard substances must be emphasized.

When considering the proper arrangement of an analytical laboratory, it is essential to have a separate balance room with a constant temperature and humidity and a well ventilated hood for work with acids to avoid contamination of the atmosphere of the laboratory with their vapour.

The most important instrument in a micro-analytical laboratory is the micro-balance, providing an accuracy of at least ± 5 ^ig. Sartorius, Mettler and other companies produce micro-balances with an accuracy of ± 2 jig, and the total weight placed on the balance should not exceed 10-20 g. Careful and cautious handling of the very sensitive micro-balances is of primary importance for successful work in the laboratory.

Within the balance room, a vibration-free location of the balance is essential, (when a separate balance room is not available, the balance is placed in a well isolated glass box to protect it from the atmosphere of the laboratory). A vibration-free location of the balance is ensured by the use of a table fixed on separate holders. The construction of a table with reduced vibration is shown in Fig. 38, this is suitable for damping vibrations of various frequencies within a certain range. Vibrations caused by machines may be too large and an office building is usually more favourable than a workshop building for the location of a micro-analytical laboratory. The balance room should be heated with non-radiating, low-temperature heating devices, and the space should be large enough to accommodate several operators without changing the temperature or humidity.

o

(a) O oj 2

Fig. 38. Vibration-free micro-balance table

(a) P l ane ; / — b a l a n c e ; 2—cantilever; (b) side view;

/ — b a l a n c e ; 3—beam; 4 - latex; 5—iron or l ead ; 6—sand

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The equilibrium of the atmosphere in the balance box and in the balance room must be ensured by opening both side-doors of the box for 20-30 min every morning. No desiccant (silica gel, etc.) should be placed in the balance box so that the humidities in the balance box and the balance room are identical. Otherwise, the amount of moisture adsorbed on the surface of glass vessels would change during weighing and the equilibrium state would be attained after a longer time. Preferably, all objects to be weighed, particularly the glass vessels, are kept in the balance box for 10-20 min prior to weighing.

The equilibrium position of a good micro-balance (zero point) does not change over a period of several hours ; if longer periods are involved it must be checked. Volatile and hygroscopic samples must be weighed in closed vessels.

It is well known that not the absolute weight but a difference in weight is measured on a micro-balance. In weighing, a tare weight is used, the weight of which is several milligrams less than that of the sample. The tare weight is preferably made from aluminium or, when weighing absorption vessels, a similar piece made from glass is used to eliminate the ocassional errors caused by a surface moisture film.

Samples are placed on the balance plates by means of long-stemmed pincers so that the operator 's hands do not reach into the inner space of the balance container.

In addition to micro-balances, some analytical balances are also necessary in the laboratory for weighing chemicals for the preparation of standard solutions.

Gravimetric micro-analytical methods are obsolete today, except for the determination of carbon and hydrogen in one sample and the determination of phosphorus in the form of an ammonium molybdophosphate precipitate. In the literature after 1960, only a few papers deal with gravimetric procedures for any elements other than carbon and hydrogen. Gravimetric methods have been replaced by simpler and faster micro-volumetric and spectrophotomet-r y methods.

The advancement of volumetric micro-analysis seems to have reached a plateau. It has been accepted that better results can be obtained with relatively concentrated standard solutions and finely calibrated burettes that allow readings to be made to within 0.1 Usually such a precise determination of the volume of a standard solution is not necessary, and burettes with total capacities of 2-5 c m 3 and 0.02 c m 3 divisions are used. When the volume of the standard solution consumed in the determination is a few cubic centimetres, the error of the readings will be less than 1%, and this can be compensated for in repeated determinations. The lower limit of the concentration of standard solutions is 0.01 N, and usually, in these instances chemical indicators can still be used. In certain electrometric indication

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methods, the concentration of the standard solution may be 0.001 N or even lower. With very small samples, it is preferable to titrate in small volumes with relatively concentrated standard solutions. In concentrated solutions the indication of the end-point is often disturbed by foreign ions present, especially when chemical indicators are used. Ion selective electrodes are not sensitive for these. In titrations on the ultramicro-scale, the volume of the solution to be titrated is less than 0.5-1.0 cm 3 , the titrant is added from a forced-flow burette with a total volume less than 1 cm 3 , and the volume of the titrant consumed is 100-10 ^1 or less.

With burettes with a total volume of 0.05-0.1 c m 3 and fine calibrations, the volume of the titrant can be read to within 0.1 \il In these titrations, the very thin metal tip of burette is immersed in the solution being titrated.

The advancement of spectrophotometers in the last 15 years has allowed more precise reading of absorbance values in the ultraviolet and near infrared ranges. Many new reagents have been introduced recently [1] . The individual spectrophotometric procedures will be discussed in connection with methods for the determination of different elements.

As was mentioned in the introduction, even today mainly chemical methods are applied in elemental analysis. These are mainly suitable, for the determination of a particular element, only carbon and hydrogen being determined simultaneously. In the past, the development of procedures for the simultaneous determination of several (4 or 5) elements in one sample was attempted. These procedures are very complicated and are used only when very small amounts of sample are available. In most methods of this type the determination of an important heteroelement (halogens, sulphur, phos-phorous) is combined with the determination of carbon and hydrogen. The simultaneous determination of the three most important elements, carbon, hydrogen and nitrogen, in one sample, can be regarded as too complicated for application in automated analyzers involving gas chromatographic or other separations which were designed mainly for serial analyses. Other methods based on chemical separation are of no real importance today, when total analysis or the simultaneous determination of several heteroelements is rarely necessary.

It must be emphasized again that in writing the sections on the quantitative determination of elements and functional groups we have relied considerably on the chapter of CAC IB published in 1960 dealing with organic chemical analysis in detail. Therefore, methods that have not been modified since 1957-1960 will not be discussed here, and we refer readers to the book (CAC). Of course, papers published after 1960 to deal with such procedures and our own experience in connection with the old and new methods will be discussed.

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2. Determination of carbon

3. Determination of hydrogen

There is more importance attached to the individual determination of hydrogen than to carbon, as the results are more accurate and reliable than the gravimetric determination of water according to the classical method. Olson et al. [16] achieved an accuracy of ± 0 . 0 1 % in a coulometric method involving measurement of water adsorbed on phosphorus pentoxide. A similar procedure was suggested by Anisimova and Klimova [17]. Greenfield and Smith [18] used sulphuric acid as the absorbing liquid and measured the

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It is rarely necessary that only the carbon content of an organic compound is to be determined. N o old procedure is known for this purpose and this problem was not studied by Pregl and co-workers when developing micro-analytical methods.

Many papers dealing with such problems have been published since 1960. These mainly involve combustion of the sample in a stream of oxygen, yielding carbon dioxide, which is then absorbed in sodium hydroxide, barium hydroxide or barium perchlorate solution and the change in the conductivity of the solution is measured [2 -5 ] . Roemer et al. [6] reported on an automated volumetric [7] and coulometric [6] technique designed for the de-termination of carbon. The determination of carbon is important in metal-containing compounds [8] . Several papers have discussed the problem of how to determine the carbon dioxide content of the absorption liquid after combustion in an oxygen flask and to calculate the carbon content. The sample was packed in a metal foil or a glass textile instead of a piece of filter-paper, and the excess of the absorbing liquid (barium hydroxide, strontium hydroxide, sodium hydroxide and barium chloride solution) was titrated in the presence of an indicator [9, 10]. However, work in this direction was discontinued because, although the method is faster and simpler than gravimetry, it cannot compete with it in accuracy. An interesting procedure was elaborated by Mlinko [11, 12], involving pyrolysis of the sample in a stream of ammonia, conversion of the products into hydrogen cyanide at 1150°C in the presence of a platinum catalyst and subsequent determination of hydrogen cyanide by the Deniges method. Dobbs et al. [13] suggested an oxidation-reduction gas chromatographic method for the determination of carbon (representing organic matter) in water, using a hydrogen flame ionization detector. Japanese workers [14,15] developed a procedure for the determination of carbon-14.

change in its conductivity. The sample size was 1 mg and the accuracy was ±0.19 ng. Mlinko [19] removed by freezing the water produced during combustion, converted it into hydrogen sulphide with carbon disulphide in the presence of an aluminium oxide catalyst, and was then measured the hydrogen sulphide iodimetrically. In a procedure elaborated by Maly [20], the sample was heated in a sealed quartz vessel at 800°C for 6 h, during which period hydrogen left the vessel quantitatively by means of diffusion and the weight loss of the vessel represented the hydrogen content. Liebetran et al. [21] heated the sample in a nitrogen atmosphere, the pyrolysis products were led through a calcium chloride and then a palladium chloride layer, and hydrogen chloride equivalent to hydrogen was liberated. Mlinko and Kerecsenyi-Hermann [22] proposed several procedures for the determi-nation of hydrogen, based on the fact that hydrogen in organic matter will yield sulphur trioxide when allowed to react with sulphur, sulphur dioxide or carbon disulphide. The sulphur trioxide was then converted into sulphuric acid, which was distilled and titrated acidimetrically. In another procedure [23], the organic sample was mixed with sulphur powder, heated to 1200°C in a stream of nitrogen in the presence of a quartz-wool-platinum catalyst to form hydrogen sulphide, which was absorbed in an appropriate liquid and titrated iodimetrically. Fedoseev and Bajdulina [24] used magnesium activated with iodine. The sample was combusted and the water formed was allowed to react with magnesium iodide. As a result, the water decomposed completely and hydrogen gas was liberated and measured volumetrically. The absolute error was less than ±0.15%. Pauschmann [25] reported a gas chromatographic method using helium as the carrier gas.

4. Simultaneous determination of carbon and hydrogen

Lavoisier can be regarded as the originator of organic chemical elemental analysis, as he was the first to recognize that organic compounds consist mainly of carbon and hydrogen, as indicated by the fact that they yield carbon dioxide and water on combustion. About 50 years later, in 1837, Liebig succeeded in achieving the quantitative determination of carbon and hydrogen in organic compounds, and later Dumas developed a method for the determination of nitrogen. It was only at that time that Liebig found a suitable catalyst for the complete conversion of carbon and hydrogen in organic compounds into carbon dioxide and water at not too high temperatures (in a glass tube). Copper oxide was used for this purpose, as it is capable of oxidizing the pyrolysis products of organic compounds (including

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even methane) relatively rapidly. Among the pyrolysis products, carbon monoxide and hydrogen predominate.

Liebig effected the pyrolysis of organic matter in a stream of air in a glass tube about 1 m in length and 20-30 mm in diameter, containing a granulated copper oxide packing in a length of about 0.5 m and kept at the required temperature by a series of gas flames. The pyrolysis products leaving with the carrier gas were absorbed in vessels filled with ignited calcium chloride (for water) and granulated potassium hydroxide (for carbon dioxide). In this form the method was suitable only for the analysis of the simplest organic compounds consisting of carbon, hydrogen and oxygen. The interfering action of nitrogen oxides, halogen elements, sulphur, etc., which increased the weight of the absorption vessels and caused erroneous results for carbon and hydrogen, had to be eliminated by the application of suitable agents in the combustion tube (lead dioxide, lead chromate, metallic silver, etc.).

This macro-analytical method used in the last century and the early part of this century, was time consuming, troublesome and required large samples. One determination took about 2-3 h and about 1 g of sample was used. Skilled operators could achieve an accuracy of 0.2% for carbon and 0.1% for hydrogen. At that time, the analysis of a new compound was considered to be as important as its preparation.

Since the beginning of this century, as a result of research on natural organic substances, available sample sizes have usually been much smaller, so that the introduction of organic elemental and functional group micro-analytical methods became necessary.

The first analytical technique on the micro-scale was developed by Pregl, who received the Nobel Prize for his pioneering work. The Pregl method made possible the determination of carbon and hydrogen in 6-10 mg of sample, and nitrogen, sulphur, halogens and other elements and functional groups can also be determined today in similarly small samples. These methods are not simply the old methods for the macro-scale reduced to smaller sample sizes, but new principles based on careful experiments were also utilized to reduce the working time significantly while leaving the precision and accuracy unaltered. The development of micro-balances with a sensitivity of 1-2 jig and a reproducibility of better than 5 |ig allowed a high precision of weighing of these small samples and, with the advent of apparatuses equipped with electrical tube furnaces, gas purifying and regulating devices, etc., the initial operation problems have been eliminated and the technique has gained widespread application.

The main components of the Pregl apparatus designed for the de-termination of carbon and hydrogen on the micro-scale are the following:

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1. Gas source—later oxygen was used exclusively instead of air. 2. Gas purifying, pressure and flow rate regulating devices. Oxygen gas had

to be made free from all organic matter, carbon dioxide and moisture. Oxygen entered the combustion tube at a pressure higher than atmospheric by a few centimetres of water at a flow rate of 4 cm 3 /min.

3. The combustion tube was a heat-resistant glass tube (500 mm long, 10 mm i.d.), with one end narrowed and the other end closed with a rubber stopper. Oxygen gas was introduced through a side-tube near the open end.

About one third of the length of the combustion tube was packed with the following packings, starting from the narrow end:

(a) Silver wire, to prevent condensation of water vapour at the end of the combustion tube.

(b) Lead dioxide-asbestos, heated to about 200°C, to decompose and bind nitrogen oxides.

(c) Silver wool, to ensure thermal transition between the two furnaces and to bind halogens.

(d) Copper oxide-lead chromate, the so-called universal oxidizing packing. This oxidized the pyrolysis products of the organic sample completely into carbon dioxide and water and bound sulphur oxides in the form of lead sulphate. This section was heated to 550°C.

( e) Silver-wool, half-way out of the furnace heating the universal oxidizing packing; its temperature was 200-400°C and served for binding hydrogen halides and elemental halogens formed on pyrolysis of halogen-containing compounds.

(f ) The sample was weighed in a small platinum or quartz boat and placed in the combustion tube. Volatile liquids were weighed in glass capillaries than placed in the boat. The sample was pyrolysed with a small gas flame or a furnace, constructed from identical halves so that it can be opened, heated to about 500°C electrically.

4. Absorption tubes. The first absorption tube, packed with ignited calcium chloride or phosphorus pentoxide, was attached to the narrow end of the combustion tube. Today magnesium perchlorate is used almost exclusively for this purpose. The second absorption tube was attached to the first by means of a rubber tube, the packing was sodium asbestos (Ascarite) and a desiccant. The task of the latter was to bind water released in the reaction of sodium hydroxide and carbon dioxide in order to retain it in the absorption tube.

The two absorption tubes were somewhat different in length; the glass walls were very thin and could be closed with a ground-glass stopper. At both

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ends a system of chambers and capillaries was constructed so as to allow only forced flow (under the influence of suction or pressure). The weight of the absorption tubes when packed must be less than 20 g.

5. As the packing of the combustion tube and the absorption tubes present a considerable barrier to the gas flow, usually a Mariotte bottle is attached to the end of the absorption tubes to produce a slight suction.

In the original Pregl method, the greatest drawback was the frequent need to refill the combustion tube (after 20-30 measurements) and the long time required to attain the "moisture equilibrium" of the system that is necessary for correct results. The use of lead oxide was particularly disadvantageous in the decomposition of nitrogen oxides, as it requires a particular temperature and moisture content for efficient action. Therefore, for example, samples with high and low hydrogen content could not be measured consecutively as the change in hydrogen concentration in the sample affected the results obtained with the subsequent sample. Coupling of the absorption tubes to the combustion tube and each other with rubber tubing was another source of error.

There were so many problems with the method that Boetius [26] wrote a whole book on them in 1931.

In the 70 years since the development of this method, several modifications have been introduced but the fundamental principles have been retained.

Today, the oxidizing packing contains more powerful oxidizing agents, such as cobalt oxide, nickel oxide, the decomposition product of silver permanganate, etc., instead of lead chromate and copper oxide. Nitrogen oxides are converted into elemental nitrogen not with lead dioxide but with chromic acid-sulphuric acid or manganese(IV) oxide placed after the combustion tube. In this way, the working time has been shortened, owing to the increased flow rate of the gas (10-20 cm 3 /min, instead of 4 cm 3 /min) . Combustion of the sample and absorption of the pyrolysis products can thus be achieved in 15-20 min, instead of 45-50 min. Monographs published before 1960 discussing the determination of carbon and hydrogen can be regarded as obsolete, but can be very instructive for beginners in organic elemental micro-analysis.

Apparatus for the determination of carbon and hydrogen is manufactured today by many companies. Such apparatus is equipped with well regulated furnaces and the absorption tubes are coupled through ground-glass joints, etc. The apparatus produced by Haereus Co. can easily be converted into one suitable for the determination of nitrogen and oxygen. For technical details, we refer readers to the operating manuals.

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In this manner, the determination of carbon and hydrogen in simple substances can be accomplished in 25-30 min with an accuracy of ±0 .2-0 .3% for carbon and ±0 .05-0 .1% for hydrogen (absolute error). The term "simple substances" refers to compounds with not too high or too low carbon and hydrogen content (40-70% C, 3-15% H) and with the elemental constituents oxygen, nitrogen, halogens (except fluorine) or sulphur. No other elements should be present. When compounds containing phosphorus, silicon, boron or metals are combusted, the sample must be covered with tungsten oxide in the sample boat. This compound has an oxidizing and flushing action and prevents retention of carbon or hydrogen by the residue in the boat. In combustion of mercury compounds (mercury is retained only partially by tungsten oxide) a packing made from gold foil or wire must be placed before the absorption tubes. The sulphur oxides were bound by lead oxide, forming lead sulphate, in the Pregl packing. Silver is used for this purpose now, but it is not sufficiently effective in the presence of large amounts of sulphur. Therefore, with compounds with a high sulphur content (higher than 30%), an adsorber packed with manganese dioxide is employed before the absorption tubes. Difficulties often arise during combustion of heterocyclic nitrogen compounds with high nitrogen content. In these instances, higher temperatures must be applied, and the use of tungsten oxide for facilitating combustion is recommended.

A significant modification of the Pregl method was suggested by Belcher and Ingram [27, 28] in which an empty tube with a vertical furnace heated to about 900°C is used. In this arrangement, the pyrolysis products are in prolonged, intimate contact with oxygen and intense mixing is ensured, so that complete combustion is achieved. A silver packing at 400-450°C is applied only after the combustion tube to retain halogens and sulphur oxides.

On pp. 443-458 of Volume IB of CAC, Goulden described two procedures designed for the determination of carbon and hydrogen. The first is the so-called "slow" method, being essentially a modern variation of the Pregl method. The same principle is applied in the commercial apparatus (Haereus Co. etc.) available today. The other is the Belcher-Ingram "rapid" method, which is very suitable for the rapid serial determination of the carbon and hydrogen contents of organic compounds that are relatively easy to combust. Descriptions of the apparatus and of the sample preparation and combustion techniques are given in the original publication.

In the following, those papers published after 1960 that deal with improvements in detail to the Pregl micro-method for the determination of carbon and hydrogen, will be discussed.

Thomas [29] reported a simple apparatus. Hadzija [30] used nitrogen as a carrier gas, and the packing in the oxidizing tube was copper oxide. Nitrogen

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oxides were bound on a metallic copper packing, while the sulphur oxides formed from sulphur-containing compounds were retained on a silver oxide -manganese oxide layer. Binkovski [31] subjected organic samples to pyrolysis at 1050-1070°C, using copper oxide as the packing in the oxidizing tube. At this high temperature the flow rate of the gas could be increased to 20 cm 3 /min, and thus one measurement could be finished in 14-16 min. Oda et al. [32] utilized a special combustion tube, the sample was added as a drop at 800-850°C and was combusted immediately, with explosion-like rapidity. The Pregl method was also improved by the special tube packing elaborated by Gustin and Trefft [33]. The packing designed for binding the halogens consisted of silver tungstate and silver oxide mixed with an equal amount of Chromosorb W. This was followed by a magnesium oxide-based packing mixed with silver oxide, silver tungstate and Chromosorb P. The absolute error was ±0 .03% for carbon and ±0.04% for hydrogen. Trutnovsky [34] described an apparatus with the absorption tubes hanging in a micro-balance. The combustion products were led there through thin capillaries and the change in weight was measured continuously. Kainz and Horvatich [35] studied the "empty tube" method developed by Belcher and Ingram and stated that the temperature of the furnace for the combustion tube should not be less than 750°C, but the actual temperature also depends on the nature of the sample. Ingram [36] further improved the "empty tube" technique. The combustion tube was a quartz vessel of length 150 mm and diameter 33 mm, the boat with the sample was introduced into it by means of a magnet and combustion took place there. Butterworth [37] employed an induction furnace for combusting the sample, Mitsui et al. [38] effected combustion in an atmosphere of nitrogen gas and oxygen produced by electrolysis was mixed with it; further, a special packing in the tube and metal absorption tubes were used. The method was regarded as suitable for automation. Kainz and Scheidl [39] studied the combustion of methane in an oxygen atmosphere. It was stated that in the empty tube method the temperature applied can be reduced from 800-850°C to about 750°C, when a star-shaped piece of platinum or finely divided platinum metal is placed in the combustion tube. The effect of platinum was found to be nearly identical with that of granulated copper oxide. The same workers [40] reported a combustion tube constructed so as to allow the introduction of vapours of the sample and oxygen through a nozzle. Addition of hydrogen gas to the gas mixture was necessary only when the halogen content of the sample was higher than about 50%. The procedure on both the semi-micro- and micro-scale proved to be very suitable for the determination of carbon and hydrogen, and also as of halogens, sulphur and phosphorus. Rittner and Culmo [41] constructed an apparatus consisting of four relatively thin combustion tubes in the furnace.

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This made possible four simultaneous measurements of carbon and hydrogen, and one measurement took only 7.5 min. Kirsten [42] developed a rapid automatic procedure, in which the quartz tube was packed with silver permanganate, cobalt(II, III) oxide and lead oxide. Using a rapid flow of gas, one measurement could be accomplished in 10 min. Gelman and Sheveleva [43] placed a thin quartz test-tube containing 4-8 mg of in the larger combustion tube and the quartz test-tube was placed with the open end towards the gas stream. This method has been known for some time, and its advantage is that the pyrolysis products mix with oxygen slowly but completely. Secor and White [44] used capsules made from indium metal for weighing the samples, which made possible the determination of the carbon and hydrogen contents of volatile samples. Sels and Demoen [45] modified the Coleman nitrogen analyzer into a carbon and hydrogen analyzer, and one measurement could be effected in 15 minutes.

Alternative types of tube packing with different properties have been developed. It is well known that the Pregl universal packing has a relatively low efficiency, and the flow-rate of the gas should not be higher than about 4 cm 3 /min, so that pyrolysis of the sample and transport through the tubes takes 25-30 min. This time can be reduced by increasing the flow rate of the gas, but this is possible only when the Pregl universal packing is replaced with more powerful oxidizing agent(s).

Another factor is the length of the packing, and shortened packings lead to a shorter working time. Decomposition products of silver permanganate and cobalt(II, III) oxide had been used successfully before 1960. The use of this product as an oxidizing packing was discussed in detail by Vecera [46], who emphasized its advantages. Kakabadse and Manohin [47] and also Potman et al. [48] used a cerium oxide mixed catalyst as the oxidizing agent and a silver layer was also placed in the tube. In this way, the determination of carbon and hydrogen was effected in 15 min at a flow rate of the gas of 25 cm 3 /min. The mixed catalyst was found to be superior to pure cerium oxide. Fluorine, phosphorus, boron and metals do not interfere. Abramyan et al. [49] prepared a paste made from potassium permanganate, asbestos and water as the oxidizing packing, this was heated at 200°C for 2 h, 500°C for 2 h and 800-900°C in a stream of oxygen for 4 h. The packing, of length 100 mm was heated to 400-450°C during the measurement. The packing will retain halogens, phosphorus, sulphur and nitrogen and is also capable of binding 140 mg of fluorine. Campiglio [50] used a barium chromate packing. The sample was pyrolysed in a stream of nitrogen gas then combusted in a stream of oxygen in 15 min. Abramyan and Karapetyan [51] effected the combustion of 3-6 mg of sample in a stream of oxygen at 850-900°C, then the pyrolysis products were led through an asbestos bed containing decomposition

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products of silver permanganate at 40O-500°C Halogens and sulphur oxides were adsorbed on a layer of silver. Kissa and Seepera-Yllo [52] studied the efficiency of tungsten oxide with pure metal carbonates decomposed by tungsten oxide. Combustion was carried out at temperatures higher than 900°C; at lower temperatures, additives that promote melting must be used.

Pella [53] reported investigations which indicated that in the first section of the combustion tube reducing conditions predominate, and thus no nitrogen oxides are formed in reduction by carbon and hydrogen. Horacek et al. [54] investigated the performance of 28 catalysts in the determination of carbon and hydrogen by measuring the lowest temperature at which the oxidation of methane was still complete. Cobalt(II,III) oxide was found to be the best, methane being oxidized at 311°C. Abramyan and Afashian [55] observed that cobalt oxide not only is an excellent catalyst, but also can bind sulphur oxides. The Pregl universal packing, consisting of copper oxide and lead chromate, oxidized methane only at 650°C.

Kainz and Mayer [56] studied the efficiency of manganese oxide and lead dioxide precipitates with respect to their activity in decomposing nitrogen oxides. The activity of manganese dioxide was found to be greatly dependent on the conditions of drying of the precipitate, the completely dry substance being almost inactive. Manganese dioxide becomes dry under the influence of the gas stream, so that it must be wetted again. Dry-prepared manganese dioxide has no nitrogen oxide-decomposing activity. Kainz and Horvatitsch [57] established that the decrease in methane concentration along the oxidizing packing follows a simple correlation, and a similar statement applies to the flow rate of the gas. The same authors [58] carried out experiments with an empty combustion tube and stated that the least combustible substances are the members of homologous series with the lowest molecular weight; at a flow-rate of 5 cm 3 /min the required temperature is at least 750°C. Vecera et al. [59] studied systematically the kinetics and mechanism of combustion. They developed a rapid procedure for the determination of carbon and hydrogen in a few minutes, and the combustion method was suitable for complete automation.

Kainz and Mayer [60] studied the efficiency of the manganese dioxide packing as a function of the amount of nitrogen oxides, flow-rate and the thickness of the manganese dioxide bed, and calculated the break-through capacity. Kainz and Horvatitsch [61] examined the elution of carbon dioxide and water from various oxide layers. Dry-prepared copper oxide does not retain the combustion products, whereas the wet-prepared product does. This effect can be eliminated by thermal treatment. Retention is caused by the hydroxyl groups present. The same authors [62] studied oxide and metal catalysts in various gas streams for the determination of carbon, hydrogen

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and nitrogen. In an oxygen stream, metallic palladium proved to be the best, followed by colbalt(II,III) oxide and manganese dioxide, active at 100~200°C. In a stream of carbon dioxide, in the Dumas determination of nitrogen, manganese dioxide is the best, followed by copper oxide, cobalt(II,III) oxide and iron oxide, the minimum temperature for their operation is 100-350°C. In reductive pyrolysis in hydrogen gas, the order is palladium, platinum, chromium oxide, cerium oxide and zinc oxide, at 300-500°C.

Kainz and Mayer [63] investigated the efficiency of manganese dioxide in the decomposition of nitrogen oxides as a function of the particle size and humidity of the catalyst. The product with small particle size and small amounts of water adsorbed was found to be the best. The temperature should not be higher than 50°C, otherwise the manganese(II) nitrate formed in the reaction decomposes. The same authors [64] prepared very active lead dioxide for the decomposition of nitrogen oxides so that lead(II) salt solutions were oxidized with alkaline hydrogen peroxide. This material was significantly more active than manganese dioxide. A mixed catalyst consisting of lead dioxide and manganese dioxide was also prepared, and the migration of the absorption zone was also observed. The high activity of the lead dioxide compound was explained by the presence of P b O ( O H ) 2 , the hydroxyl groups in which can bind water, drying of the compound thus being prevented.

Newman and Tomlinson [65] investigated the conditions under which the external catalyst layer (placed outside the combustion tube) used for the binding of nitrogen oxides will not retain water. They used manganese dioxide, and the conditions were adjusted so that accuracies of ±0 .35% and ±0 .25% for carbon and hydrogen, respectively, could be achieved, even with substances with a high nitrogen content.

Kainz and Mayer [66] found that lead dioxide is very suitable for the decomposition of nitrogen oxides, with optimal conditions, temperature and length of the layer. The method for the preparation of the very efficient lead dioxide compound was also given. Later they studied the carbon dioxide and water retaining power of the lead dioxide layer by gas chromatography, and observed reversible adsorption of the combustion products of carbon and hydrogen on lead dioxide [67]. This effect is more significant with water than with carbon dioxide. The extent of adsorption depends on the method of preparation of lead dioxide, the product obtained by the wet procedure being poorer in this respect than the dry-prepared product. Adsorption can be attributed to two factors: the presence of isolated hydroxyl groups, and the absence of anions in the lattice.

Childs [68] examined the effect of various catalysts in the determination of carbon and hydrogen. Kainz and Zidek [69] studied the interfering action of

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nitrogen oxides in the micro-determination of carbon and hydrogen. Hydrogen peroxide, concentrated sulphuric acid, a mixture of sulphuric acid and potassium permanganate, and sulphuric acid and various anions were examined in this respect. Kainz and Chromy [70] used gas chromatography to study the carbon compounds formed in the determination of carbon and hydrogen as a function of the experimental conditions, and the completion of their oxidation into carbon dioxide.

Many papers have dealt with the problem of the interference of certain elements in the determination of carbon and hydrogen. This is still a problem in the construction of automated analyzers.

A method for the determination of the carbon and hydrogen content of bromine-containing organic compounds was developed by Margolis and Bibileishvili [71]. The packing was barium silicate at 750°C, instead of the usual silver layer, and the flow-rate of oxygen was 13 cm 3 /min . In the combustion of organic compounds with high chlorine and bromine contents, Awad et al. [72] employed a layer of silver-wool with a length of about 100 mm in an empty combustion tube at 450°C. Abramyan et al. [73] carried out the combustion of fluorine-containing samples with a packing consisting of the thermal decomposition products of silver permanganate. Tonkovic and Mesaric [74] used a packing prepared from pure thorium oxide or a mixture of thorium oxide and silicon oxide in the combustion tube in the analysis of fluorine-containing organic samples, heated to about 600°C. The packing retains only hydrogen fluoride.

Pechanek and Horacek [75] examined the possibility of binding halogens and sulphur compounds by means of a silver packing. According to their studies, silver binds iodine most readily and sulphur oxides less readily. The binding power increases with increasing surface area, and decreases with increasing temperature. Spongy silver was found to be the best material when mixed with 0 .1-1% of aluminium oxide. The optimal temperature is 100-550°C for halogens and 400-550°C for sulphur oxides. At these temperatures, the binding efficiency of silver is 85-95% of the theoretical value.

Volodina et al. [76] carried out the combustion of fluorine-containing samples for the determination of carbon and hydrogen in a stream of ammonia, and fluorine was bound on iron(III) oxide. Bishara et al. [77] developed a method for the determination of the carbon and hydrogen content of fluoro and perfluoro compounds. Wojnowski et al. [78] studied sulphur-containing organosilicon compounds and developed a semimicro gravimetric procedure. The sample was combusted after wrapping it in an aluminium foil, and the packing was cobalt(II,III) oxide in a vertical tube at 920-950°C. The samples were combusted at this temperature in an explosion-like reaction and one measurement took 10 min.

289

Celon and Bredasola [79] investigated the determination of the carbon and hydrogen content of polymers containing boron and silicon. The sample was heated at high temperatures in a non-flowing oxygen atmosphere for a prolonged period and a manganese dioxide, chromium oxide and tungsten oxide catalyst mixture was employed. Gawargious and Macdonald [80] studied the analysis of silicon-containing substances. The sample was evaporated very slowly and cautiously, then led through a magnesium oxide layer at 850°C with oxygen as the carrier gas. Under these conditions, no silicon carbide was formed, but magnesium silicate was obtained. The determination of carbon and hydrogen in organophosphorus compounds was studied by Binkovski and Vecera [81]. The oxidizing packing was cobalt(II,III) oxide and the phosphorus compounds were bound on a pumice layer impregnated with silver. Binkovski [82] eliminated the interfering action of phosphorus by modifying the combustion process. Pietrogrande and Dalla Fini [83] suggested the use of silver permanganate and cobalt oxide catalysts in the determination of carbon and hydrogen in organophosphines. Bishara and Attia [84] carried out rapid combustion in the empty tube in the determination of carbon, hydrogen and phosphorus in one sample. The phosphorus content was calculated from the weight of the P 2 O s residue.

Buis and Pieters [85] determined carbon and hydrogen in germanium-containing organic substances by covering the sample with tungsten oxide in the boat in a very slow combustion procedure, the oxidizing packing being the decomposition product of silver permanganate.

Lebedeva et al. [86] examined complex organic substances containing thallium. The sample was placed in a quartz test-tube (90 mm long) in the combustion tube, which was filled with quartz powder treated with base and acid then ignited (particle size 0.5-1 mm). The oxidizing packing was cobalt oxide. Thallium was bound by quartz as silicate and exerted no interfering action in the determination of carbon and hydrogen.

Gelman et al. [87] developed a simultaneous procedure for the determination of carbon, hydrogen and tin. Holmes and Lauder [88] investigated mercury-containing samples, and in the determination of the carbon and hydrogen content, a piece of asbestos impregnated with gold bromide in diethyl ether then dried was placed in a Flaschentraeger tube and the pyrolysis products were led through it at a rate of 150 cm 3 /min ; 7-10-mg samples could be combusted in 5 min. Newman and Tomlinson [89] used uranium persulphate in the combustion of alkali and alkaline earth metal-containing substances using an apparatus described earlier [90].

Gawargious and Macdonald [91] studied the determination of the carbon and hydrogen contents of organic substances containing metallic and non-metallic elements. They used tungsten oxide in the boat, the oxidizing catalyst

290

being cobalt oxide, and obtained excellent results in the determination of 27 compounds.

Saran et al. [92] obtained lower carbon content than the calculated values in the combustion of polycyclic compounds. Similarly, the carbon and hydrogen values were not correct in the combustion of perhydropolycyclic compounds carrying an angular methyl group. These substances were subjected to pyrolysis at 900°C with the use of an oxidizing packing prepared from lead dioxide, silver, copper oxide, platinum sieve and platinized asbestos. When heated to 700°C, correct results were obtained. Marzadro et al. [93] in the determination of the carbon and hydrogen contents of certain nitrogen-containing organic substances, obtained too high values for hydrogen when nitrogen oxides were decomposed after the combustion tube. When, for example, compounds of type

|« I/* N C - C - C - C N

contained hydrogen at the a and /? positions, too high hydrogen contents were obtained, whereas the result was correct when these carbon atoms carried phenyl or hydroxyl groups.

Gawargious and Farad [94] achieved the satisfactory combustion of steroids even by the rapid method when a cobalt oxide oxidizing packing was employed. Pella [95] studied volatile and flammable samples, which were weighed in a quartz tube to allow slow, gradual contact with oxygen in the course of the pyrolysis process. Combustion was effected with an automatic furnace moving forwards and backwards at a constant rate.

The methods discussed so far, designed for the determination of the carbon and hydrogen content of organic compounds, were gravimetric methods, that is, the pyrolysis products were led through absorption tubes of known weight and the gain in weight was used to calculate the results. Weighing and previous conditioning of the surface of the glass absorption tubes was the crucial factor and a time-consuming step in the measurement. Binkovski [96] used special absorption tubes that could be transferred rapidly to a special semi-micro-balance to save time. Although the use of more powerful oxidizing packings made possible the use of higher gas flow-rates with a consequent reduction in the analysis time from 30-35 to 10-15 min, the time required for the treatment and weighing of absorption tubes did not decrease significantly. Therefore, after 1960, some workers tried to introduce a more rapid procedure than gravimetric measurement in the determination of carbon and hydrogen. The first successful automatic carbon and hydrogen analyzer was described by Malissa [97], in which the combustion products of organic samples were rapidly determined volumetrically in a recording apparatus.

21 291

The apparatus consisted of an oxygen-purifying device, a combustion tube, a pump, measuring cells and a recorder. The combustion tube was made from glass, porcelain or quartz and one end was closed. No oxidizing packing was applied, and therefore a very high temperature was required, (about 1200°C). The combustion products were transported from the combustion tube to the measuring cell by means of a pump at a constant rate (the flow rate of the gas was about 3 d m 3 / m i n and the pressure was about 10554 Pa). Absorption of the combustion products took place in the measuring cell. Two measuring cells were used, both containing the same absorbing liquid, and their electrical conductivities were measured. The measuring cells were carefully therm ostated, as a 1°C change in temperature brought about a 3% change in conductivity. Into one cell the carrier gas (oxygen), and into the other the carrier gas containing the combustion products were introduced, and the difference in the conductivities in the two cells was amplified electrically and recorded. When the two cells contained fresh absorption liquid, the recorder indicated the zero line.

In the measuring cells 0.005 N sodium hydroxide solution was placed. Before the determination of the hydrogen content, water was converted into acetylene by reaction with calcium carbide, then combusted in a stream of oxygen to give carbon dioxide, which was led into the measuring cell. Sulphur oxides were absorbed in an acidic solution that does not dissolve carbon dioxide [the solution may contain sulphuric acid-potassium pyrochromate, thallium(III) chloride or a mixture of sulphuric acid and hydrogen peroxide]. The apparatus was checked with suitable test samples weighing about 1 mg, and in this way all systematic errors in the apparatus and the procedure were eliminated. One measurement took about 10 min.

Kainz et al. [98] used 0.01 N sodium hydroxide solution containing 2% of ethanolamine for the absorption of carbon dioxide. Water was absorbed in glacial acetic acid containing 2% sulphuric acid. The sample was combusted at 900°C in a stream of oxygen, water was adsorbed on calcium chloride, which was then removed at 350°C and introduced into the absorption cell, called the mixing cell as there is a magnetic stirrer in it to ensure rapid mixing. In both solutions the change in conductivity was measured and the results were calculated from these data, after adjustment with suitable substances.

Merz [99] effected combustion of the sample in a boat covered with manganese dioxide in a stream of oxygen in a vertical quartz tube, and combustion was completed on a copper oxide packing. Water was removed from the gas stream by freezing, carbon dioxide was absorbed.

The water removed by freezing was evaporated in a stream of nitrogen and led through a carbon bed at 1120°C, where carbon monoxide was formed. This was oxidized into carbon dioxide with copper oxide and its amount

292

measured in the above manner. When double freezing vessels and measuring cells were used, water and the corresponding hydrogen content could be determined simultaneously with carbon dioxide. One measurement (carbon and hydrogen) took 8 min.

A similar procedure was suggested by Roemer et al. [100], but only the carbon content was determined automatically. The absorption liquid for carbon dioxide was 0.1 mole /dm 3 barium perchlorate in a mixture of tert.-butanol and water. The pH of the solution was 9.5, which changed while carbon dioxide was absorbed. The original pH was re-established by titrating it automatically with 0.03 mole /dm 3 sodium hydroxide solution in tert. butanol-water. Later the method was developed further [101], carbon dioxide being absorbed in 0.075 N sodium hydroxide in tert.-butanol also containing barium ions. Water was evaporated after freezing and converted into carbon dioxide in a reaction with l-isocyanatonaphthalene-1,4-diazabicyclo[2, 2, 2]octanone at about 80°C, and measured as above. Floret [102] also employed freezing to separate carbon dioxide and water. The former was titrated conductimetrically, then water was evaporated and led through a layer of N,N'-carbonyldiimidazol which reacted with the formation of stoichiometric amounts of carbon dioxide.

Wachsberger et al. [103] applied an automatic analyzer and, after combustion of the sample in a stream of oxygen, the combustion products were separated by selective absorption and desorption using helium carrier gas, then carbon dioxide, water and nitrogen were fed into a cell for measurement of thermal conductivity. The potential signals were trans-formed into frequency, evaluated by means of an electronic digital integrator and printed out. The accuracy of the method is equal to that of the chemical methods.

Kainz and Wachsberger [104] also applied a thermal conductivity detector after separation of water and carbon dioxide on calcium chloride. Vecera [105] used an automatic apparatus for the determination of carbon and hydrogen with thermal conductivity detectors, measuring the thermal conductivity of oxygen gas containing carbon dioxide. Hydrogen was determined by converting the combustion product (water) into hydrogen with iron turnings at 650°C in a stream of nitrogen, followed by measurement of the thermal conductivity of nitrogen gas containing hydrogen.

Monar [106] added oxygen containing ozone to the helium carrier gas during combustion, then the components of the gas mixture containing carbon dioxide, water and nitrogen were separated and the amounts determined on the basis of thermal conductivity data. Rezl and Kaplanova [107] effected combustion by the Pregl -Dumas method, followed by frontal gas chromatographic separation. The method suggested by Pella and

21* 293

Colombo [108] is based on a similar principle, combustion being effected in a vertical quartz furnace at 1050°C in helium carrier gas containing oxygen;

Van Leuven and Gouvernier [109] employed a manometric technique for the determination of the carbon and hydrogen content of organic substances after separation of carbon dioxide and water by freezing. A photoelectric method was utilized to adjust the level of the mercury in the micro differential manometer. Ihn et al. [110] applied a similar principle. One measurement takes about 20 min and the standard errors are 0.42% and 0.27% for carbon and hydrogen, respectively.

Another manometric technique was described by Simon and Miillhofer [111], using samples smaller than 0.3 mg. Simon et al. [112] gave a comprehensive survey of the possibilities for the completely automated determination of carbon and hydrogen in organic substances.

In connection with the automatic determination of the carbon and hydrogen content of organic substances, various workers have described techniques for coupling the combustion apparatus with a gas chromato-graph. The first such method was described by Sundberg and Maresh [113], involving the separation of carbon dioxide and acetylene obtained from water with calcium carbide on a silica gel column. This determination was carried out in a similar manner by Haber and Gardiner [114]. Berezkin et al. [115] suggested combustion of the sample in a sealed ampoule mixed with copper oxide at 650-700°C, the combustion products being transferred to the gas chromatographic column with helium carrier gas. Rezl et al. [116] used a micro combustion apparatus coupled to a gas chromatograph for the determination of hydrocarbons.

Kuck et al. [117] determined the carbon dioxide content of gas mixture on the basis of the IR absorption, while water converted into hydrogen was measured by the change in thermal conductivity. Thurauf and Assenmacher [118] also employed IR measurement in the determination of carbon dioxide and water. The latter was converted into carbon monoxide on a layer of carbon, then oxidized to carbon dioxide. The apparatus was equipped with an integrator and a recorder. One measurement took 7 min and the standard errors were ±0 .23% and ±0 .16% for carbon and hydrogen, respectively.

In addition to carbon and hydrogen, nitrogen must also be determined frequently in organic samples. For this reason, workers have endeavoured for many years to construct apparatus suitable for the simultaneous de-termination of these three elements in one sample. The simplest solution seemed to be an arrangement in which the gas mixture leaving the absorption tubes containing only nitrogen and the carrier gas is led, after removal of oxygen, into a nitrometer with carbon dioxide carrier gas and then measured.

294

In certain constructions, the apparatus for the determination of carbon and hydrogen was built together with the nitrogen-determining apparatus [119], and the two measurements, on two samples, were effected simultaneously. However, this is rather troublesome and not accurate enough, and thus has not gained widespread application. Subsequent determination of the three elements in one sample by gas volumetric method has also been tried [120, 121]. The method proposed by Hozumi and Kirsten [122] is very simple. The sample was combusted in a sealed tube in an oxygen atmosphere, then selective absorbents were introduced into the tube through a mercury layer and the volume changes were measured gas volumetrically. Similar principles were applied by Frazer and Stump [123] using a manometer. The method elaborated by Kirsten et al. [124] proved to be more satisfactory, water and carbon dioxide being removed by freezing, nitrogen being measured first manometrically, then the two other components. Fedoseev and Baidulina [125] used a similar method.

Kainz and co-workers [126, 127] examined the efficiency of various catalysts in the simultaneous determination of carbon, hydrogen and nitrogen. Coupling of the combustion apparatus with a gas chromatograph was started at the beginning of this work. Nightingale and Walker [128] separated nitrogen, carbon dioxide and acetylene obtained from water with molecular sieves and analyzed by means of a thermal conductivity detector. Chumacenko and Pachomova [129] developed a similar technique, using helium carrier gas, in the separation of nitrogen, carbon dioxide and acetylene. Rezl suggested a similar procedure [130]. Combustion of the sample in a closed vessel filled with oxygen at 90O-950°C in 1-2 min was achieved by Chumacenko et al. [131], with subsequent transfer of the combustion products into the gas chromatographic column with helium carrier gas.

Frazer and Crawford [132] described a method for the simultaneous determination of the carbon, hydrogen and nitrogen content of volatile substances. Campbell et al. [133] carried out the analysis of organometal perchlorates and nitrates in the Coleman C H N analyzer. Wright [134] developed a method for the cautious pyrolysis of explosive organic substances for the determination of carbon, hydrogen and nitrogen contents. Chumacenko and Pachomova [135] improved their earlier procedure [129]. Jaenicke and Walish [136] investigated the composition of combustion products formed in the determination of carbon, hydrogen and nitrogen by means of mass spectrometry. Five C H N determinations were carried out by them in 1 h on about 300-|ig samples. Various workers have compared commercial C H N analyzers with the classical Pregl and Dumas methods with respect to their accuracy and reliability.

295

Many papers have been published on the determination of elements other than carbon and hydrogen in one sample [137-144]. Detailed description of these methods will be omitted here, as they have practical importance only when the total sample size available is only a few milligrams, which rarely happens. The accuracy and reliability of these methods is less than that of methods specially designed for the determination of the particular heteroele-ment in separate samples. Here only the relevant literature is listed.

The following references can be cited: determination of carbon, hydrogen and sulphur [145-148]; carbon, hydrogen and halogens [149-151]; carbon, hydrogen and boron [152]; carbon, hydrogen and mercury [153, 154]; carbon, hydrogen and aluminium [155]; carbon, hydrogen and germanium [156]; carbon, hydrogen, silicon and germanium [157] ; carbon, hydrogen and thallium [158]; carbon, hydrogen and metallic elements [159]; carbon, nitrogen and sulphur [160] ; carbon, hydrogen, nitrogen and oxygen [161]; carbon, hydrogen, oxygen and sulphur [162]; carbon, hydrogen, iodine and sulphur [163]; carbon, hydrogen, sulphur and phosphorus [164] ; carbon, hydrogen, nitrogen and oxygen [165] ; and carbon, hydrogen, nitrogen, sulphur and halogens [166].

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(1972). 117. Kuck, J. A., Berry, J. W., Andreatch, A. J., Lentz, P. A.: Anal. Chem., 34, 403 (1962). 118. Thurauf, W., Assenmacher, H.: Z. anal. Chem., 262, 262 (1972). 119. Knobloch, W., Knobloch, F., Mai, G.: Mikrochimica Acta, 4, 576 (1961). 120. Frazer, J. W.: Mikrochimica Acta, 993 (1962). 121. Koch, C. W., Jones, E. E.: Mikrochimica Acta, 734 (1963). 122. Hozumi, K., Kirsten, W. J.: Anal. Chem., 35, 1522 (1963). 123. Frazer, J. W , Stump, R.: Mikrochimica Acta, 1324 (1968). 124. Kirsten, W. J , Hozumi, K., Kirk, L.: Z. anal. Chem., 191, 161 (1962). 125. Fedoseev, P. N., Baidulina, G. O.: Izv. vyssh. ucheb. Zavod. Khim. khim. Tekhnol., 14 (7), 1061

(1971); Ref., Anal. Abstr., 23, 3206 (1972). 126. Kainz, G., Horvatitsch, H.: Mikrochimica Acta, 7 (1962). 127. Kainz, G , Zidek, K.: Mikrochimica Acta, 725 (1967). 128. Nightingale, C. F., Walker, J. M.: Anal. Chem., 34, 1435 (1962). 129. Chumacenko, M. N , Pachomova, I. E.: Dokl. Akad. Nauk SSSR, 170, 125 (1966); Ref., Z.

anal. Chem., 235, 284 (1968). 130. Rezl, V.: Microchem. J., 15, 381 (1970). 131. Chumacenko, M. N., Pachomova, I. E., Ivandichova, R. A.: Izv. Akad. Nauk SSSR, Ser.

Khim., 1219 (1970); Ref., Anal. Abstr., 20, 3868 (1971). 132. Frazer, J. W., Crawford, R. W.: Mikrochimica Acta, 676 (1964). 133. Campbell, A. D., Loo Shai Harn, Monk, R, Petrie, D. R.: Mikrochimica Acta, 836 (1968). 134. Wright, H.: Explosivstoffe, 14, 274 (1966); Ref., Anal. Abstr., 15, 1495 (1968). 135. Chumacenko, M. N , Pachomova, I. E.: Dokl. Akad. Nauk SSSR, 170,125 (1966); Ref., Anal.

Abstr., 14, 7503 (1967). 136. Jaenicke, O., Walish, W.: Talanta, 22, (4-5) 345 (1975). 137. F. M. Sci. Corp. New Model 180 CH Analyser. Anal. Chem., 35, 21A (1963). 138. Clerc, J. T., Dohner, R., Sauter, W., Simon, W.: Helv. Chim. Acta, 46, 2369 (1963). 139. Weitkap, H., Korte, F.: Chem. Ing. Technik, 35, 429 (1963); Z. anal. Chem., 207,141 (1965). 140. Walisch, W., Scheuerbrandt, G., Marks, W.: Microchem. J., 11, 315 (1966). 141. Condon, R. D.: Microchem., J., 10, 408 (1966). 142. Culmo, R.: Mikrochimica Acta, 175 (1969). 143. Childs, O. E., Hemer, E. B.: Microchem. J., 15, 590 (1970). 144. Ebel. S.: Z. anal. Chem., 262, 352 (1972). 145. Rittner, C. R, Culmo, R.: Microchem. J., 11, 264 (1966). 146. Hadzija, O.: Mikrochimica Acta, 619 (1968). 147. Abramyan, A. A., Kocharyan, A. A., Megroyan, R. A.: Izv. Akad. Nauk Armyan SSR. Ser.

Khim., 20, 15 (1967); Ref., Anal. Abstr., 15, 2029 (1968). 148. Radmacher, W., Hoverath, A.: Z. anal. Chem., 181, 77 (1961). 149. Mamaril, J. C , Meloan, C. E.: J. Chromatogr., 17, 23 (1965). 150. Fadeeva, V. P., Diakur, L. N.: Izv. sib. Otdel. Akad. Nauk SSSR Ser. Khim., (2) 139 (1969);

Ref., Anal. Abstr., 19, 1335 (1970). 151. Campbell, A. D., Macdonald, A. M. G.: Anal. Chim. Acta, 26, 275 (1962). 152. Rittner, R. C , Culmo, R.: Anal. Chem., 34, 673 (1962).

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153. Lebedeva, A. J., Kramer, K. S.: Izv. Akad. Nauk SSSR. Otdel. Khim. Nauk, (7) 1305 (1962); Ref., Anal. Abstr., 10, 1042 (1963).

154. Pechanec, V.: Coll. Czech. Chem. Comm., 27,1817 (1962); Ref., Anal. Abstr., 10,1835 (1963). 155. Gelman, N. E., Brjuska, I. I.: Zhur. Anal. Khim., 19, 369 (1964); Z. anal. Chem., 206, 141

(1965). 156. Pieters, H., Wim J. Buis: Microchem. J., 8, 383 (1964). 157. Aranyi-Halmos, T., Erdey-Schneer, A.: Magyar Kern. Lapja, 20, 164 (1965). 158. Lebedeva, A. I., Nikolaeva, B. A., Shikmann, E. V.: Zhur. Anal. Khim., 20 (7) 832 (1965);

Ref., Anal. Abstr., 14, 1429 (1967). 159. Gelman, N. E.: Talanta, 14, 1423 (1967). 160. Pennington, S., Meloan, C. E.: Anal. Chem., 39, 119 (1967). 161. Monar, I.: Mikrochimica Acta, 934 (1966). 162. Malissa, H., Schmidt, W.: Microchem. J., 8, 180 (1964). 163. Hadzija, O.: Mikrochimica Acta, 1114 (1969). 164. Papay, M., Mazor, L.: Mikrochimica Acta, 299 (1967). 165. Stoffel, R.: Z. anal. Chem., 262, 266 (1972). 166. Nakkaaki Oda, Masaji Kubo: Japan Analyst, 11,411 (1962); Ref., Anal. Abstr., 17,11 (1964).

5. Determination of oxygen

U p to 1937, methods suitable for the determination of the oxygen content of organic compounds on the micro-scale were time consuming and inaccurate, just as in the older indirect method in which all of the other elemental constituents of the sample were determined and the oxygen content was calculated by subtracting their sum from 100%. The errors made in the determination of the individual elements were cumulated to give a large overall error for oxygen, so that the accuracy was several per cent, and the measurement was very uncertain with oxygen contents less than 2-3%. Even the presence or absence of oxygen could not be decided reliably in some instances.

A concise summary of the older methods is given in Volume IB of CAC (Chapter 3 f. F . H. Oliver: The direct micro determination of oxygen, pp. 567-577).

A suitable micro method for the determination of oxygen in organic compounds was described by Unterzaucher [1] , based on the work of Schiitze. This is the method used today, after some modifications. The other procedure, hydrogenation, suggested by Ter Meulen, has not gained widespread application.

In the Schutze-Unterzaucher method, the organic sample is pyrolysed in a stream of an inert gas (nitrogen, argon, helium or sometimes hydrogen), and the oxygen reacts with carbon and hydrogen present in the organic substance to yield carbon monoxide, carbon dioxide and water. The gas carrying the pyrolysis products is led through a layer of carbon at high temperature (about M00°C), where carbon dioxide is reduced:

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C 0 2 + C = 2 C O

and water is also converted into carbon monoxide:

H 2 0 + C = CO + H 2

that is, the oxygen content of the sample is converted into carbon monoxide. The amount of the latter can be determined and the oxygen content calculated from it. Quantitative determination of carbon monoxide can be effected in various ways. In the original Unterzaucher method, a volumetric (titrimetric) procedure was employed. Nitrogen gas containing carbon monoxide was first led through an absorber filled with a base to absorb the acidic decomposition products, halogens and hydrogen halides. Then the gas mixture entered an oxidizing tube packed with crystalline anhydroiodic acid ( H J 3 O g ) at 120°C.

Carbon monoxide reacts with anhydroiodic acid according to the following equation:

15CO + 2 H I 3 0 8 = 1 5 C 0 2 + 3 I 2 + H 2 0

Iodine was absorbed from the gas stream in an absorber packed with a base:

I 2 + 2 0 H = I + O I + H 2 0

The slightly alkaline solution containing iodide and hypoiodite ions was acidified with acetic acid then oxidized with bromine in glacial acetic acid solution:

I + 3 H 2 0 + 3 B r 2 = I 0 3 + 6 H B r

1CT + 2 H 2 0 + 2 Br 2 = I 0 3 + 4 HBr

Then elemental iodine was liberated with iodide ions from the solution acidified slightly with sulphuric acid:

I 0 3 - + 5 r + 6 H + = 3 1 2 + 3 H 2 0

and the iodine was titrated with sodium thiosulphate in the presence of starch indicator. The iodimetric amplification method renders the procedure very sensitive, 1 c m 3 of 0.02 N sodium thiosulphate standard solution is equivalent to 0.1333 mg of oxygen. For example, when 10 mg of a substance containing 5% of oxygen is tested, the volume of standard 0.02 N sodium thiosulphate standard solution consumed in the titration will be 3.75 c m 3 .

The indicator error in the iodimetric titration, when starch indicator is used, is ± 2 drops (about 0.1 cm 3 ) . When the average volume of the titrant consumed is 5 cm 3 , this means a ± 2% error, and a ±0.013 mg deviation in the calculated oxygen content. This error, as will be seen later, is much lower than

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other errors involved in the method proper (e.g., blank value). When the blank value can be eliminated, the method is as accurate as the gravimetric method.

The major parts of the apparatus designed for use of the iodimetric method are as follows.

1. Gas source (nitrogen, argon, helium, sometimes hydrogen), obtained from a gas cylinder through a reducing valve. Nitrogen and argon are equivalent in application. Helium is used as the carrier when carbon monoxide or carbon dioxide is measured gas chromatographically with a thermal conductivity detector. The same holds for hydrogen, in which carbon monoxide can readily be measured, owing to the great difference in their thermal conductivities. This cannot be achieved in nitrogen and argon. However, hydrogen has other disadvantageous properties. In modern G C apparatus the carrier gas used is mainly helium.

2. Gas purifying device. The carrier gas must be free from trace amounts of oxygen, moisture and organic vapours. The gas to be purified is led through layers of metallic copper, platinized asbestos and phosphorus pentoxide heated to suitable temperatures. The removal of moisture from the carrier gas is essential, and coupling of several towers packed with phosphorus pentoxide in series is recommended. The sample must also be dried (89% of water is oxygen). Fluctuation in the blank value may be due to impurities in the carrier gas, and fluctuating results for the test substance indicate inadequate drying of the sample. If air and moisture enter the reaction tube while introducing the sample they must be flushed out with dry carrier gas.

3. Rotameter for gas flow-rates of 0-25 cm 3 /min. In simple instances, a gas scrubber filled with concentrated sulphuric acid or paraffin oil can also be used, in which case the number of bubbles per minute is stated.

4. Reaction tube made from high-quality quartz (length 50-60 cm, i.d. about 10 mm and wall thickness about 1 mm). One end is closed with a ground-glass stopper or a system of stopcocks allowing the introduction of the sample boat without opening the tube to the atmosphere. The carrier gas enters the reaction tube through a side-tube. A by-pass tube is connected here to the system, which makes it possible to reverse the flow of the gas in order to flush the tube after the introduction of the sample boat. The other end of the reaction tube is narrowed and attached to an alkaline absorber with a ground-glass joint. At this end of the reaction tube is placed a carbon layer of length 100-120 mm.

Granulated carbon of the desired quality is available commercially. The carbon layer is fixed in position by a quartz stopper or by a layer of granulated quartz. Vertical mounting of the reaction tube prevents the formation of channels due to sinking of the carbon layer.

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One of the most important sources of error in this procedure is the reaction between carbon and quartz at high temperatures, in which case silicon oxides and carbides with oxidation numbers lower than four are formed and carbon monoxide is liberated. According to experience, this effect occurs with even the highest quality quartz tubes after 20-30 h of operation. A greyish layer appears on the inner walls of the quartz tube as a result of the reaction, and the blank value increases.

In order to eliminate this effect, the use of platinized carbon was suggested, as carbon containing 40-50% of platinum is an active reducing agent at 900°C instead of the temperature formerly applied of 1120°C, and the reaction between carbon and quartz is much slower, so that there is virtually no blank value. A better solution is to use a nickel tube instead of the quartz tube, but then pyrolysis of the sample cannot be checked visually.

5. Absorber packed with alkali. This is a glass tub (about 100 mm long and 10-15 mm in diameter) attached to the reaction tube with a ground-glass joint. Its task is to absorb reducing (oxygen-free) pyrolysis products, halogens and hydrogen halides. The packing is granulated sodium hydroxide or sodium asbestos (Ascarite). The tube is kept at room temperature.

6. Oxidizing tube. This is a glass tube (length about 100 mm and i.d. 10-12 mm) attached with ground-glass joints to the alkaline absorber and to the iodine absorbing tubes at its ends. There are glass- or quartz-wool plugs at both ends and the tube is filled with large crystals of anhydroiodic acid ( H I 3 0 8 ) . As anhydroiodic acid is consumed during the reaction utilized in the determination, vertical mounting of this tube is also advantageous. The anhydroiodic acid reagent, is also available commercially, or can be prepared in the laboratory by recrystallization of iodine pentoxide from hot concentrated nitric acid. When nitric acid is cooled slowly, large crystals that produce no blank value are obtained. Anhydroiodic acid of poor quality at temperatures lower than the prescribed value, does not ensure complete oxidation, and thus the amount of iodine formed will be lower than the stoichiometric value. Positive errors can be due to slow spontaneous decomposition of anhydroiodic acid involving the formation of iodine. In the opinion of some workers hydrogen also reduces anhydroiodic acid, while others did not observe this effect with anhydroiodic acid of appropriate quality. In the pyrolysis of relatively large samples with high hydrogen content (e.g., in the determination of oxygen contaminants in hydrocarbons), the effect of hydrogen must be taken into account.

7. Absorber tube for iodine. This is a glass tube with a large inner surface, attached to the tube filled with anhydroiodic acid by a ground-glass joint. The inner surface is wetted with an alkaline solution to bind iodine with the formation of iodide and hypoiodite ions.

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The carbon packing in the reaction tube is heated to 900-1120°C by an electric furnace (140-150 mm long). When platinized carbon is used, a temperature of 900-1000°C is sufficient.

Pyrolysis of the sample is effected with a small furnace at about 900°C, which can be opened into two halves and can be moved at a controllable rate forwards and backwards along the tube.

The velocity of the moving furnace and the flow-rate of the gas must be harmonized. The usual gas flow-rate is 5-10 cm 3 /min and the usual velocity of the furnace is 0.5-1 cm/min. Pyrolysis is accomplished in the manner described for the determination of carbon and hydrogen, that is, the sample in the boat is approached slowly (0.2-0.3 cm/min) and the furnace is allowed to stand for several minutes over the section containing the boat. Pyrolysis techniques are, of course, different for samples that sublime, vaporize or decompose completely at the site (see the determination of carbon and hydrogen, p. 295).

A detailed description of the procedure is given in Volume IB of CAC, pp. 573-577. Reagents are also specified there.

In view of the errors in the Schiitze-Unterzaucher iodimetric method, caused primarily by anhydroiodic acid, today the gravimetric method is favoured. Here the gas mixture containing carbon monoxide is first led through a metallic copper bed at 900°C to remove primarily the sulphur compounds (H 2 S, C S 2 , COS), then through a copper oxide layer at 650°C, which oxidizes carbon monoxide into carbon dioxide. The gas is dried in a gas scrubber containing concentrated sulphuric acid, then led through an absorber tube packed with sodium asbestos and magnesium perchlorate antl the weight gain is determined [2] . The advantage of this version is its higher reliability (when neglecting the slowly but continuously increasing blank value caused by the reaction between quartz and carbon). However, it is slower and troublesome, owing to the weighing step.

In the last 15 years, methods for the determination of oxygen have greatly improved and automatic apparatuses are now available for this purpose. Various papers have dealt with improvements to the original Schiitze-Unterzaucher method and the elimination of the errors involved.

For instance, Pella [3] examined the use of helium as carrier gas and stated that nitrogen and argon have more favourable properties, the conversion of oxygen into carbon monoxide taking place more rapidly and completely.

Pansare and Mulay [4] effected pyrolysis of 3-7 mg samples in a stream of nitrogen gas. In the reaction tube, star-shaped pieces of platinum, p la t inum-rhodium wool, platinized asbestos and silver-wool were used at 700°C. The gas leaving the tube still contained carbon dioxide and water, in addition to carbon monoxide. The gas mixture was led through a copper oxide layer at

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300°C, carbon dioxide and water were absorbed in tubes packed with adsorbents and the weight gain was determined and used for calculating the oxygen content. According to a British Patent [5] , pyrolysis of the organic sample is carried out in a stream of nitrogen at 92O-1000°C. The pyrolysis products are led through a layer of carbon at 900-1000°C containing more than 50% of platinum. Carbon monoxide is oxidized into carbon dioxide with copper oxide, dried over magnesium perchlorate and the weight gain of an absorber packed with sodium absestos is determined. Korshun and Bondarevskaya [6] stated that the special pyrolysis technique developed by them ensures about 90% conversion into carbon monoxide, water and carbon dioxide in nitrogen gas, so that a much shorter (about 5 cm long) carbon packing is sufficient for complete conversion of the products into carbon monoxide. The blank value caused by the silicon dioxide—carbon reaction is also lower.

When platinized carbon was used, the temperature required was not higher than 900°C. Oita [7] , who first suggested the application of platinized carbon, modified the method for determining oxygen contamination in crude oil products. The sample (0.5-5 c m 3 ) was placed in a spiral quartz tube, which was then rotated slowly while approaching the packing, consisting of platinum and platinized carbon layers, during pyrolysis. At the beginning, accurate control of the temperature of the carbon layer presented some difficulties. Campiglio [8] described an automatic temperature regulator suitable for maintaining the temperature at 1120°C with an accuracy of + 1°C. Ehrenberger [9] developed an arrangement with a vertical carbon layer at 1140°C, with a platinum layer before it and a copper and a silver layer after it, at 900°C.

Mizukami et al. [10] studied the purification of nitrogen used as the carrier gas with respect to the reduction of the blank value. The layer of metallic copper at 500°C was replaced with a layer of Raney nickel at room temperature. The same workers [11] based the determination of oxygen on the measurement of the weight loss of an absorber tube packed with anhydroiodic acid ( lmg of anhydroiodic acid = 0.2354 mg of oxygen). Data for the apparatus found to be the most suitable for the determination of oxygen were given in the Microchemical Journal [12].

Various papers have dealt with oxygen determination techniques, paying special attention to the chemicals and packings used. Kainz and Scheidl [13] examined the efficiency of anhydroiodic acid prepared in different ways. The method of preparation was found to be of decisive importance for the reactivity of the substance, large crystals being more efficient. In the presence of carbon monoxide autocatalytic activation takes place, and at higher temperatures the activity of the substance decreases. The same workers [14]

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also studied the effect of certain substances, (ethylene, acetylene, carbonyl oxysulphide, hydrogen sulphide and hydrogen) formed during the pyrolysis of organic substances on anhydroiodic acid and iodine pentoxide at 118°C. Anhydroiodic acid proved to be more favourable, as it does not release iodine under the influence of hydrogen, so that the carrier gas can be hydrogen. Ethylene is less efficient, but the other compounds listed liberated significant amounts of iodine even from anhydroiodic acid. By decreasing the temperature of anhydroiodic acid, hydrogen, acetylene and ethylene were made ineffective, but sulphur compounds liberated iodine even at 50°C. At 50°C, the conversion of carbon monoxide into carbon dioxide is still satisfactorily rapid and complete.

In related work [15], the same group varied the length of the iodine pentoxide layer and the flow rate of the gas. It was established that the minimal length of the iodine pentoxide layer is 100 mm, and the flow rate should not be high. They also examined the effect of varying the length and temperature of the carbon layer and the flow rate of the gas containing the pyrolysis products of various organic compounds [16]. The minimal temperature of the carbon layer was found to be 1100°C to ensure conversion of carbon dioxide into carbon monoxide. The conversion of water into carbon monoxide requires a temperature of 1000°C, while oxygen reacts with the formation of carbon dioxide at 650°C. Nitrogen oxide first yields carbon dioxide, which is then reduced to carbon monoxide. Similar problems were discussed in another paper [17].

Schoniger [18] pointed out that the transparent quartz tube cannot be replaced with the non-transparent type, as the latter is permeable to gases. The Emich carbon dioxide amplification procedure was employed in the measurement. Vecera and Lakony [19] studied the efficiency of various carbon preparations by the use of carbon dioxide and water under different experimental conditions. It was established that under conditions suitable for the determination of oxygen it is impossible to achieve the degree of conversion corresponding to the thermodynamic parameters in the sta-tionary state.

Belcher and Ingram [20] investigated the error caused by the carbon packing, and the fluctuation in the blank value was attributed partly to the moisture content of nitrogen gas and partly to errors in weighing the absorption tube. "Carbon-wool" containing 10% of platinum was found to be suitable in place of carbon containing 50% of platinum. Belcher and co-workers [21,22] also examined the pyrolysis products of organic compounds by mass spectrometry and, when the carrier gas was nitrogen, cyano derivatives were also found in the gas mixture. The pyrolysis products were different when a platinized carbon packing was used.

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The widespread application of automatic elemental analysis techniques led to discussion of the possibilities of automated oxygen determination, and many papers have dealt with this problem.

The simplest method is the determination of the reaction products formed in the Unterzaucher procedure by means of automatic titration. Calme and Keyser [23] applied this version to the Monar method [24] with automatic titration of the iodine liberated. Karman and Karlsson [25] flushed the iodine with a stream of nitrogen into the cathode chamber of an electrolysis cell, where iodine was reduced at a rotating platinum electrode at controlled potential and the amount of electricity consumed was measured. The standard deviation was 0.08% with 0.5-6-mg samples. Salzer [26] employed a relative conductimetric method, the apparatus being suitable for determi-nation of the oxygen content in the 50-0.02% range.

Nakamura et al. [27] reported on the use of a modified Keidel cell (plat inum-phosphorus pentoxide cell). The organic sample was pyrolysed in stream of nitrogen gas, then the products were led through platinum, platinized carbon and copper layers. Carbon monoxide was oxidized to carbon dioxide with copper oxide, then converted into water with lithium hydroxide, and water was led into an electrolytic cell. A manometric method was suggested by Frazer [28], in which the sample (3-5 mg) was mixed with oxygen-free organic reducing agents and heated at 1055°C for 30 min in a quartz bomb, then cooled to below 650°C. The carbon monoxide and hydrogen content of the gas mixture in the bomb were determined by the manometric technique and the oxygen content was calculated from these values.

The most suitable technique for automatic determination of oxygen is gas chromatography, as the end-product of the reactions, carbon monoxide or carbon dioxide, can be determined very sensitively in a suitable carrier gas with a gas chromatographic detector, primarily a thermal conductivity detector. Pella and Colombo [29] have developed such a procedure, using a specially activated carbon packing, the carbon monoxide and nitrogen being separated gas chromatographically. In this way, not only the oxygen content but also the ratio of oxygen to nitrogen could be established.

Pipper and Romer [30] reported on a gas chromatographic procedure designed for the determination of low oxygen contents in hydrocarbons. The carrier gas was hydrogen, and the carbon monoxide content was measured with a thermal conductivity cell after separation on molecule sieves. Ehrenberger and Weber [31] pyrolysed the organic sample in helium carrier gas in a furnace, used molecular sieves for the separation of carbon monoxide and the signals from the thermal conductivity cell were integrated for calculating the oxygen content. The sensitivity of the method was increased

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by Klesment [32] in the following manner. The carbon monoxide formed was separated gas chromatographically in argon gas containing 8% of hydrogen, then hydrogenated in the presence of a nickel catalyst and the amount of hydrogen consumed was determined from the thermal conductivity values.

Boos [33] also pyrolysed of the sample in helium carrier gas and applied separation on molecular sieves and thermal conductivity measurement. Chumacenko and Khabarova [34] mixed 1-3 mg samples with carbon obtained by ashing sugar in a quartz tube and the mixture was heated in helium at 1100°C for 3-5 min. The pyrolysis products were flushed from the tube with helium on to a G C column and carbon monoxide was determined with a thermal conductivity detector.

Some workers have tried the use of automatic apparatus designed for the determination of carbon and hydrogen in the determination of oxygen. For example, Culmo [35] replaced the combustion tube of the Perkin-Elmer apparatus with a quartz tube packed with platinized carbon. The organic sample was pyrolysed in helium carrier gas, the gas containing carbon monoxide leaving the carbon layer was led through a copper oxide layer at 610°C and the amount of carbon dioxide in the gas was measured in a thermal conductivity cell. Ebeling and Marcinkus [36] achieved the pyrolysis of samples in the Coleman CH apparatus using a tube made from alumina containing 50% of platinum at 940°C, carbon monoxide was converted into carbon dioxide and this product and water were measured gravimetrically.

Smith and Krause [37] modified the Nitrox-6 oxygen determination apparatus and used it for the determination of the oxygen content of organic substances, organometallic compounds and inorganic compounds with ± 0 . 3 % (absolute) error in 5-25-mg samples. Merz [38] constructed an automated version of the apparatus used by Ehrenberger et al. [39] and coupled it to a computer.

Kuck et al. [40] applied infrared absorption measurement to the determination of the oxygen content of organic samples in the 0.5-1.4 mg range. Oxygen was converted in the usual manner into carbon monoxide and its infrared absorption was measured in nitrogen carrier gas. Olson and Kulver [41] utilized the isotope dilution technique in the determination of oxygen. The sample was mixed with succinic acid labelled with oxygen-18 and heated to 850°C under reduced pressure. The gases formed were analyzed by mass spectrometry.

Some oxygen determination methods based on principles entirely different from that applied in the Schiitze-Unterzaucher method have also been published. Gal'pern et al. [42] heated 5-30 mg samples in a stream of dry hydrogen to 1000°C and the pyrolysis products were converted into water and methane with the hydrogen carrier gas in copper and nickel catalyst

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layers. Water was separated from the gas mixture by freezing, then evaporated again, allowed to react with calcium hydride and the volume of hydrogen gas formed was measured.

Stefanac and co-workers [43, 44] covered about 2 mg samples with about 45 mg of fine carbon in a platinum boat, which was heated rapidly to 1120°C in a quartz tube, and the carbon monoxide formed from the total oxygen content was flushed with a stream of nitrogen (10 cm 3 /min) into a tube filled with anhydroiodic acid. In this way, the reaction between the walls of the quartz tube and carbon was eliminated. Mlinko [45] suggested a method for the gas volumetric determination of oxygen, re-examined the Ter Meulen hydrogenation method and eliminated the errors involved. Water was formed during hydrogenation, which was allowed to react with barium cyanate (cyanamide) to yield ammonia :

BaNCN + 3 H 2 0 = 2 N H 3 + B a C 0 3

which was converted with copper oxide into water and nitrogen, and the volume of the latter was measured with a micro-nitrometer.

Stallwood et al. [46] applied the rapid neutron activation method in the determination of oxygen in organic samples. Although this method is, in principle, suitable for any oxygen concentration, its application is favourable primarily in the measurement of low oxygen contents, where the accuracy and sensitivity of the method exceed those of the chemical methods.

The Unterzaucher method may be sensitive to the interfering action of certain heteroelements. Halogens can be bound in the alkaline absorber tube and fluorine can be converted into stable magnesium fluoride with magnesium nitride at elevated temperatures. Bernhardt [47] studied the effect of pyrolysis products containing no oxygen on iodine pentoxide and with the elimination of their interfering action.

Sulphur compounds present special problems in the determination of oxygen, as the sulphur compounds formed (hydrogen sulphide, carbon disulphide and carbonyl sulphide) pass through the alkaline absorber and liberate iodine from anhydroiodic acid. Haraldson [48] studied these problems in detail and analyzed the gases formed during pyrolysis and those leaving the carbon layer gas chromatographically. The gas mixture leaving the carbon layer was found to contain mainly hydrogen sulphide, elemental sulphur and carbon disulphide, plus 2 - 3 % of carbonyl sulphide. Sulphur dioxide and large amounts of carbonyl sulphide found in the initial mixture are reduced in the carbon layer. Haraldson [49] also examined the reaction of sulphur compounds and anhydroiodic acid.

Campiglio [50] pointed out that the sulphur compounds were bound when the sulphur-containing gas mixture was led through a metal layer at

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elevated temperatures, and zinc proved to be the most suitable at 350°C. In the method proposed [51], the pyrolysis step was carried out in hydrogen or nitrogen containing 10% of hydrogen, and all the sulphur compounds were converted into hydrogen sulphide, which was bound with sodium asbestos. Later, Campiglio [52] suggested freezing out of carbon disulphide and carbonyl sulphide at —196°C and binding on zinc or nickel layers at 350°C and 600°C, respectively.

Lebedeva and Nikolaeva [53] determined the oxygen content of organo-mercury compounds by their earlier method [54] in which mercury vapour was absorbed in a pumice bed then carbon monoxide was converted into carbon dioxide and measured gravimetrically. Karpon and Brandt [55] modified the Unterzaucher method to make it suitable for the analysis of organometallic compounds. As metals bind the organically bound oxygen in the form of metal oxides, pyrolysis was effected in a boat in the presence of copper (I) chloride and carbon. In this instance, the oxygen content was converted completely into carbon monoxide and metals were retained in the pure state.

Ehrenberger [56] suggested vacuum melting for the determination of oxygen in organic and inorganic substances. In this procedure, the sample was melted with a cerium-nickel alloy in a graphite crucible and the amount of carbon monoxide formed was measured with the Balzers Exhalograph apparatus. Chizkov and Sinitsyna [57] described a pulseless circulation gas chromatographic method for the separation of mixtures containing hydrogen and deuterium.

References to Section 8.5.

1. Unterzaucher, J.: Ber. chem. Ges., 73, 39 (1940). 2. Dixon, J. P.: Modern Methods in Organic Microanalysis, van Nostrand Co. Ltd., London,

1968. pp. 87-96. 3. Pella, E.: Mikrochimica Acta, 13 (1968). 4. Pansare, V. S., Mulay, V. N.: Mikrochimica Acta, 606 (1961). 5. British Patent 1 051 425. Anal. Abstr., 14, 2581 (1967). 6. Korshun, O. M., Bondarevskaya, E. A.: Zhur. Anal. Khim., 14, 123 (1959); Z. anal. Chem.,

172, 303 (1960). 7. Oita, I. J.: Anal. Chim. Acta., 22, 439 (1960). 8. Campiglio, A.: Mikrochimica Acta, 796 (1961). 9. Ehrenberger, F.: Mikrochimica Acta 265 (1962).

10. Satoshi Mizukami, Tadayoshi Ikey, Kazue Numoto: Mikrochimica Acta, 183 (1960). 11. Satoshi Mizukami, Tadayoshi Ikey: Mikrochimica Acta, 188 (1960). 12. Report on Recommended Specifications for Microchemical Apparatus. Oxygen in Organic

Compounds. Microchem. J., 8, 424 (1964). 13. Kainz, G., Scheidl, F.: Z. anal. Chem., 202/5, 349 (1964).

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14. Kainz, G., Scheidl, F.: Z. anal. Chem., 204/1, 8 (1964). 15. Kainz, G., Scheidl, F.: Mikrochimica Acta, 539 (1964). 16. Kainz, G., Scheidl, F.: Z. anal. Chem., 208, 27 (1965). 17. Kainz, G., Scheidl, F.: Mikrochimica Acta, 624 (1966). 18. Schoniger, W.: Mikrochimica Acta, 679 (1965). 19. Vecera, M., Lakony, J.: Mikrochimica Acta, 370 (1966). 20. Belcher, R., Ingram, G.: Microchem. J., 11, 350 (1966). 21. Belcher, R, Ingram, G., Mayer, J. R.: Mikrochimica Acta, 418 (1968). 22. Belcher, R., Davies, D. H., West, T. S.: Talanta, 12, 43 (1965). 23. Calme, P , Keyser, M.: Mikrochimica Acta, 1248 (1969). 24. Monar, I.: Mikrochimica Acta, 209 (1965). 25. Karman, K. J., Karlsson, R.: Talanta, 19, 67 (1972). 26. Salzer, F.: Mikrochimica Acta, 835 (1962). 27. Nakamura, K. I., Nishimura, M., Mitsui, T.: Microchem. J., 15, 461 (1971). 28. Frazer, J. W.: Mikrochimica Acta, 679 (1964). 29. Pella, E., Colombo, B.: Anal. Chem., 44, 1563 (1972). 30. Pipper, G , Romer, S.: Mikrochimica Acta, 1039 (1966). 31. Ehrenberger, F., Weber, O.: Mikrochimica Acta, 513 (1967). 32. Klesment, I.: Mikrochimica Acta, 1237 (1969). 33. Boos, R. N.: Microchem. J., 8, 389 (1964). 34. Chumacenko, M. N., Khabarova, N. A.: Izv. Akad. Nauk, SSSR, Ser. Khim., 971 (1970). 35. Culmo, R.: Mikrochimica Acta, 811 (1968). 36. Ebeling, M , Marcinkus, D.: Microchem. J., 8, 213 (1964). 37. Smith, S. K., Krause, D. W.: Anal. Chem., 40, 2034 (1968). 38. Merz, W.: Mikrochimica Acta, 71 (1971). 39. Ehrenberger, F , Gorbach, S., Mann, W.: Mikrochimica Acta, 778 (1958). 40. Kuck, J. A., Andreatch, A. J., Mohns, J. P.: Anal. Chem., 39, 1249 (1967). 41. Olson, P. B., Kulver, S.: Mikrochimica Acta, 403 (1970). 42. Gal'pern, G. D., Chudakova, I. K., Egorushkina, M. V.: Zhur. Anal. Khim., 19, 598

(1964); Ref., Anal. Abstr., 12, 4568 (1965). 43. Stefanac, Z., Sliepcevic, Z., Rakovic-Tresic, Z.: Microchem. J., 15, 58 (1970). 44. Sliepcevic, Z., Stefanac, Z : Mikrochimica Acta, 362 (1971). 45. Mlinko, S.: Mikrochimica Acta, 833 (1961). 46. Stallwood, R. A., Mott, W. E., Fanale, D. T.: Anal. Chem., 35, 6 (1963). 47. Bernhardt, A.: Mikrochimica Acta, 468 (1967). 48. Haraldson, L.: Mikrochimica Acta, 650 (1962). 49. Haraldson, L.: Mikrochimica Acta, 1068 (1966). 50. Campiglio, A.: Mikrochimica Acta, 631 (1972). 51. Campiglio, A.: Mikrochimica Acta, 169 (1973). 52. Campiglio, A.: Mikrochimica Acta, 317 (1974). 53. Lebedeva, A. I., Nikolaeva, N. A.: Izv. Akad. Nauk SSSR. Ser. Khim., 10, 1867 (1965); Ref.,

Anal. Abstr., 14, 729 (1967). 54. Lebedeva, A. I., Nikolaeva, N. A.: Zhur. Anal. Khim., 18, 984 (1963). 55. Karpon, M., Brandt, M.: Anal. Chem., 33, 1762 (1961). 56. Ehrenberger, F.: Z. anal. Chem., 26^, 21 (1973). 57. Chizkov, V. P., Sinitsyna, L. A.: Zhur. Anal. Khim., 29, (3), 600 (1974).

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6. Determination of nitrogen

In organic compounds, nitrogen is the most important and most frequently appearing heteroelement, being present in most naturally occurring organic substances. The quantitative determination of nitrogen has made possible the nutritional evaluation of food products of animal and vegetable origin.

There are three main procedures, based on different principles, for the determination of the nitrogen content of organic substances. The Kjeldahl method [ 1 ] is still the most widely applied method based on complete decomposition of the organic sample with concentrated sulphuric acid by boiling it in the presence of catalysts and other oxidizing agents. Carbon, hydrogen and nitrogen are converted into carbon dioxide, water and ammonium hydrogen sulphate, respectively, by the active oxygen in sulphuric acid and sulphur trioxide, while sulphur dioxide leaves the system. The residual clear solution is made strongly alkaline and ammonia is removed by steam distillation. Ammonia is condensed in a condenser or absorbed in a known amount of standard acidic solution or in boric acid solution, then titrated with an alkaline or acidic standard solution, respectively, in the presence of a suitable indicator.

The Dumas method [2] is the oldest technique, in which the organic sample is mixed with copper oxide powder and combusted at 550-650°C in a pure carbon dioxide atmosphere. The nitrogen content of the organic substance is mainly converted into elemental nitrogen gas, but nitrogen oxides are also formed. These are reduced into nitrogen by the hot metallic copper layer. Carbon dioxide carrying nitrogen is absorbed in concentrated potassium hydroxide solution and the volume of nitrogen is measured in a nitrometer.

The third method is the hydrogenation method suggested by Ter Meulen [3] , in which organically bound nitrogen is converted into ammonia in a stream of hydrogen gas in the presence of nickel catalyst, and the ammonia is measured by titration or another method.

The three methods are not equivalent; none of them can be regarded as the best and none is suitable for the analysis of any organic nitrogen compound. The Kjeldahl and Dumas methods are the most widely applied, whereas the hydrogenation procedure suggested by Ter Meulen has not gained widespread application, in spite of its advantages.

Certain organic nitrogen compounds (pyridine, aniline, quinoline, acridine and certain primary, secondary and tertiary aliphatic, aromatic and heterocyclic amines) are strong bases, and can therefore be titrated in glacial acetic acid with perchloric acid in glacial acetic acid solution in the presence of

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a suitable indicator or by the use of potentiometric end-point indication. In this way, the so-called basic nitrogen content can be determined. This titration in a non-aqueous medium is faster and more convenient than both the Kjeldahl and the Dumas method and, in most instances, is equivalent with respect to accuracy. As this is not a stricktly elemental analysis method, it will be discussed in the section on quantitative functional group analysis (p. 458).

A description of the Kjeldahl and Dumas methods adapted to the micro-scale can be found on pp. 467-509 of [4] . Thus in the following section these two methods will be discussed in terms of their advantages and disadvantages and only results published after 1960 will be reviewed. It should be noted that no significant advancement has been achieved since the work of Pregl, who adapted the Dumas method to the micro-scale, but recent studies have made the method suitable for the analysis of certain nitrogen compounds which, when analyzed according to the original Pregl method, produced too low results. Similar advances have been made with the Kjeldahl method also. The Ter Meulen method will be discussed here in detail because according to recent experience, this method is, in some instances more advantageous, rapid and reliable than the Kjeldahl or Dumas method.

The Kjeldahl method has been widely applied, which can be attributed to its simplicity and applicability to routine serial analysis. It was almost the only method used when the nitrogen content of organic substances occurring in nature was to be determined. In these compounds, nitrogen is present in the aminoidal form, that is, in amino or imino groups, and excellent results could be obtained in the analysis of these substances by the Kjeldahl methods. Another advantage of the method over the Dumas and Ter Meulen methods is that low nitrogen contents can also be determined very precisely, as several grams of sample can be decomposed and analyzed. Also, complicated apparatus is not required, the usual purity of reagents is satisfactory and 3 0 -50 samples can be processed simultaneously in series. Steam distillation, when using the Wagner-Parnass apparatus with automatic discharge, can be completed in 5-8 min. The sensitivity of the method can be increased by using spectrophotometry instead of titration for the determination of ammonia. The Kjeldahl nitrogen determination method has been completely automated today, and will be discussed in the section on automatic analytical methods (p. 514).

In the Kjeldahl decomposition procedure, however, various nitrogen compounds (e.g., nitro, nitroso, azo, azoxy and hydrazo compounds) do not yield ammonium hydrogen sulphate, but, partly or completely, nitric acid, nitrogen oxides or elemental nitrogen gas, which can leave the hot (338°C) sulphuric acid solution. Compounds that contain nitro, nitroso and other non-aminoidal nitrogen atoms can be decomposed by the reducing technique

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and compounds that contain amino groups are formed. Reduction can be effected in pre-decomposition with hydrogen iodide, phenol and sulphuric acid, sodium thiosulphate, etc. When decomposition with sulphuric acid is subsequently applied, no loss of nitrogen occurs.

During sulphuric acid decomposition while, e.g., the nitrogen in the amino group forms ammonium hydrogen sulphate, probably several simultaneous and successive reactions take place. These have been studied by several workers but no significant results have been achieved. In his book on the Kjeldahl nitrogen determination, Brandtstreet [5] stated the following.

The organic nitrogen compounds and sulphuric acid participate in a first-order reaction at the boiling point of sulphuric acid or at higher temperatures. In this reaction, the aminoidal nitrogen yields directly ammonium hydrogen sulphate directly, in the case of simple compounds, and sulphuric acid and sulphur trioxide are reduced to sulphur dioxide and leave the system:

C 2 H 5 N H 2 + 7 H 2 S 0 4 = 2 C 0 2 + 6 S 0 2 + 8 H 2 0 + N H 4 H S 0 4

A similar reaction takes place with other simple aliphatic nitrogen compounds and, of course, with compounds that contain no nitrogen, where ammonium hydrogen sulphate is not formed. With more complex compounds that contain nitrogen with a nature other than aminoidal, when decomposition takes place without loss of nitrogen, the end-product is again ammonium hydrogen sulphate, but sulphonation, esterification, ester hydrolysis, dehydration and polymerization reactions may also occur, in addition to simple oxidation. Further, intermediates may also be formed [6] , for example, certain amino acids undergo cyclization during decomposition.

The book by Brandstreet dealt with the method in detail, mainly with respect to practical problems, and listed nearly 500 references.

The Kjeldahl decomposition was effected earlier, on the macro-scale, in a long-necked flask of capacity 500-1000 cm 3 , the so-called Kjeldahl flask (evaporating sulphuric acid is cooled in the long neck and returns to the flask), containing 50-100 cmy of concentrated sulphuric acid. This is suitable for the decomposition of 5-10 g of organic sample. In the procedure on the micro-scale, the shape of the flask has been retained, but the capacity is 50 c m 3

(maximum 100 c m 3 ) and the volume of sulphuric acid is 3-5 c m 3 (maximum 10 cm 3 ) .

It is well known that decomposition with sulphuric acid is a time-consuming procedure, although the reaction time can be very different with identical amounts of different substances. Therefore, catalysts that accelerate decomposition are applied. Of the several possibilities, today mainly mercury metal, copper sulphate and selenium or a mixture of two or all of them are

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used. Catalysts consisting of mainly sodium or potassium sulphate pressed into pellets are available commercially, as these raise the boiling point of sulphuric acid and decomposition is more efficient at higher temperature. Lake et al. [7] increased the boiling point of sulphuric acid to 380°C by the addition of potassium sulphate, and the nitrogen in the pyridine ring could then be converted into ammonium hydrogen sulphate in about 1 h. However, an excessive increase in the boiling point of sulphuric acid should be avoided, as losses of ammonium hydrogen sulphate may occur. According to experience, the temperature of sulphuric acid should not exceed 350-360°C, and rather the reaction time is increased.

The amount of sulphuric acid required for the decomposition depends greatly on the substance treated; usually a 6-20-fold excess is necessary. In the decomposition of proteins, an 8-10-fold excess is usually sufficient, whereas fatty acids and carbohydrates require an 18-20-fold excess. In general, the decomposition of substances that contain larger amounts of oxygen can be achieved with smaller amounts of sulphuric acid. It must be borne in mind that in the decomposition procedure there is some loss of sulphuric acid, owing to evaporation, to the extent of 3-4 g during the usual 1.5 h reaction period. On the other hand, the use of too much sulphuric acid is disadvantageous, as the salt concentration and volume of the alkaline solution obtained after decomposition will be increased unfavourably.

In contrast with earlier opinions, certain nitrogen compounds, such as carbazole, piperidine, pyrrole, phenothiazine, 2,4,5-triphenylimidazoline and substituted triazines, can be decomposed without losses and produce excellent results for nitrogen.

The slow Kjeldahl decomposition procedure can be accelerated by the addition of oxidizing agents. Kjeldahl added potassium permanganate to the sulphuric acid solution in the final period of the decomposition, and several workers have studied the properties of potassium permanganate as an oxidant, but the results were unsatisfactory. Today this reagent is replaced with hydrogen peroxide which leaves the solution without any residue. A 30% solution of hydrogen peroxide is added dropwise to the flask, very cautiously, as the reaction is very vigorous. In the decomposition of large amounts (about 5 g) of substances of vegetable or animal origin, the sample should be mixed with 25 c m 3 of concentrated hydrogen peroxide before adding 40 c m 3 of concentrated sulphuric acid with shaking. As has been stated, about 80% of nitrogen is converted into ammonium hydrogen sulphate at this stage.

Another oxidant that has been suggested is perchloric acid (1-2 c m 3 added to 25 c m 3 of sulphuric acid), which is added only after complete carbonization of the organic matter. Other workers have recommend the use of dipotassium peroxydisulphate (1-2 g added to about 25 c m 3 of sulphuric acid) [4] .

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As has been mentioned, the period required for decomposition depends greatly on the nature of the sample. Of the amino acids, tryptophan and lysine are difficult to decompose, whereas carbohydrates and cellulose can be decomposed more easily.

Decomposition is usually regarded as complete when the solution which initially contained black carbon particles becomes clear. It is advisable, however, to continue the treatment for 0.5-1.5 h, sometimes at a lower temperature, to ensure complete conversion of certain colourless nitrogen compounds into ammonium hydrogen sulphate. Care must be taken to avoid excessive concentration of the solution as a result of evaporation of sulphuric acid during heating after the appearance of the clear solution, as losses in ammonium hydrogen sulphate may occur when the temperature of the solution exceeds 400°C.

Compounds that contain nitrogen atoms with a non-aminoidal character cannot be decomposed directly with sulphuric acid and a catalyst, and must therefore be reduced prior to or during decomposition. Reduction can be effected with zinc and hydrochloric acid, elemental sulphur, powdered copper, titanium trichloride, sodium dithionite, etc.

On the micro-scale, Steyermark et al. [8] used the following treatment with compounds that contain nitro, azo, oxime and isoxazole groups, and also with hydrazines and hydrazones before the decomposition procedure. A 5-8 mg amount of sample was mixed with 0.2 c m 3 of formic acid and 0.1 c m 3 of concentrated hydrochloric acid, and the solution was heated until clear. Then 80 mg of zinc powder were added, the mixture was heated for 5 min with stirring, 40 mg of iron powder, 0.1 c m 3 of concentrated hydrochloric acid and 0.15 c m 3 of methanol were added and the mixture was heated on a water-bath for 5 min. During heating, about 0.1 c m 3 of concentrated hydrochloric acid was added in small portions until the iron powder was dissolved. The next step was the addition of 1 c m 3 of concentrated sulphuric acid and heating to remove hydrochloric acid. Then 0.65 g of potassium sulphate, 0.016 g of mercury(II) oxide and 0.5 c m 3 of concentrated sulphuric acid were added. Decomposition was continued for about 4 h at a temperature such that sulphuric acid condensed half way up the neck of the flask. However, the procedure failed in the decomposition of 1,2-diazines, 1,2,3-triazines and pyrazolones.

Pre-decomposition with hydrogen iodide and red phosphorus, lithium aluminium hydride, ethylene-bismercaptoacetate and alkaline tin(II) chlo-ride solution [9] have also been suggested. Other workers used salicylic acid as a reducing agent, as it decomposes when heated with sulphuric acid, finely dispersed carbon is formed, then sulphur dioxide with a reducing action is

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obtained. About 2 g of the substance are used with about 30 c m 3 of sulphuric acid. Pyrogaliol, thiosalicylic acid and sugar have similar action.

Cepciansky and Chromova [10] suggested the use of 96% sulphuric acid plus 65% orthophosphoric acid (3:1) for the decomposition of heterocyclic compounds. The mixture was divided into two parts, one was mixed with 1 g of copper sulphate for every 100 cm 3 , the other with the same amount of selenium, and both fractions were heated until the additive dissolved. After cooling, the two solutions were mixed and heated again to boiling. In the decomposition of 100-150 mg of sample, 15-22 c m 3 of the acid mixture was used.

Ashraf et al. [11] reduced nitro, nitroso and azo compounds with glucose or with zinc and methanolic hydrochloric acid. The procedure is not suitable for the reduction of hydrazines and picrolonic acid. In another paper by the same group [12], glucose is recommended as a reducing agent. Griepink and Terlow [13] described the decomposition of organic samples with sulphuric acid in a sealed quartz tube, with subsequent treatment of the diluted solution on an ion exchanger in the anionic form and titration of ammonia with perchloric acid using end-point indication with a pH meter. In this way, 30 |!g of nitrogen were determined with an absolute error of ±0.25%. Mlejnek [14] treated the organic sample with copper oxide and cobalt oxide in a sealed glass ampoule and analyzed the gas formed gas chromatographically; carbon and hydrogen were also determined, in addition to nitrogen. Kozak et al. [15] decomposed diazo compounds with chromic-sulphuric acid in a scaled vessel and nitrogen was flushed into a nitrometer and measured.

In a decomposition method applied by Fedoseev et al. [16], the organic sample (10-25 mg) was mixed with a few mgs of Devarda's alloy in a glass tube, then about a further 0.5 g of Devarda's alloy were added together with some diethyl ether (to remove air), and the tube was heated at 55O-600°C, for 15-17 min. After emptying the tube, the substance was brought to boiling with 5 c m 3 of 0 . 1 N sulphuric acid and, after making the solution alkaline, ammonia was distilled in the usual manner. Later, the method was developed further [17] and ammonia liberated with the base was absorbed by a piece of filter-paper impregnated with boric acid in a Petri dish. At the same time, halogens could also be determined in the solution by Volhard titration.

Horacek and Sir [18] studied the influence of certain elements on the determination of the nitrogen content of samples decomposed with sulphuric acid in a sealed tube. Sulphur dioxide was removed by boiling after decomposition at 420°C for 1 h, then potassium bromide was added and the reducing substances were titrated biamperometrically.

Alkalimetric or acidimetric titration of ammonia distilled off is not very accurate on the micro-scale, even when using the mixed indicator that has the

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most favourable properties for this purpose (methyl red-bromocresol green-p-nitrophenol). Similarly, iodimetric determination of the excess of acid is not sufficiently accurate, and therefore many attempts have been made to determine ammonia in another manner.

The oxidimetric-iodimetric determination of ammonia is significantly more sensitive, as was suggested first by Willard and Cake [19]. In this procedure, ammonia was oxidized with sodium hypobromite, added in a known excess and the amount unconsumed was determined iodimetrically. Belcher and Bhatty [20] and Ashraf et al. [12] used sodium hypochlorite and arsenous acid instead of sodium hypobromite. A method that is widely applied today is based on the procedure suggested by Hashmi et al. [21] and will be discussed in detail later (p. 319).

If, after the Kjeldahl decomposition, ammonia is separated by distillation or in another way (e.g., by means of a stream of air, or the diffusion method in a Conway cell) from the catalysts used in the decomposition procedure, it can be oxidized with sodium hypobromite:

2 N H 3 + 3 B r C r = 3 B r - + N 2 + 3 H 2 0

then the excess of hypobromite can be measured iodimetrically:

B r C T + 2 1 + 2 H + = Br + I 2 + H 2 0

by titration of iodine liberated with standard thiosulphate solution. As the titration can be carried out even with 0.002 N sodium thiosulphate solution and the oxidation is an amplification reaction, the method is very sensitive. For example, Harwey [22] was able to measure 5-30 jag of ammonia in this way.

According to Dixon [23], the measurement is carried out as follows. In a Kjeldahl flask of capacity 30 cm 3 , the following amounts of substances are added:

Sample, m g Cata lys t , g A c i d , c m 3 D e c o m p o s i t i o n

t ime , after

d i s c o l o r a t i o n ,

m i n

0.2-5 0.50 2.0 15

5.0-50 1.00 3.0 20

50 -200 1.50 4.0 30

One or two blank tests are also carried out. The composition of the catalyst is 15 g of potassium sulphate, 1 g of mercury(II) oxide and 0.5 g of selenium, finely ground and thoroughly mixed. The decomposition treatment is continued until the colour of the solution disappears, plus a further 15, 20 or

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30 min, depending on the sample size. One drop of 40% iron(III) chloride solution (acidified with 1-2 drops of hydrochloric acid) is added to the cool solution and washed with about the same volume of water into the flask of the distillation apparatus. An Erlenmeyer flask of capacity 100 c m 3 containing 10 c m 3 of 1% hydrochloric acid is placed under the tube of the condenser. The acidic solution in the flask of the distillation apparatus is made alkaline with 6, 9 or 12 c m 3 of 50% sodium hydroxide solution [depending on the amount of sulphuric acid (2, 3 or 4 c m 3 ) used for decompositon]. Steam distillation is continued until 25 c m 3 of liquid are in the flask.

The condensate is mixed with 5 c m 3 of sodium hypobromite reagent, which is prepared as follows: A 1-g amount of sodium hydroxide and 23 g of sodium carbonate are dissolved in water, 0.5 c m 3 of bromine is added and the volume is made up to 1000 c m 3 with water. A standard solution is prepared from this stock solution by dilution (1:1) and 5 c m 3 of the standard solution are added to the flask. Then buffer solution is added (40 g of potassium hydroxide and 6 g of boric acid dissolved in 100 c m 3 of water) until the yellow colour of bromine disappears, which usually requires 5-6 drops. After mixing, 1 g of potassium iodide and 2 c m 3 of dilute glacial acetic acid (1 + 1) are added and iodine is titrated with 0.002 N sodium thiosulphate solution in the presence of starch indicator.

The blank solution consumes about 25 c m 3 of thiosulphate solution, the volume of standard solution consumed by the sample solution is subtracted from this value and the result is multiplied by 0.0093 to obtain the amount of nitrogen, while multiplication by 0.0113 gives the amount of ammonia in the sample.

The method provides reliable results, particularly when the nitrogen content of the sample is low, if the blank and sample solutions are treated in an identical manner.

Ammonium ion forms a precipitate with sodium tetraphenylborate [ N a B ( C 6 H 5 ) 4 ] . Strukova and Fedorova [24] based their method on this reaction, and precipitated ammonium tetraphenylborate in a solution with the pH adjusted to 3-5 with a buffer and measured it gravimetrically. Titration of the precipitate or the excess of reagent can also be applied. As several other ions, e.g., K + , H g 2 + , C u 2 + , also react with sodium tetraphenyl-borate, the method is used mainly for the determination of ammonium ions in the distillate. Shah and Bhatty [25] measured the amount of ammonium tetraphenylborate precipitate by a turbidimetric method after the decomposi-tion.

An old colorimetric method for the determination of ammonium ions is based on the appearance of a yellow colour with Nessler reagent. This has also been utilized in the measurement of ammonia after the Kjeldahl

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decomposition [26, 27], but, owing to the fundamentally unreliable results, the method is not widely applied.

Today the reaction involving the formation of Indophenol Blue dye is applied most frequently in the spectrophotometric determination of ammonia. The procedure was suggested by Van Slyke and Hiller [28] and improved by Borsook [29] and Russel [30]. Bolleter et al. [31] proposed the following reaction mechanism:

N H 3 + HOC1 NH 2C1 + H2O (fast)

Indophenol Blue

The above-described method with phenol-hypochlorite was employed by Lewis and Mann [32] in the determination of the nitrogen content of samples of biological origin. The most favourable wavelength was 630 nm, and 0 . 1 -1.5 ng /cm 3 of nitrogen were determined. The method was developed further by Tetlow and Wilson [33], who used acetone as catalyst. Ammonia was measured in boiler water, the detection limit of ammonia in this reaction was found to be 0.0013 ppm, and 0.05-0.5 ppm of ammonia could be measured in this way. Harwood and Huyser [34] examined the effects of the conditions of the reaction on the intensity and stability of the colour, and found sodium nitroprusside to be superior to acetone.

The procedure is disturbed by certain metal ions (Cu 2 + , H g 2 + ), and therefore, if the determination is effected in the solution obtained on decomposition, without distillation, as is usual, e.g., in automatic analyzers, the catalyst must be selenium. Hashmi et al. [35] recommended ruthenium trichloride-triphenylphosphine reagent for the spectrophotometric measurement of ammonia.

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The Dumas method, adapted by Pregl to the micro-scale, is suitable for the analysis of a wider range of organic nitrogen compounds than the Kjeldahl method, as most nitro, nitroso, azo, etc. compounds that contain nitrogen atoms of non-aminoidal character can be measured without interferences or losses of nitrogen. Its disadvantage in comparison with the Kjeldahl method is that on the micro-scale only a few milligrams of sample are treated, and even on the semimicro-scale the sample size cannot be higher than 50-100 mg. Therefore, it is not suitable for the analysis of samples with relatively low nitrogen contents.

A detailed description of the micro-method is given on pp. 467-491 of Volume IB of CAC, and the construction of the apparatus and the detailed procedure are also given there. Advances in the technique up to 1960 are also reported; of these improvements, the most important is that combustion of the sample is now ensured not by in copper oxide alone, but some oxygen is also added to the carrier gas (carbon dioxide), and this is retained on a larger packing consisting of metallic copper. In this way, the behaviour of the substance during combustion can be observed visually in the boat. Automation of the method has been attempted for a many years, the main advance being the replacement of the micro-nitrometer with a digital device for measurement of volumes. Later, thermal conductivity and flame-ionization detectors were applied in automatic apparatus designed for the determination of carbon, hydrogen and nitrogen, using mainly helium as the carrier gas.

Earlier, the carbon dioxide carrier gas in the Dumas method was prepared by the reaction of hydrochloric acid with marble. Removal of air from the marble was very time consuming, and therefore large crystals of potassium hydrogen carbonate were used, carbon dioxide being obtained from them by reaction with sulphuric acid in an automatically regulated apparatus by e.g., Hohenegger [36]. Today, dry-ice is employed almost exclusively, after appropriate purification.

In the original Dumas method, the sample was mixed with powdered copper oxide and placed in a combustion tube as a "temporary filling". The disadvantage of this technique was that the behaviour of the sample during combustion could not be observed visually, and it has therefore been modified so as to ensure combustion by the addition of oxygen to the carbon dioxide carrier gas. Copper oxide is often replaced with more powerful oxygen-transferring catalysts, e.g., cobalt(II, III) oxide [37-40], potassium per-manganate mixed with copper oxide [41], vanadium pentoxide and nickel oxide [42], lead(II) oxide [43], platinized asbestos [44], magnesium oxide and cerium dioxide [45]. In this way, nitrogen compounds impossible to analyze by the classical Dumas method could also be combusted (e.g.,

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aromatic nitro compounds, vinyl polymers, carborane polymers). Oxygen added to the carbon dioxide carrier gas is prepared either electrolytically [46] or by the decomposition of hydrogen peroxide [47].

The so-called permanent filling, which earlier consisted of granulated copper oxide and metallic copper, has also been modified, as a much longer metallic copper packing is necessary for the removal of the excess of oxygen gas. The use of metallic nickel [48] instead of metallic copper has been suggested and, in the reduction of nitrogen oxides, Raney nickel proved advantageous [49]. Lapteva et al. [50] achieved increased rate of combustion and analysis by using two combustion tubes simultaneously, and developed a programmable furnace to suit the properties of the sample. Mizukami and Miyahara [51] described an apparatus containing eight parallel tubes. In the apparatus suggested by Merz [52] the combustion tube was mounted vertically, and the volume of nitrogen gas transported by carbon dioxide carrier gas was measured with an automatic nitrometer, so that one sample could be processed in 2.5 min. Mitsui [53] constructed a nitrometer with no levelling bulb and containing the alkaline solution in an amount sufficient for 150 measurements. A semi-automatic apparatus was utilized by Ching Siang Yeh [54], Hachenberg and Gutberlet [55] and Stewart et al. [56], with gas chromatographic finish.

The third method for the determination of nitrogen is the Ter Meulen hydrogenation procedure, which has not gained such widespread application as the other two methods. The original version of the method was not suitable for practical application, owing to rapid poisoning of the catalyst. Later Ter Meulen [3] modified the catalyst with nickel and magnesium, and this needs to be renewed only after about 100 measurements. Vapour of the sample pyrolysed in a stream of hydrogen is led through the catalyst bed at 350°C, ammonia is absorbed in boric acid solution, then the alkali content is titrated with 0.01 N hydrochloric acid (1 c m 3 of 0.01 N hydrochloric acid is equivalent to 0.14 mg of nitrogen). The mixed indicator is prepared from identical volumes of solutions of 0.1 g of Methyl Red, 0.3 g of Bromocresol Green and 0.9 g of p-nitrophenol dissolved in 100 c m 3 of 95% ethanol.

A schematic diagram of the apparatus is shown in Fig. 39, and the procedure was described in detail by Dixon [57]. The advantage of the method is that the apparatus is simple, the measurement is rapid, taking only 12 min, and the compounds that can be analyzed are the same as in the Kjeldahl method. Another advantage is that volatile substances (weighed in a capillary) can also be treated whereas this is not possible with the Kjeldahl method. According to Dixon, several compounds that yield unreliable results by the Kjeldahl method can be analyzed successfully by the Ter Meulen procedure (for example, pyridine, carbazole, indole, 2-formylpyrrole, 5-

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Fig. 39. Schematic diagram of the apparatus for the determination of N by Ter Meulen's method

1—Hydrogen source ; 2 — r o t a m e t e r ; 3—pyrolysis t ube (d -10 m m , / = 450 m m ) ; 4—absorber; 5—Ni-Mg cata lys t ; 6, 7—glass

gr ind ing ; 8—separating co lumn

amino-3-phenyl-l,2,4-triazole, N-trimethylaminododecane imide and potassium tricyanomethane).

Some special methods for the determination of nitrogen should also be mentioned here. For example, Hozumi [58] combusted 0.3-0.5 mg samples in a tube filled with oxygen and the volume of nitrogen formed and the weight of mercury excluded by nitrogen were determined. This complicated method is recommended only for the examination of some substances that are hardly combustible. Gouverneur et al. [59] applied the Wickbold [60] combustion in an oxygen-hydrogen flame for the determination of nitrogen. Norris and Flynn [61] studied the determination of trace amounts of nitrogen in petroleum and combusted the sample in a stream of oxygen over a platinum catalyst. The nitrogen was completely converted into nitrogen oxide, which was absorbed in Griess-Ilosvay reagent and measured spectrophoto-metrically. Jacobs [62] suggested decomposition of the sample with sulphuric acid in a closed or open vessel for the determination of the nitrogen content of serum, proteins, etc., the ammonia obtained being measured spectrophotometrically with ninhidrin reagent even in very small amounts. Oita [63] described a catalytic hydrogenation procedure, measuring ammonia with an automatic coulometer.

Automation of nitrogen determination has been attempted for many years. However, the Coleman nitrogen analyzer, which is based on the Dumas method, cannot be regarded as a real automatic apparatus, the volume of nitrogen being measured with a plunger moving in a cylinder coupled to a counter device. Tefft and Gustin [64] reported on their experience with such apparatus. Thiirauf and Assenmacher [65] effected combustion of the sample

23 323

in a stream of pure oxygen at 800-1000°C, then the pyrolysis products were led through a copper oxide bed at 950°C followed by a copper layer at 550°C with helium as carrier gas. Carbon dioxide and moisture were absorbed in 50% potassium hydroxide solution and the helium, now containing only nitrogen, was analyzed in a thermal conductivity cell.

Aue et al. [66] used an alkali flame-ionization detector with a rubidium sulphate coating. The sensitivity of this detector to, e.g., N-trifluoroacetyl derivatives and butyl esters of amino acids, was 100-200 times higher than that of the simple flame-ionization detector, and the increase in sensitivity to aniline was 308-fold. Barsdate and Dugdale [67] carried out combustion in the Coleman apparatus with the modification that carbon dioxide carrier gas was frozen out and residual nitrogen was pumped directly into the mass spectrometer. In this way, contamination of the gas with atmospheric nitrogen could be avoided, and the error of the determination was not higher than that of the mass spectrometer. Pennington and Meloan [68] carried out the determination of nitrogen and also of carbon and sulphur in liquids by coupling the micro combustion apparatus with a gas chromatograph and a recorder. The sample volume was only 2 jig, the combustion chamber was heated to 840°C, and the nitrogen and also the carbon dioxide and sulphur dioxide formed were measured. This was the first method to be described for simultaneous determination of carbon, nitrogen and sulphur. Diehn et al. [69] reported on a reaction-radiochromatographic method for the analysis of substances with low 1 4 C or U C activities, with wet combustion and measurement of nitrogen in the form of nitrogen oxide after absorption in manganese dioxide.

Today several types of automatic analyzers are available commercially which determine carbon, hydrogen and nitrogen in one sample. Combustion of the sample is achieved chemically, carbon dioxide, water and elemental nitrogen gas being separated gas chromatographically and measured successively, usually with thermal conductivity detectors. The construction and operation of such apparatus will be discussed in a separate section later (p. 499).

References to Section 8.6

1. Kjeldahl, C : Z. anal. Chem., 22, 366 (1883). 2. Dumas, L.: Ann. Chim. Phys., 2, 198 (1931). 3. Ter Meulen, H.: Bull. Soc. Chim. Beige, 49, 103 (1940). 4. Wilson, C. L. and Wilson, D. W. (Eds.): Comprehensive Analytical Chemistry. Elsevier,

Amsterdam, 1960, Vol. IB, Chapter VIII. 3B. Kirsten. W.: The Dumas Determination of Nitrogen, pp. 467-494. Chapter VIII. 3C. Jones, A. S.: Determination of Nitrogen by Kjeldahl's Method, pp. 495-509.

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5. Brandstreet, R. B.: The Kjeldahl Method for Organic Nitrogen. Academic Press, New York, London, 1965.

6. Gorsuch, T. T.: The Destruction of Organic Matter. Pergamon Press, Oxford, 1970, pp. 19™ 20. 7. Lake, G. R., McCuthan P., Van Meter, R., Neel, I. C : Anal. Chem., 23, 1634 (1951). 8. Steyermark, A., McGee, B. M., Bass, E. A., Kaup, R. R.: Anal. Chem., 30, 1561 (1958). 9. Lunt, G. T.: Analyst, 88, 466 (1963).

10. Cepcianski, I., Chromova, L.: Chem. Promysi, 9, 188 (1959); Z. anal. Chem., 1 72, 301 (1960). 11. Ashraf, M., Bhatty, M. K., Shah, R. A.: Anal. Chim. Acta, 25, 448 (1961). 12. Ashraf, M., Siddiqui, M. A., Bhatty, M. K.: Talanta, 15, 559 (1968). 13. Griepink, B., Terlow, J. K.. Mikrochimica Acta, 624 (1968). 14. Mlejnek, O.: Chemicke Zvesti, 2^ (3), 421 (1973); Ref., Anal. Abstr., 26, 178 (1974). 15. Kozak, P., Slamova, I., Jurecek, M.: Mikrochimica Acta, 1024 (1966). 16. Fedoseev, P. N., Vladimirova, V. M., Osadchii, V. D.: Izv. vyssh. Ucheb. Zaved. Tekhnol. legk.

Prom., (1) 55 (1971); Ref., Anal. Abstr., 23, 341 (1972). 17. Fedoseev, P. N., Vladimirova, V. M., Osadchii, V. D.: Izv. vyssh. Ucheb. Zaved. Khim. khim.

Tekhnol., 15 (12), 1885 (1972); Ref., Anal. Abstr., 26, 3281 (1974). 18. Horacek, J, Sir, Z.: Coll. Czech, chem. Commun., 40, 1143 (1975); Ref, Anal. Abstr., 26,605 (1975). 19. Willard, H. H , Cake, W. E.: J. Am. Chem. Soc, 42, 2646 (1920). 20. Belcher, R., Bhatty, M. K.: Mikrochimica Acta, 1183 (1956). 21. Hashmi, M. H., Ehsan, A., Umar, M.: Anal. Chem., 34, 988 (1962). 22. Harwey, W. H.: Analyst, 7 6 , 657 (1951). 23. Dixon, J. P.: Modern Methods in Organic Microanalysis, Van Nostrand Co. Ltd. London

1968. pp. 70-73. 24. Strukova, M. P., Fedorova, G. A.: Zhur. Anal. Khim., 21, 509 (1966); Ref., Anal. Abstr., 15,

1418 (1968). 25. Shah, R. A., Bhatty, N.: Mikrochimica Acta, 81 (1967). 26. Burck, H. C : Mikrochimica Acta, 200 (1960). 27. Morrisson, G. R.: Analyt. Biochem., 43, 527 (1971); Ref., Anal. Abstr., 23, 514 (1972). 28. Van Slyke, D. D., Hiller, A.: J. Biol. Chem., 102, 499 (1933). 29. Borsook, H.: J. Biol. Chem., 110, 481 (1935). 30. Russel, J. A.: J. Biol. Chem., 156, 457 (1944). 31. Bolleter, W. T., Bushman, C. J., Tidwell, P. W.: Anal. Chem., 33, 529 (1961). 32. Lewis, T., Mann, S.: Anal. Chem., 35, 2179 (1963). 33. Tetlow, J. A., Wilson, A. L.: Analyst, 89, 453 (1964). 34. Harwood, J. E., Huyser, C : Water Research. Pergamon Press, 1970. Vol. 4, pp. 500-515. 35. Hashmi, M. H., Ajmal, A. I., Rashid, A.: Mikrochimica Acta, 860 (1968). 36. Hohenegger, M.: Mikrochimica Acta, 431 (1961). 37. Vecera, M.: Mikrochimica Acta, 896 (1962). 38. Borda, P., Hayward, L. D.: Anal. Chem., 39, 548 (1967). 39. I Shien Sha, Chang I Wang: Acta Chim. Sin., 31 (5) (1965); Ref., Anal. Abstr., 14, 730 (1967). 40. Vecera, M , Synek, L.: Mikrochimica Acta, 208 (1960). 41. Abramyan, A. A., Pogosyan, L. E.: Izv. Akad. Nauk Armyan SSR Khim. Nauk 19, 188 (1966);

Ref., Anal. Abstr., 14, 4750 (1967). 42. Pippel, G , Romer, S.: Chem. Techn. Leipzig, 15, 173 (1963); Ref., Anal. Abstr., 11, 1312 (1964). 43. Gelman, N , Larina, N. I., Cekaseva, I. S.: Zhur. Anal. Khim., 21, 612 (1972); Z. anal. Chem.

264, 318 (1973). 44. Saran, P. N., Khanna-Subriti Banerji: Mikrochimica Acta, 252 (1972). 45. Kakabadse, G. J., Manohin, B.: Mikrochimica Acta, 855 (1965). 46. Swift, H.: Mikrochem. J., 11, 193 (1966). 47. Kakabadse, G., Manohin, B.: Analyst, 86, 512 (1961).

23* 325

48. Abramyan, A. A., Kocharyan, A. A.: Izv. Akad. Nauk Armyan SSR Khim., Nauk, 15, 225 (1962); Ref., Anal. Abstr,, 10, 1832 (1963).

49. Mizukami, S., Miyahara, K., Numoto, K.: J. Pharm. Soc. Japan, 7 8 , 842 (1958); Ref., Z. anal. Chem., 168, 212 (1960).

50. Lapteva, A. S , Novikov, A. V., Bondaravskaia, E. A.: Zavods. Lab., 38, 32 (1972); Z. anal. Chem., 262, 221 (1972).

51. Mizukami, S., Miyahara, K.: Mikrochimica Acta, 705 (1962). 52. Merz, W.: Z. anal. Chem., 237, 272 (1968). 53. Mitsui, T.: Microchem. J., 7, 277 (1963). 54. Ching Siang Yeh: Microchem. J., 11, 229 (1966). 55. Hachenberg, H., Gutberlet, J.: Brennstoff-chemie, 44, 235 (1963). 56. Stewart, B. A., Porter, L. K , Beard, W. B.: Anal. Chem., 35, 1331 (1963). 57. Dixon, J. P.: Modern Methods in Organic Microanalysis. Van Nostrand Co. Ltd., London,

1968, pp. 67-70. 58. Hozumi, K.: Anal. Chem., 38, 641 (1966). 59. Gouverneur, P., Snoek, O. I., Heeringa-Kommer, M.: Anal. Chim. Acta, 39, 413 (1967). 60. Wickbold, R : Angew. Chemie, 64, 133 (1952). 61. Norris, T. A., Flynn, J. E.: Anal. Chem., 3^, 152 (1965). 62. Jacobs, S.: Analyst, 85, 257 (1960). 63. Oita, I. J.: Anal. Chem., 40, 1753 (1968). 64. Tefft, M. L., Gustin, G. M.: Microchem. J., 10, 175 (1966). 65. ThUrauf, W., Assenmacher, H.: Z. anal. Chem., 250, 111 (1970). 66. Aue, W. A., Gehrke, C. W., Tindle, R. C , Stalling, D. L., Ruyle, C. D.J. Gas Chromatography,

5, 381 (1967); Ref., Anal. Abstr., 15, 6019 (1968). 67. Barsdate, R. J., Dugdale, R. C : Analyt. Biochem., 13, 1 (1965); Ref., Anal. Abstr., 14, 731

(1967). 68. Pennington, S., Meloan, C. E.: Anal. Chem., 39, 119 (1967). 69. Diehn, B., Wolf, A. P , Rowland, F. S.: Z. anal. Chem., 204/1, 112 (1964).

7. Determination of halogens

As was mentioned in Part I in the chapter on the detection of halogens, the detection reactions of the four halogens in practice are similar. However, there are some significant differences between them and also between methods for their quantitative determination.

Therefore, in the following discussion of the methods developed for determination of these elements, it seems reasonable to deal first with the very similar methods suitable for the determination of three halide ions (chloride, bromide and iodide), and then with procedures specific for the individual halide ions. A separate section is devoted to the decomposition methods applied in the mineralization of most organic fluorine compounds and to the methods suggested for the determination of fluoride ion, which are entirely different from those used in the analysis of the other three halide ions.

In organic halogen compounds, the properties that determine the accuracy and simplicity of methods for the determination of the halogen content depend on, among other factors, the nature of the halogen present, the other

326

component linked to the halogen atom, the nature of the molecule moiety (e.g., aliphatic or aromatic) and the nature, number and location of other functional groups.

There are few halogen compounds that release halide ions by dissociation or hydrolysis in aqueous or alkaline solutions. Most halogen compounds have to be subjected to dry or wet decomposition procedures in order to convert the organically bound halogen into halide ions. Most of the chemical methods (gravimetric, volumetric, spectrophotometric) developed for the determination of halogens are based on the determination of halide ions.

The analytically most important properties of the four halogen elements are shown in the following table.

Element Fluorine Chlorine Bromine Iodine

Atomic weight 18.9984 35.453 79.909 126.9044 Boiling point, °C - 1 8 8 . 2 - 3 4 . 7 58.78 184.35 Melting point, °C - 2 1 9 . 6 - 1 0 1 . 0 - 7 . 2 113.7 Electronegativity 4.0 3.0 2.8 2.5 Standard electrode,

potential, V 2.85 1.36 1.08 0.58 Energy of C—halogen

bond, kJ 2 5 C 434.92 279.05 222.07 162.15

The energy of the carbon-halogen bond decreases considerably from fluorine to iodine, that of fluorine being extremely high. In the conversion of covalently bound halogens into ionic halogen compounds, this energy has to be invested. However, this is not the only fact to be accounted for in deciding the decomposition method, as other conditions (e.g., the presence of other substituents) can alter the energy of the carbon—halogen bond; thus, there are iodine compounds that are more stable than the corresponding chlorine compounds, and the fluorine a tom can be removed from some fluoro compounds with the application of a relatively small amount of energy.

The methods applied to the determination of the halogen content of organic halogen compounds will therefore involve two steps. One is the conversion of halogen into halide ions, which requires a more or less intense treatment depending on the bond strength and other conditions, and the other is the quantitative determination of the halide ion (or the elemental halogen). In favourable instances the two procedures are accomplished rapidly in sequence in the same apparatus in order to minimize sample losses.

327

(A) M E T H O D S FOR THE LIBERATION O F THE CHLORINE, BROMINE A N D

IODINE C O N T E N T O F O R G A N O H A L O G E N C O M P O U N D S

The decomposition methods can be classified into three groups in order of increasing vigour of the treatment:

1. Methods involving hydrolysis and dehalogenation in aqueous or organic solutions by chemical reactions.

2. Decomposition methods, in which the sample is treated with oxidizing or reducing agents, often at elevated temperatures, and the organic molecule is partly or completely converted or decomposed while the halogen is liberated.

3. Combustion methods, which involve the decomposition of the organic substance in oxidizing or reducing atmospheres or gas streams at elevated temperatures yielding the elemental components or simple inorganic compounds.

With unknown compounds, when the strength of the bond involving the halogen is not known, methods in the third group are recommended. The additional advantage of these methods over the first two types is that the active agent is a gas, usually available in very pure state (or easily purified), and the excess does not contaminate the solution of halide ions or increase the foreign ion concentration.

1. Hydrolysis methods. There are several organohalogen compounds that release halogens in hydrolysis reactions, yielding halide ions in aqueous solutions. Thus, for example, halide salts of protonated organic bases dissociate. When the base is sufficiently weak, the strong acid component of the salt can be titrated with a base, particularly when the organic base liberated during the titration is extracted continuously from the aqueous solution with an immiscible organic solvent. Several halogen compounds used as drugs can be titrated with standard silver nitrate solution using ion-selective membrane electrodes [1] .

Certain halogen compounds (e.g., acid halides and alkyl halides) can be saponified with aqueous or ethanolic potassium hydroxide solution and the halide ions titrated in the solution or the excess of base measured. The method can be extended to compounds that are less liable to saponification by applying elevated temperatures and solvents with higher boiling point (e.g., benzyl alcohol) instead of water or ethanol.

Reduction of several halogen compounds with alkali metal yields halide ions in, e.g., an ethanolic medium. This method (the Stepanov or Umhoefer method) has been known for many years, and is still used on the semimicro-scale. Finely dispersed sodium metal in an indifferent solvent exerts a

328

powerful reducing action at room temperature [2, 3]. Volatile compounds cannot be analyzed by this method as heating of the solution is necessary. Organosodium compounds have been found to be less sensitive and more reliable than sodium metal for use in this treatment, and several papers have dealt with the preparation, storage and the application of diphenylsodium [4-7]. Halogen compounds can also be decomposed with bases in dimethyl sulphoxide [8] or with piperidine in methanol [9] . The carbon—halogen bond can be cleaved with sodium borohydride in alkaline medium in the presence of a palladium salt catalyst [10] and with Raney nickel in alkaline solution [11].

2. Decomposition methods. Mineralization methods, involving treatment with oxidizing acids or acid mixtures, have been known for many years and the Carius method is reliable for the decomposition of volatile organic compounds on the semimicro- and micro-scales. Similarly, decomposition with sulphuric acid and potassium dichromate is also a very old method, but it is still used in a modified form and will be discussed in detail later. Polymers containing chlorine can be easily decomposed with a mixture of sulphuric acid and cerium(IV) sulphate in a closed apparatus; the chlorine liberated is absorbed and titrated.

The Piria method is suitable for the determination of the halogen content of involatile solid halogen compounds, but its applicability is limited. The decomposition agent can be granulated magnesium oxide, magnesium metal or an alkali metal. Essentially, a Lassaigne-type reaction takes place, which, when carried out free from losses (in a metal bomb, sealed and evacuated glass tubes or flasks equipped with electric heating to heat the small test-tube containing the sample and the alkali metal to a few hundred degrees Celsius [12]) is suitable for quantitative purposes the halide ions being measured in the absorption liquid. Involatile compounds can also be decomposed with sodium metal in open test-tubes if the pyrolysis products are passed through the molten alkali metal layer [13]. An advantage provided by the reductive procedures is that halide ions are always formed, whereas the product in oxidative decomposition may be elemental halogen, particularly with iodine and bromine compounds.

The most widely used oxidizing agent in decomposition procedures is sodium peroxide. The reaction is carried out in a Parr bomb, the melt is dissolved in water and the halide ions in the solution are determined gravimetrically on the micro-scale or titrated argentimetrically. Removal of sodium ions from the solution can be effected in an ion-exchange column.

3. Combustion methods. As was mentioned earlier, the most reliable decomposition method with the lowest risk of losses and contamination is treatment in an oxidizing (occasionally reducing or inert) gas atmosphere.

329

In one group of methods, pyrolysis and complete decomposition of the pyrolysis products are effected in a stream of gas which is also the carrier gas, and the hydrogen halide (occasionally, elemental halogen) content is absorbed in an appropriate absorbent liquid. Quantitative determination of halide ions is carried out in this solution. Hydrogen chloride and hydrogen bromide can be titrated directly acidimetrically, while iodimetric determination of hydrogen iodide and iodide ions has been found to be more advantageous. Several suggestions have been made in connection with the use of catalysts serving to lower the temperature of combustion in the combustion tube, and to ensure complete combustion of the pyrolysis products and an increased reaction rate. Kainz and Mueller [14] and Scheidl and Toome [15] employed a platinum catalyst. In another method, an empty combustion tube is used. When the temperature is sufficiently high (about 950°C) and the pyrolysis products come into intimate contact with the walls of the combustion tube for a sufficiently long period, complete combustion can be achieved without the presence of an oxidizing catalyst [16-19] . The Grote-type combustion tube and absorber are also applicable to the rapid combustion of organic halogen compounds, particularly chloro and bromo compounds [20-22] .

On the basis of an earlier suggestion, Wickbold [23] developed a very powerful combustion method. .The vapour from the sample is burnt in a stream of town gas enriched with oxygen. The flame burns in a water-cooled quartz chamber; the pyrolysis products are cooled in an attached quartz condenser and are absorbed in a Reitman absorber. One of the advantages of this method is the possibility of using larger amounts of sample for substances with low halogen contents [24].

The method suggested by Ehrenberger [25] involves the combustion of the sample in a mixture of hydrogen and oxygen. The vapour from the sample is introduced into the flame via the hydrogen stream, and the high temperature of the flame ensures rapid and complete combustion. The amount of the condensate is small, and therefore sensitive and precise measurements are possible. As the method is particularly important in the analysis of organic fluorine compounds, a detailed discussion will be presented in a separate section. Combustion in a hydrogen-oxygen flame was applied in the method developed by Martin and Floret [26]. In the determination of the halogen content of crude oil products, the hydrogen stream is replaced with the evaporated sample, which itself acts as the combustible gas [27, 28].

Recently, Mazor developed a combustion method that is suitable for the decomposition of almost all organic compounds containing any of the four halogen elements. The method utilizes the process of "hydropyrolysis", applied earlier in the determination of the halogen contents of inorganic

330

compounds (silicates, glasses, uranium compounds). At temperatures higher than 1000°C, these compounds release hydrogen fluoride, hydrogen chloride or boric acid under the influence of superheated water vapour, preferably in oxygen as carrier gas. The pyrohydrolysis products are collected in a condenser and measured by an appropriate method. This method was applied to the hydropyrolytic decomposition of organic halogen compounds and it was established that organic compounds decompose under the influence of water vapour present in a stream of inert gas (nitrogen) at much lower temperatures (700-800°C) and hydrogen halides can be distilled off. The decomposition process taking place under the influence of steam at elevated temperature involves the following steps [29, 30] :

1. In several aliphatic and aromatic compounds, particularly when the halogen atom is loosened owing to the presence of, e.g., electron-attracting substituents, exchange of the halogen atom for the hydroxyl ions of water takes place in a nucleophilic substitution reaction, with the formation of, e.g., alcohol, phenol and halide ions. Such behaviour is shown, for example, by allyl chloride:

OH H +

C H 2 = C H C H 2 C 1 — • C H 2 = C H C H 2 O H + C P . . . H +

or, at 500-600°C, by chlorobenzene:

~OH H +

C 6 H 5 C1 — > C 6 H 5 O H + C P . . . H +

the latter reaction being the basis of the Raschig phenol production process. Such nucleophilic substitution reactions can take place not only with the

complete molecule, but also with pyrolysis products obtained at lower temperatures.

2. Halogen elimination reactions can occur in the range 300—600°C, which result in the decomposition of the molecule into two (or more) parts, while the halogen is partly or completely converted into hydrogen halide.

3. In the range 40O-700°C range, thermal decomposition processes take place with the decomposition of the molecule into several fragments. These thermal decomposition processes usually yield compounds of lower molecular weight. The molecular fragments or new molecules can participate in recombination reactions when the thermal stability of the new molecule formed is higher.

4. In the range 700-1000°C, and sometimes at lower or higher temperatures, the molecules or molecular fragments retaining the halogen atoms are converted into carbon oxides by the oxygen in water vapour, while

331

the halogen atoms form hydrogen halides. These oxidation reactions are greatly facilitated by surface catalysts. In the experience of the author, platinum is an excellent catalyst because, for instance, the carbon—chlorine bond breaks at its surface at 70O-800°C, even with carbon tetrachloride which is greatly resistant to thermal decomposition. Higher temperatures have been found to be necessary only with compounds that contain the thermally very stable trifluoromethyl group (e.g., trifluoroacetic acid). The so-called "empty tube" method used in the determination of halogens also indicates that prolonged and intimate contact between the vapour from the sample and the walls of the quartz tube at 80O-900°C ensures complete thermal decomposition, oxidation and release of halogens in an oxygen atmosphere.

In the procedure suggested by Mazor, the water vapour required for the reactions is supplied in a stream of inert gas (nitrogen or argon) in a quartz tube of length about 500 mm and i.d. 8-10 mm, attached to a condenser where water vapour condenses and can be collected in a suitable vessel. A platinum sieve (100-150 mm long) to act as the catalyst is placed in the quartz combustion tube near the condenser. When the sample is an in volatile solid, it is introduced into the tube in a platinum boat, volatile liquids are weighed in a glass capillary, and gases are injected into the tube through a side-tube closed with a Teflon plug. The sample is pyrolysed by means of a moveable furnace at 600-700°C, and the section containing the platinum catalyst is heated with another furnace. The flow-rate of the gas stream is 4-6 cm 3 /min, and the amount of water vapour transported in one measurement is equivalent to a total of 10-15 c m 3 of water, the main part of which is required for flushing of the system (condenser) in the final period of combustion (Fig. 40).

2

Fig. 40. Apparatus for pyrohydrolysis

/ — S t e a m genera to r ; 2—vertical p ipe-s tab with Teflon s topper for injection of gases a n d volati le l iquids; 3—inlet for rising gas ;

4—movable electric furnace; 5—platinum b o a t ; 6 c a t a l y s t (rolled u p p l a t inum sieve); 7—condenser

332

The advantage of the method is that in the same apparatus any kind of sample present in any physical state can be analyzed; only the temperature of the catalyst layer and the rates of the gas flow and the pyrolysis process need to be adjusted according to the nature of the sample.

The method is considered to be oxidative in nature, but it has the advantage over combustion methods in which a stream of oxygen or an oxygen atmosphere is applied, that even iodo compounds yield hydrogen iodide instead of elemental iodine. When the compound is poor in hydrogen (e.g., iodoform), additives with a high hydrogen content (paraffin oil, octanol, decanol, etc.) are required in the combustion process. The boiling point of the additive should not be too different from that of the sample.

lS When the temperature of the platinum catalyst layer is sufficiently high, the organic compound is completely combusted, that is, no organic or inorganic contaminants appear in the condensate, and the halide ions can then be determined by any suitable method without interferences. When the halogen content of a b r o m o or iodo compound is to be determined, the oxidizing agent converting bromide and iodide ions into bromate and iodate ions, respectively, can be added to the vessel in which the condensate is collected.

A detailed description of the method has been given elsewhere [30, 31]. A method has been suggested [32, 33] that involves the combustion of

organic samples in a stream of ammonia gas, ammonium halides being obtained which are absorbed in water for measurement of halide ions. Korbl [34] heated organic iodine compounds in a stream of chlorine at 1000°C. The interaction of iodine and chlorine yielded iodine dichloride which was absorbed in water, oxidized to iodate ions and determined iodimetrically. Volodina et al. [35] applied high-frequency heating for the decomposition of organic halogen compounds in an atmosphere consisting of hydrogen and nitrogen (1+4) . Decomposition was completed in 10 minutes with the formation of ammonium halides.

The combustion of organic samples in an oxygen flask on the macro-scale was first suggested by Hempel [36]. Later the method was modified by several groups and finally Schoniger adapted it to micro-scale [37].

In the procedure, the sample (5-10 mg) is packed in a piece of filter-paper cut to an L-shape with an extension serving as a fuse (Fig. 41). The small package is placed in a platinum gauze or coil fixed to the ground-glass stopper of a 300-500 c m 3 flask.

The flask containing the absorbent liquid (usually 20 c m 3 are sufficient; it may be water or an appropriate reagent solution, depending on the nature of the halogen to be determined) is filled with oxygen through a glass tube that extends to the bot tom of the flask. On the micro-scale, purification of the oxygen is not necessary.

333

70 mm

e E o - 4

e E o - 4

30 mm m

Fig. 41. L-shaped piece of filter-paper for combustion in the oxygen flask

The tip of the filter-paper fuse is ignited and the stopper is inserted quickly into the neck of the flask while holding it firmly against the increased pressure in the flask. Preferably, some water is poured around the stopper after it has been inserted to ensure absorption of eventually released gases.

When the sample is not too large (not more than about 20 mg) and not too large a piece of filter-paper was used (20-25 c m 2 of "black ribbon" type), the increase in pressure is not high enough for explosion of the flask to be a risk. However, it is advisable for the operator to be protected and safety spectacles must be worn.

The sample burns in the oxygen atmosphere with a white flame. N o solid residue should appear in the absorbing liquid. At the high temperature produced (1000-1300°C), all of the fluorine and chlorine in the sample are converted into hydrogen fluoride and hydrogen chloride, respectively. With bromo compounds, most of the bromine is converted into hydrogen bromide, but some elemental bromine and bromate ions are also formed. Iodine compounds yield mainly iodine indicated by the violet colour of the vapour in the flask. In the absorbent liquid iodine is converted partly into hypoiodite and partly into iodate ions. The flask is shaken for 20-30 minutes to achieve complete dissolution of the pyrolysis products or allowed to stand for 2-3 h.

The method has become widely accepted in a short period and, as it is simple and requires no complex devices, it is well suited to routine analyses [37]. In his first paper, Schoniger suggested the use of the mercury(II) oxycyanide method for the determination of chloride and bromide and the iodimetric measurement of iodine. In the 25 years that have elapsed, innumerable papers have dealt with the modification, improvement and application of the method. Several suggestions have been published regarding the shape and inner arrangement of the flask and the ignition of the sample [38-42] . Other absorbents instead of bases or alkaline hydrogen peroxide

334

have also been recommended [43-49] and spectrophotometric [50] , polarographic [51] or turbidimetric [52] finishes have also been tried in place of the titration of halide ions. The oxygen flask method was unsuitable at first for the decomposition of volatile substances and still has some deficiences in this respect; however, for the combustion of such compounds in glass capillaries [53] and vessels made from nitrocellulose [54] or polyethylene capillaries [55] have been suggested.

There are various organic compounds that cannot be completely burnt. In some instances, combustion can be promoted by additives such as saccharose or glucose, but naphthalene or phthalic acid [56, 57], or 4-5 times the sample weight of solid paraffin [58] can also be used. Samples with high chlorine content often give low results. This can be due to incomplete absorption of the pyrolysis products, which can be avoided by the use of, for example, a dilute ammonia solution as absorbent [59]. Some workers consider that free halogens are formed even during the combustion of chlorine compounds which can be accomodated by using reducing absorbents such as sodium hydrogen sulphite or sulphurous acid [60, 61].

Procedures for the analysis of substances with very low bromine contents [62], and modifications of the shape of the flask and a procedure for the determination of 0.1 jxg of chlorine in 100 mg of sample, have also been reported [63]. Bennewitz [64] studied the reproducibility of the method on the semimicro-scale.

(B) DETERMINATION OF CHLORIDE, BROMIDE A N D IODIDE IONS

O N THE MICRO-SCALE

Organic compounds are decomposed today almost exclusively on the micro-scale, and therefore the liberated constituents (ions or simple compounds) must also be determined by microanalytical methods.

There are several methods available for the quantitative determination of chloride, bromide and iodide ions (occasionally, of elemental bromine and iodine), and the choice of the most appropriate method depends partly on the analytical properties of the ion to be determined and partly on the amount of sample available. In respect of the latter it must be remembered that results are regarded as reliable only when the values obtained in three independent measurements are in good agreement. When mineralization and measure-ment in two separate steps are used, two samples are mineralized and both solutions are divided into two parts to obtain four quantitative results. When decomposition and determination are effected in the same process, minerali-zation should be carried out on three separate samples with different sample weights. In gravimetric measurement on the micro-scale the error should not

335

be larger than ± 0 . 5 - 1 % (absolute), in volumetric methods this may be ± 2-5% (absolute), and that of spectrophotometric methods depends greatly on the reaction and instrument applied, the error usually being in the range 2 -10% absolute.

General information on gravimetric, volumetric and spectrophotometric methods are given in Volumes IA and IB of Comprehensive Analytical Chemistry [65] and in books by Hecht and Donau [66], Pregl [67] and Mika [68]. In the following, these problems will be discussed only to the extent relevant to halogen determinations.

Determination of chloride ions. Owing to their frequent occurrence and industrial importance, chloride ions have long been investigated analytically, and the methods are highly developed compared with those for the other halogens.

The oldest method for the quantitative determination of chloride ions, which is still widely applied on both the macro- and micro-scales, is gravimetric measurement as silver chloride.

Silver chloride precipitates as a colloid, but a small excess of silver ions promotes coagulation, yielding a microcrystalline precipitate. Agitation, heating and standing facilitate coagulation of the precipitate. Preferably, the precipitate is produced in a solution containing nitric acid to prevent formation of other silver salts (phosphate, carbonate) which would precipitate in neutral solution, causing adsorption of cations on precipitates with a large surface area. When multivalent cations (iron, aluminium), which have a tendency to be adsorbed, are present in the solution, larger amounts of nitric acid should be used. Under the usual conditions, 2-3 ml of 2 N nitric acid are added to 100 ml of the solution prepared for precipitation. When the cations mentioned are present, 10 ml of acid are necessary.

Silver chloride is only sparingly soluble (Ks = 1.56x 1 0 " 1 0 mole 2 d m - 6 at 25°C), but the solubility product increases rapidly with increasing temperature, being 13.2 x l 0 ~ 1 0 and 21.5 x l O " 1 0 at 50°C and 100°C, respectively. Therefore, the precipitate should be filtered from a cold solution: the solubility product is only 0.37 x 1 0 ~ 1 0 at 10°C. In the presence of an excess of 10% of the precipitant the maximal error from solubility is —0.2%. The presence of foreign ions affects the solubility of the precipitate only slightly and can be neglected. As silver chloride is liable to peptize, the working liquid should be acidified with nitric acid.

Silver chloride is light-sensitive, becoming violet and then black under strong illumination, with loss of halogen. Slight discoloration of the precipitate, however, can be neglected.

Dennstedt [69] began the development of a special technique for the gravimetric determination of chloride ions. The organic sample is burnt in a

336

stream of oxygen in a closed system and chlorine and sulphur oxides are absorbed on a known weight of finely dispersed silver placed in a boat in the combustion tube, when silver chloride and silver sulphate are formed. The gain in weight of the boat gives a direct measurement of the chlorine or sulphur content of the compound examined. This procedure was later modified by several workers and applied to the micro-scale determination of chloride in organic compounds, with simultaneous determination of carbon and hydrogen. Finely powdered silver was replaced with a roll of silver gauze placed in a glass tube to prevent mechanical loss of silver chloride during weighing. The weighed absorption tube was kept at 425°C, which was found to be the most favourable temperature for the absorption of chlorine and hydrochloric acid from the gas stream. This procedure, however, was unsatisfactory, because absorption was not quantitative, owing to some loss of silver chloride by volatilization. Furthermore, the increase in weight is relatively small and a constant weight of the heated glass tube cannot be ensured. The method cannot be applied to the determination of bromine and iodine. Halogen compounds also contain sulphur in the form of silver sulphate, which can be separated from silver chloride by dissolution in hot water.

Some workers tried to eliminate the weighing errors by extracting the silver chloride from the silver gauze with ammonia solution followed by precipitation of silver iodide by the addition of a known excess of potassium iodide and back-titration of unconsumed iodide after oxidation to iodine. This modified procedure was not satisfactory, because of the formation of silver oxide on the surface of the silver gauze heated in a stream of oxygen, which caused a positive error.

Titrimetric methods. Chloride ions, when obtained in the form of hydrochloric acid, such as after combustion of organic compounds and absorption in a suitable solution, can be satisfactorily titrated with 0.01 N solutions of bases [70].

When the solution contains another acid or it is initially neutral or alkaline, the Viebock method [45] can be used. This is based on the following reaction:

Hg(OH)CN + NaCl = HgClCN + NaOH .

After neutralization of the halide solution in the presence of the same indicator as is used in the titration, the so-called mercury(II) oxycyanide is added and the base liberated by the above reaction is titrated. Both mercury compounds are soluble but undissociated, methyl red-methylene blue mixed indicator is usually employed. This procedure was utilized by Schoniger [71, 72] in the determination of chlorine and bromine in organic compounds after combustion by the oxygen flask method.

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The poor solubility and rapid precipitation of silver chloride allow chloride ions to be determined by precipitation titrations. In most argentimetric chloride determinations, specific indicators are used, in both direct and back-titrations. Well-known titration procedures include the Mohr method, in which the red silver chromate precipitate is used as indicator, and the Volhard titration, in which chloride ions are precipitated with a known amount of silver nitrate and the excess of silver is back-titrated with thiocyanate ions, using the red iron (III) thiocyanate complex as indicator. Although several suggestions have been made [73] regarding the application of these two methods on the micro-scale (that is, using 0.01 N solutions), these have not been widely accepted, as the silver chromate precipitate does not give a sharp end-point in dilute solutions, and the Volhard back-titration method is unsuitable in principle for micro-scale titrations.

End-point detection on the micro-scale can be carried out more conveniently by the use of adsorption indicators, particularly in solutions of properly adjusted pH, when the silver chloride colloid is stabilized and when alcoholic or acetone solvent mixtures are used. In acidic acetone solutions, dithizone has also been found to be a suitable indicator [74].

Chloride ions can also be titrated argentimetrically in the presence of redox indicators. Thus, for example, the redox potential of Variamine Blue 4B (4-amino-4'-methoxydiphenylamine sulphate) is + 0.47 V in solutions of pH 4.5. When silver ions appear in the solution after the equivalence point, the redox potential of the solution increases ( £ A g / A g + = 0.80 V) and this is indicated sharply by the blue colour of the oxidized indicator. Accurate results are obtained even when standard 0.005 N silver nitrate solutions are used [75].

In addition to techniques in which indicators are employed, numerous electrometric methods are also available for end-point detection in the argentimetric titration of chloride ions. These are used extensively on the micro-scale, mainly owing to their high sensitivity. When the concentration of foreign ions in the solution is high, the potentiometric technique gives favourable results; at lower concentrations of foreign ions other electrometric methods can also be used.

In solutions of high ionic strength the method is not sensitive enough, because the potential change at the equivalence point is small. Malmstadt and Winefordner [76] applied a special potentiometric technique, so-called zero-point potentiometry, to the argentimetric titration of chloride ions in aqueous solutions. In this procedure, silver or silver chloride electrodes are immersed in the sample and reference solutions, and a solution containing chloride ions is added in small increments to the solution to be determined until its chloride ion concentration becomes equal to that of the reference solution and the

338

potential difference between the electrodes in the solutions drops to zero. Bishop and Dhaneshwar [77] suggested the use of differential electrolytic potentiometry in the determination of halides. Silver or silver chloride electrodes were immersed in the solution and were polarized with a very small voltage by means of a stabilized d.c. source. At the end-point of the titration the potential difference between the electrodes appears as a differential peak, which allows very precise establishment of the equivalence point. N o blank value appears in glacial acetic acid, so that 0.5 jig of chloride ions can be determined with only a slight error. Schwab and Tolg [78] titrated about 1 jig of chloride ions with 0.0002 N silver nitrate solution in a glacial acetic acid medium by this procedure.

Chloride ions were titrated in organic solvents by Cunningham et al. [79] by means of a bimetallic electrode system. Polarization voltage titration was first used by Schmidt [80]. Olson and Krivis [81] titrated chloride ions coulometrically after oxygen flask combustion, and Solomon and Uthe [19] suggested a similar titration procedure. Coulson and Cavanagh [82] constructed an automatic apparatus for the determination of chloride ions based on coulometry. The dead-stop end-point detection technique was employed by Clippinger and Foulk [83] in the argentimetric titration of chloride ions. Greenfield et al. [84] titrated chloride ions using amperometric end-point detection. The conductimetric technique was used for the determination of chloride ions by Jander and Innig [85] ; for a detailed description of the method we refer to the book by Jander and Pfundt [86].

The halide ion activity of solutions can be measured with ion-selective electrodes, these can be used for all the four halide ions. The first electrodes of this type were prepared from ion-exchange resins in the chloride form, then from collodion membranes. They are m a d e | r o m silver halides incorporated in membranes such as silicone rubber [87, 88]. In comparison with other electrometric methods, the advantage of those based on ion-selective electrodes is their great selectivity.

In the microtitrimetry of chloride ions, mercurimetry, using mercury(II) ions as titrant, is very useful. In this reaction mercury(II) chloride is formed, which dissociates only to a very limited extent. The procedure was suggested by Votocek [89], who used sodium nitroprusside as a precipitation indicator. Erdey and Banyai [90] found iodate ions to be more advantageous for this purpose, as the mercury(II) iodate precipitate formed at the end-point indicates the equivalence point more sharply. The end-point can also be detected in mercurimetric chloride titrations by the redox indicator phenanthroline. However, the most suitable indicator for this purpose is diphenylcarbazide, a very sensitive colour reagent for mercury(II) ions. Bognar and Jellinek [91] worked on mercurimetric titrations employing

24 339

diphenylcarbazide as indicator, and succeeded in making the procedure the most sensitive of micro methods for chloride ions using visual end-point detection [92, 93]. Sahla, Alfy and Abul Taleb [94] and Lalacette and Steyermark [95] suggested 5 mole /m 3 mercury(II) perchlorate standard solution for use in mercurimetric titrations.

Of the instrumental methods of end-point detection, high-frequency titrations are well known and widely applied in the determination of chloride ions [96].

Nephelometry and turbidimetry are sensitive techniques for the determination of chloride ions, but they are used mainly in trace analysis in which larger errors are acceptable.

Chloride ions form few coloured compounds. The chloro complexes of iron(III) and palladium chloride are such substances, but their molar absorptivities are relatively low. Iron(III) perchlorate and mercury(II) thiocyanate have been used as reagents in the spectrophotometric determination of chloride ions after combustion in an oxygen flask [97].

Indirect methods are more sensitive, but less reliable. Kirsten [98] employed silver dithizonate which released dithizone in the presence of chloride ions, and the dithizone was measured spectrophotometrically. Bertolacini and Barney [99] introduced mercury(II) chloroanilate, which releases purple chloranilic acid in equivalent amounts under the influence of chloride ions, and the chloranilic acid is measured spectrophotometrically.

Instrumental methods have been applied most frequently for the measurement of chloride ions, as other methods suitable for routine measurement are unsatisfactory. The methods are suitable primarily for water analysis.

As polarography cannot be^readily used for the direct measurement of chloride ions, Gladysev and Kalvoda [100] applied oscillopolarography for this purpose. Maddox et al. [101] suggested the use of cathodic stripping voltammetry based on the fact that the surface of a mercury electrode becomes coated with a layer of calomel when connected as the anode in a solution containing chloride ions, and in the measuring step this electrode becomes the cathode, when the calomel is reduced. The chloride content is measured from the current.

Radioactive isotopes were employed first by Johannesson [102] in the determination of chloride ions in water. Radioactive precipitation exchange reactions were developed by Banyai et al. [103], which allowed the measurement of 30-3500 ng of chloride ion by shaking the solution with a suspension of mercury (II) iodate labelled with 2 0 3 H g , and the activity of the mercury(II) chloride solution obtained was measured.

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Gas chromatography is not suitable for the direct determination of the halogen content or organic halogen compounds, because in gas chromato-graphic separations the physical rather than the chemical properties of compounds are the principal determining factors. Halogen-containing and halogen-free compounds having identical physical characteristics cannot be separated in this way. Separation can be achieved, however, after degrading the molecules (e.g., by pyrolysis), and the free halogens, hydrogen halides or molecular fragments containing halogens can be separated from those free from halogens.

The use of gas chromatography in the determination of the halogen content of organic compounds was first suggested by Mamaril and Meloan [104]. In their method, the carbon, chlorine, bromine and iodine contents of compounds were determined after combustion in a stream of oxygen according to the modified Pregl method; the water formed was absorbed and carbon dioxide and the halogens were frozen out with liquid nitrogen. These were then swept by helium carrier gas into a Chromosorb P column wetted with silicone grease in order to decrease the volatility; a thermistor detector was used.

Determination of bromide ions. The analytical characteristics of bromide ions are similar to those of chloride ions; for example, they give precipitates with silver and mercury (I) ions, which are less soluble than the corresponding chlorides and mercury(II) bromide also dissociates to only a limited extent. Thus the gravimetric and precipitation titration methods reviewed in connection with quantitative micro determination of chloride ions can be applied to the determination of bromide ions virtually without alteration.

The standard redox potential of the bromine-bromide system (+1 .07 V) is less positive than that of the chlorine-chloride system (+1 .40 V), so that bromine can be liberated by chlorine molecules from solutions of bromide ions. Oxidation of bromine to hypobromite or bromate ions does not require the use of particularly powerful oxidizing agents, so that redox reactions can be readily applied to the determination of bromide ions. Free bromine is volatile and can be expelled from aqueous solutions by boiling. This offers the possibility of the separation of bromine from, for example, chloride ions. There are more numerous and reliable reagents available for the spectrophotometric determination of bromide and bromate ions than for chloride ions.

Free bromine can also be determined by gravimetric, titrimetric and colorimetric methods, but these are of minor importance in the analysis of organic compounds, because in the decomposition procedures or in subsequent steps bromine is converted into bromide ions immediately. In wet decomposition procedures involving powerful oxidants in acidic media,

24* 341

bromine is liberated and expelled from the decomposition mixture, and is converted into bromide ions in the absorbent solution. The fact that bromine can be oxidized to bromate relatively easily can be utilized in the amplification method.

The gravimetric determination of bromide ions is less favourable than that of chloride ions. Although silver bromide is less soluble than silver chloride ( K s ( A g B r ) = 4 . 1 x 1 0 - 1 3 , 18°C), it is more light sensitive and the conversion factor is less favourable (Br/AgBr = 0.4255; Cl/AgCl = 0.2474). Bromate ions, when present in the solution in addition to bromide ions, must be reduced before precipitation in order to prevent the formation of a silver bromate precipitate. Precipitation is effected in solutions made slightly acid with nitric acid, and a small excess of silver is added. When chloride ions are also present, a silver chloride-silver bromide mixed precipitate is obtained, and the amount of both halide ions in the precipitate can be determined by indirect gravimetric analysis. For this purpose, the mixed precipitate is weighed, then converted into silver chloride in a stream of chlorine gas or with chlorine water and reweighed. The two weights enable the bromide and chloride contents to be calculated. In another procedure, the mixed precipitate is converted into silver bromide with ammoniacal potassium bromide solution and the silver bromide is allowed to react with potassium iodide in a similar way, yielding silver iodide. This means that the silver halide of lowest solubility is used for the measurement.

Bromide ions can also be titrated argentimetrically using either the Mohr or the Volhard method. The results are better than those for chloride ions, because the lower solubility of silver bromide allows the end-point to be detected more sensitively. The end-point can also be indicated by adsorption and redox indicators. In coloured solutions, luminescent indicators can be employed.

In the mercurimetric titration of bromide ions, adsorption or redox indicators can be used. Mercurimetry is also suitable for micro and ultramicro measurement using electrometric end-point detection. A specific and very sensitive indicator for the mercurimetric titration of bromide ions is diphenylcarbazide or diphenylcarbazone, but the titration can also be carried out in the presence of ferroin or other redox indicators. Denney and Smith [105] used bromobenzoic acid for comparing argentimetric and mercurimetric titrations. No significant difference in the results was observed, but the mercurimetric procedure was found to be faster.

Small amounts of bromide ions can be determined by the amplification method developed by Van der Meulen [106]. The method is based on the oxidation of bromide ions into bromate ions by hypochlorite ions:

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Br + 3 O C l = 3 C 1 + B r 0 3

After decomposition of the excess of hypochlorite, with, e.g., formate ions, iodide is added and six a toms of iodine are liberated for every one of the bromate ions in the acidic solution.

The method can also be employed on the micro-scale and it is suitable for the determination of bromide ions in the presence of large amounts of chloride ions. The reaction between bromate and iodide ions is catalyzed by molybdate ions [108]. Chloride ions do not interfere, but iodide ions are oxidized to iodate ions by the hypochlorite ions.

The end-point of the argentimetric or mercurimetric titrations of bromide ions can be detected by almost all the electrometric techniques reviewed for chloride ions. According to recent literature data, zero-point potentiometry is also particularly suitable for use in micro titrations. Titrations on the ultramicro-scale have been carried out using bipotentiometric end-point indication [109].

The spectrophotometric determination of bromine liberated from bromide solutions using bromophenol blue was suggested by Stenger and Kolthoff [110]. Another suitable reagent is fluorescein, which yields red tetrabromo-fluorescein (eosin) with bromine. Liberation of bromine from a bromide solution is most conveniently achieved by means of chloramine T. Larger amounts of iodide ions interfere. Fadeeva et al. [107] used C.I. Acid Blue 7 dye in the determination of bromine after combustion in an oxygen flask.

Radioactive precipitation exchange reactions [111] can be applied to the determination of bromide ions more advantageously than to chloride ions. The procedure involves the extraction of precipitated mercury(II) bromide labelled with 2 0 3 H g with diethyl ether and measurement of the gamma-activity of the ethereal phase.

Bergmann and Martin [112] developed a gas chromatographic method for the determination of bromide ions based on the separation and detection of hydrogen halides. The procedure is suitable for the determination of bromine, chlorine and iodine in organic compounds after combustion.

Determination of iodide ions. The chemical characteristics of iodide ions from the analytical point of view are different from those of chloride and bromide ions. This can be attributed primarily to the less positive standard redox potential of the iodine-iodide system (-1-0.62 V). It follows that iodine can be formed in iodide solutions by reaction with less than moderately strong oxidants. Moreover, iodine can easily be oxidized to iodate.

Iodine is slightly soluble in water (0.28 g / d m 3 at 18°C), but its solubility is greatly increased in the presence of iodide owing to the formation of triiodide ions (I 3").

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The more electropositive halogens liberate iodine from iodide ion solutions:

2 r + c i 2 = i 2 + 2 c r

21" + B r 2 = I 2 +2 Br-

o t h e r redox systems with more positive redox potentials than that of iodine-

iodide behave similarly. Iodine is oxidized to iodate by excess of chlorine or bromine:

I 2 + 5 C l 2 + 6 H 2 O = 2IO3- + 10Cl- + 12H +

Iodate is also formed by reaction with oxygen at high temperatures. This accounts for the fact that oxidative decomposition of organic iodine compounds yields mainly iodate ions and iodine formed in the reaction of iodide and iodate. These must be reduced first when the iodine content of organic compounds is to be determined as iodide ions.

The reduction can be effected with, for example, hydrazinium salts:

2I 2 + N 2 H 4 = N 2 + 4 H + + 4 r

or metals, such as

I 0 3 + 3 M g + 6 H + = 3 M g 2 + + 3 H 2 0 + r

The only important condition is that foreign ions that interfere must not remain in the solution after reduction.

When iodide ions are oxidized to iodate ions and the excess of the oxidant is eliminated, the iodate ions, on the addition of more iodide ions, form six times the original amount of iodine originally present:

I O ; + 5 r + 6 H + = 3 H 2 0 + 31 2

In almost all instances, the iodine liberated is titrated with standard sodium

thiosulphate solution:

I 2 + 2 S 2 0 2 , - - + S 4 0 £ - + 2 1 -

The starch indicator allows the titration of even 0.005 N solutions very precisely. The use of standard arsenous acid or ascorbic acid solutions is also possible.

In the analysis of organic iodine compounds, gravimetric, titrimetric and spectrophotometric procedures can be employed, after decomposition of the compound. A very sensitive determination of iodide ions is possible on account of their catalytic effect on certain chemical reactions.

Although the gravimetric measurement of iodide ions as silver iodide is advantageous because of the very low solubility of silver iodide, being the

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lowest of the silver halides (Ks= 1.5 x 1 0 " 1 6 at 25°C), it has disadvantages, such as the tendency of the precipitate to adsorb foreign ions from the solution, its greater light sensitivity compared with the other two silver halides, and the poor conversion factor (I/AgI =0.5405).

The precipitation of iodide ions with a small excess of silver ions from solutions made slightly acidic with nitric acid is carried out in a similar manner to that of silver chloride and bromide.

When a mixture of silver halides is heated with ammonium iodide, silver iodide is obtained quantitatively. The method can be recommended only when the amounts of the three halides present in the sample and thus in the precipitate are not too different; this holds particularly for measurements on the micro-scale.

The argentimetric titration of iodide ions is facilitated by the low solubility of silver iodide and the ready coagulation of the colloidal precipitate before the equivalence point. According to Kolthoff, this permits the accurate titration of 0.04 N iodide solutions with standard 0.001 N silver nitrate solution without the use of an indicator. In acetic acid solutions, using eosin as adsorption indicator, 1 0 ~ 3 N solutions show distinct pink to violet colour changes. Fluorescein can also be used advantageously. Schulek and Pungor [113] suggested the use of p-ethoxychrysoidine as an adsorption indicator and immersed a glass electrode in the solution to be titrated. Proton exchange with silver iodide-p-ethoxychrysoidine adsorption indicator complex formed during titration occurs at the equivalence point and the pH of the solution is altered which is detected by the glass electrode. At the same time, the colour of the indicator shows a sharp change from red to yellow. In this way, 0.002-0.0005 N iodide solutions can be titrated very precisely with standard 0.01 N silver nitrate solution. Potentiometric end-point detection makes possible the titration of 5 c m 3 of solution containing 1 jig of iodide ions with standard 0.0001 N silver nitrate solution with an accuracy of 4%.

Argentimetric titrations can be carried out in the presence of dithizone [114], which exhibits a green to yellow colour change when an excess of silver ions appears in the solution after reaching the equivalence point. Chloride and bromide ions do not interfere because the stability of silver dithizonate is greater than that of silver chloride and bromide precipitates.

In the mercurimetric titration of iodide ions, mercury(II) perchlorate is used as titrant; in micro-scale titrations, 0.005 N solutions are used. Diphenylcarbazone is a suitable indicator [115]. The method has been employed for the determination of the iodine content in organic samples after combustion in an oxygen flask [116].

A useful characteristic of iodide ions is their catalytic effect on certain redox reactions when present in minute amounts. Thus, for example, the reaction

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between cerium(IV) and arsenite ions takes several weeks to proceed to completion, but it is completed in a few minutes when catalyzed by iodide ions. The reaction can be utilized for the catalytic micro determination of iodide ions. In this procedure, a solution of arsenous acid, which also contains the iodide ions to be determined, is added in excess to a solution of cerium(IV) and the time required for complete reduction of the cerium(IV) is measured. The completion of the reaction is indicated by the sharp appearance of the red colour of the o-phenanthroline-iron(II) complex. The procedure is suitable for use on the ultramicro-scale also, as 0.03-0.3 \ig of iodide ions can be measured in this way.

Microtitrimetry of iodide ions can be carried out very simply and sensitively by the amplification methods developed for the determination of iodine and bromine. In the literature this method has been attributed to Leipert, who reported it in 1929 and 1938 [117, 118]. However, it was the great Hungarian chemist Winkler who first published a procedure of this type; in 1900 he reported the oxidation of iodide to iodate ions with chlorine water [119]. Previously, potassium permanganate had been used for this purpose. Hunter [120] used hypochlorite ions for oxidation purposes in 1909 and Bugarszky and Horvath [121] described the use of bromine as an oxidant in the same year. Another Hungarian, Szabo, dealt with this subject in three papers [122-124]. In 1968, Belcher [125] reviewed amplification titration methods.

There are few spectrophotometric methods suitable for the determination of iodide ions. Although the colour of starch-iodine complex is very sensitively indicated and has already been applied to the quantitative determination of iodine, good results can be obtained only under very carefully controlled conditions. The violet colour of iodine in non-polar solvents can be more advantageously utilized in photometric measurements. The wavelength suggested for measurement is 360 nm, where Beer's law holds up to concentrations of 10 ng /cm 3 . When the iodine to be measured is oxidized to iodate ions, by the well-known Leipert procedure, free iodine is liberated, and a six-fold increase in sensitivity can be achieved [126].

Gas chromatography can be employed advantageously in the determination of the iodine content of organic samples. Iodine can be separated from the other halogens as iodoacetone; electron-capture detection is used for measurement [127]. Iodine can be isolated as acid iridium iodide; in this procedure flame photometric detectors can be used [128],

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(C) SELECTED P R O C E D U R E S FOR THE DETERMINATION O F THE CHLORINE,

BROMINE A N D IODINE C O N T E N T S O F O R G A N I C C O M P O U N D S

In this section, some micro methods that have been found to be suitable for the determination of chlorine, bromine and iodine in organic substances are reviewed, based partly on the literature and partly on the experiences of the author. Methods that can be realized by simple equipment are also considered, even if they are not suitable for the determination of each of the three halide ions or for the analysis of samples in any physical state.

The methods are described in detail, and the principles on which they are based and the reagents and equipment used are given.

It is assumed that skilled analysts will apply these methods and that therefore it is unnecessary to call special attention to the fact that microanalytical methods yield good results only in clean and precise operation, when chemicals of high purity and water distilled from glass apparatus are used.

In all instances, even when no special attention is drawn to it, checking of the results by blank tests is recommended. The data obtained in this way may be used for correction purposes only when these do not exceed 5% of the value measured. When the blank does exceed this value, the contaminated chemical should be identified and replaced.

(1) Determination of chlorine and bromine in organic compounds by wet decomposition [129, 130]

The sample is decomposed with concentrated sulphuric acid, potassium dichromate and silver dichromate, and the halogens liberated are transferred into neutral hydrogen peroxide solution with a current of air and absorbed. The hydrogen halides formed are allowed to react with mercury(II) oxycyanide in the absorbent solution and the equivalent amount of base liberated is titrated with standard sulphuric acid solution.

The method is suitable for the micro determination of itivolatile chloro and bromo compounds and is particularly convenient for routine analysis.

Apparatus. The apparatus shown in Fig. 42 is made from glass that resists heat and chemical attack. It consists of a flask (/) and an absorber (2) attached via a ground-glass joint. The flask is heated in a metal block to ensure uniform heat transfer and the reaction products are swept out of the flask with an air stream, free of carbon dioxide, sucked through the vessel.

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Fig. 42. The Ingram wet oxidation apparatus for the determination of the chlorine and bromine content of organic samples

/ — F l a s k ; 2—absorber

Reagents and solutions

Concentrated sulphuric acid, analytical grade, S.G. 1.84. A mixture of potassium dichromate and silver dichromate, 1:1 by weight.

The two substances are ground thoroughly before mixing and kept in a dark bottle.

Silver dichromate can be prepared as follows: dissolve silver nitrate (10 g) and chromic acid (6 g) in hot water (1 litre). Filter the hot solution through a glass filter and leave overnight to crystallize. Filter off the brown silver dichromate crystals on to a glass filter, wash twice with a small volume of water and dry over phosphorus pentoxide in a desiccator.

3% Hydrogen peroxide. Dilute 1 c m 3 of 30% hydrogen peroxide with 10 c m 3 of water, add 2 drops of Methyl Red-Methylene Blue mixed indicator and titrate with 0.01 N sodium hydroxide solution until the violet colour of the indicator turns to grey.

Methyl Red-Methylene Blue mixed indicator. Dissolve Methyl red (0.125 g) in ethanol (50 cm 3 ) , dissolve Methylene

Blue (0.083 g) in a similar volume of ethanol, and mix the solutions. The mixture can be used for about a week.

Saturated mercury(II) oxycyanide solution, cold. Suspend mercury(II) oxycyanide (10 g) in water (250 cm 3 ) , allow to

sediment for several days, protecting the mixture against light. Dialysis gives a clearer solution.

The strength of the standard solution can be checked as follows: add two drops of the mixed indicator solution to 10 c m 3 of the mercury(II) oxycyanide

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solution in a titration flask and add 0.01 N sulphuric acid dropwise (5-10 drops are necessary) until the indicator shows the solution to be neutral. Add 10 mg of sodium chloride dissolved in 5 c m 3 of water and titrate the green solution (the solution becomes alkaline owing to release of hydroxyl ions) with standard 0.01 N sulphuric acid (typically, 10-15 c m 3 of acid are consumed).

Standard 0.01 N sulphuric acid, prepared by dilution of 0.1 N acid. Standard 0.01 N sodium hydroxide solution, prepared by diluting

carbonate-free 0.1 N sodium hydroxide solution with carbon-dioxide-free water. Store in a polyethylene flask, with protection from carbon dioxide.

Procedure. The glass apparatus is cleaned first with a hot chromic acid-sulphuric acid mixture, washed several times with water and finally rinsed with ethanol. In order to remove all traces of moisture, the apparatus is dried at 120°C for 1 hour and allowed to cool in a desiccator.

An appropriate amount of sample containing not more than 1.5 mg of halogen (usually 4-8 mg) is weighed into the flask. Liquids are weighed into glass capillary tubes, the opening of the capillary should be below the liquid level.

Neutral hydrogen peroxide solution (11 c m 3 ) is pipetted through the side-arm of the absorber equipped with a glass stopper. This solution fills the lower part of the vessel, but some solution is transferred to the upper compartments by carefully tipping the vessel. The side-arm is closed with the stopper. The dichromate mixture (0.5 g) is added to the flask with a glass or metal spoon. Horn or plastic spoons should not be used. Then concentrated sulphuric acid (2 cm 3 ) is pipetted into the flask, the ground-glass joint is lubricated with concentrated sulphuric acid and the two parts of the apparatus are joined and secured with springs.

The apparatus is mounted on a heating stand and the side-arm of the flask is attached to the gas-cleaning device by means of a rubber tube. The gas-cleaning device consists of a scrubber filled with concentrated sulphuric acid and a U-tube packed with soda-asbestos. Suction is applied to the side-arm of the absorber; one bubble should leave the gas inlet tube of the absorber per second. A Mariotte flask is recommended for providing the air current.

The heating block is maintained at 120-130°C for 30 min. After this period, the apparatus is removed from the heating block, allowed

to cool and dismantled. The contents of the absorber are transferred into a titration flask through the side-arm and the vessel is rinsed with small portions of carbon dioxide-free water (10 c m 3 in total). Indicator solution (2 drops) is added and the acidic solution is neutralized exactly with standard 0.01 N sodium hydroxide solution. Then neutral mercury(II) oxycyanide

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solution (10 c m 3 ) is added and the now alkaline solution is titrated to a neutral end-point with standard 0.01 N sulphuric acid.

A 1-cm3 volume of 0.01 N sulphuric acid is equivalent to 0.3546 mg of chlorine or 0.7992 mg of bromine.

The method is suitable primarily for the analysis of solid samples of low volatility; careful neutralization and skilled titration are essential for achieving high precision.

(2) Oxidative decomposition of organohalogen compounds in a Parr bomb on the semimicro-scale [131]

The sample (20-25 mg) is fused with sodium peroxide (2-5 g) and ethylene glycol in a sealed metal bomb of the Wurzschmidt type. Ignition starts at relatively low temperatures (below 100°C), the organic material is completely decomposed and the halogens yield the corresponding sodium halides. The halides are determined in the solution obtained on extraction of the fused mass, by methods which are not disturbed by the high salt content of the solution. On the semi-micro-scale, a gravimetric finish is recommended.

Sodium ions can be removed by means of a cation exchange resin when necessary.

Apparatus. The Wurzschmidt-type bomb shown in Fig. 43 consists of five parts. The fusion cup (1) is made from pure nickel metal and has a capacity of about 10 cm 3 , the wall thickness is 1.5 mm. Lid (3) is made from the same material; its lower part reaches into the cup, so that the plastic sealing ring (2) ensures air-tight sealing without coming into contact with the contents of the bomb. Rings 4 and 5, made from bronze, hold the lid securely in position.

The bomb is placed in a safety box during use. There is a small burner in the case under the ring supporting the bomb.

Reagents Sodium peroxide, powder or granules Ethylene glycol Nitric acid (1 + 1)

Decomposition procedure. Ethylene glycol (8 drops, 0.16-0.17 g) is added to cup 1 of the bomb and the sample (20-25 mg) is weighed on to it. Smaller or larger amounts, up to 0.1 g, can also be analyzed; non-volatile liquids are placed in a small glass cup at the bot tom of the vessel. Then sodium peroxide (3-4 g) is added. First a small portion is added, because the sample may react with sodium peroxide in the cold. If this occurs, cover the sample with anhydrous sodium carbonate (0.2-0.3 g) before adding the sodium peroxide.

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Fig. 43. Micro Parr-Wurzschmidt bomb Fig. 44. Filtration with a filter stick

/—Nicke l c u p ; 2—Teflon sealing gaske t ; 3—nickel l id; /—Suc t ion

4 a n d 5—closing c lamps

When larger samples are to be analyzed, the amount of sodium peroxide should be increased; the bomb can be filled up to two thirds of its volume. Mixing of the materials in the bomb is unnecessary. The lid with the sealing ring is placed on the cup and secured with clamps (5 and 4\ hand-tightening the screws only.

The bomb is placed in the support in the safety box after lighting the flame and adjusting its height so as just to reach the bot tom of the bomb. The door of the safety box is closed. The mixture is ignited in the bomb within 10-30 s; a knock is heard and the gas flame shows a yellow flash. Ignition takes longer when the sample is covered with sodium carbonate. Combustion of the sample produces a large rise in temperature and sodium peroxide melts, but the increase in pressure is very small, so that there is no danger of explosion, even if larger samples are decomposed. During the combustion, only the lower part of the bomb becomes hot, and the securing screws remain almost cold. Thus, the b o m b is immediately removed from the support and cooled by

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4

dipping the bottom of cup 1 into cold water. The bomb is dismantled, opened and cup 1 and lid 3, (on which some spattered melt is always found) are placed in a porcelain dish. The fused mass is dissolved in water, the cup and the lid are removed, rinsed, the washings are added to the dish and the solution is made accurately neutral with nitric acid (1 + 1). The solution is brought to boil, filtered through a thick filter-paper and the filter-paper is washed with very dilute nitric acid. The filtrate is collected in a 50 c m 3 volumetric flask and made up to volume with water. Aliquots of this stock solution are used for gravimetric determination of the halides.

Gravimetric halide determination

Equipment. Thin-walled, 15 c m 3 beaker, filter stick with a stem 10 cm long and with a glass filter disc of 1 cm diameter of porosity 2 or 3. The total weight of the beaker and the filter stick should not exceed 15 g. Filtration apparatus as shown in Fig. 44.

Reagents Nitric acid, (1 + 1) and (1 + 100) Silver nitrate, 10% solution

Procedure. The beaker and the filter stick are cleaned thoroughly and dried at 140°C for 1 h, allowed to cool in an empty desiccator (containing no desiccant) and the filter stick is placed in the beaker. Both are allowed to stand in the microbalance case for about 30 min then weighed.

Not more than 1 0 c m 3 of the stock solution is transferred to the beaker from which the filter stick has been removed. If required, the stock solution or the solution of the fused mass may be concentrated on a water-bath, but only to such an extent that sodium nitrate does not crystallize out. The solution is acidified with 1 drop of nitric acid (1 + 1), heated but not boiled and 2-3 drops of 10% silver nitrate solution are added to precipitate the silver halide.

The beaker is covered with a watch-glass and heated on a water bath for 2-3 h. When the solution is cool and the precipitate has formed a thick layer at the bottom of the beaker, the filter stick is immersed in the solution. The suction apparatus is attached and, as described previously, the mother liquor is drawn off and the precipitate is washed five times with 0.5-1 c m 3 portions of nitric acid (1 + 100). Finally, the liquid is sucked from the stem of the filter stick. The beaker and the filter stick are dried and weighed as before. Conversion factors: Cl/AgCl = 0.24737; Br/AgBr = 0.42555; I/AgI = 0.54053.

The procedure yields very precise results when the chemicals are free from halide ions and the directions are carefully followed.

A potentiometric finish, for example, can also be applied to the determination of halides. The silver chloride reference electrode and a silver indicator electrode are used. The titration is carried out in very dilute nitric

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acid, diluted with an equal volume of acetone. The titrant is 0.05-0.01 N silver nitrate solution, the actual concentration depending on the amount of halide to be measured. When bromide and iodide ions are titrated, dilution with acetone is not necessary. In the determination of bromide and iodide ions, the conversion factors are not so favourable as for chloride ions, but weaker standard solutions can be used, owing to the large and sharp potential change at the equivalence point, particularly in the titration of iodide ions.

(3) Combustion of organic chlorine, bromine and iodine compounds in a fast stream of oxygen in an empty tube [17, 132, 133]

The combustion of organic samples is carried out in a fast (about 50 cm 3 /min) stream of oxygen in a quartz combustion tube. The tube contains no packing, but complete combustion is ensured by a temperature of 900°C. The combustion products are absorbed in a vessel packed with glass beads moistened with a suitable absorption solution. The choice of procedure for the determination of the halide ions formed depends on the halogen and the absorption solution used.

Apparatus: The two main parts are the combustion tube with the combustion chamber (Fig. 45) and the absorber connected to it by means of a ground-glass joint. The combustion tube and chamber are made of quartz and the absorber of glass.

The quartz combustion tube is 25 cm long and of 9-10 mm i.d., equipped with a side-arm for the introduction of the gas. The sample is pyrolysed in the combustion tube by means of a flame or a small electric furnace moving along the tube. A nickel-wire gauze is wrapped around the tube to ensure uniform heat transfer. There is a constriction in the combustion tube at the combustion chamber and a small quartz-wool plug is inserted before it.

Fig. 45. Empty tube apparatus for halogen determination, according to Belcher and Ingram

/—Pyro lys i s t u b e ; 2 — c o m b u s t i o n c h a m b e r ; 3—electric furnace; 4— wash-ou t device; 5—absorp t ion vessel

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The combustion tube is connected to the combustion chamber, which is 20 cm long and of 2.5 cm i.d. The internal quartz tube installed along the axis of the combustion chamber carries quartz baffle plates of about 22 mm diameter attached at 3 cm intervals. During operation, the combustion chamber is placed horizontally in the 900°C furnace. The exit tube protrudes only 15-20 mm outside the furnace; its end is constricted and continues in two directions. One tube is bent at 45° and is connected to the absorber by means of a ground-glass joint. The other is bent at approximately the same angle and carries the rinsing tube opened up into a funnel-shaped piece. A ground-quartz rod can be inserted into the opening of the rinsing tube.

The absorber is a U-tube, with limbs 12 cm long and of 10-12 mm i.d. One limb can be attached to the outlet tube of the combustion chamber (this is bent downwards) and the other is provided with a splash bulb. A drainage tube is situated at the base of the U-tube; this is a capillary tube equipped with a stopcock. The U-tube is packed with 3 mm diameter glass beads.

Oxygen is supplied from a cylinder through flexible tubing, and is led through a pressure regulator valve and a flow meter to the side-arm of the combustion tube. The flow-rate should be 50 cm 3 /min.

The apparatus is mounted on a suitable support. The combustion tube is heated by a tube furnace, of 3 cm i.d., constructed from identical halves, so that it can be opened. A muffled crucible furnace can also be used. The entire combustion chamber should preferably be within the heated space.

Absorption solutions: When a potentiometric titration or gravimetric determination of chloride

or bromide ions is required, a mixture of 35% sodium hydrogen sulphite and* IN sodium hydroxide solutions (1:2) is used as the absorption solution. For the iodimetric titration of bromide ions, a 1:2 mixture of 30% hydrogen peroxide and 1 N sodium hydroxide solution is suitable.

When the amplification titration procedure is applied in the determination of iodide ions, 1 N sodium hydroxide solution or bromine in glacial acetic acid should be used; the latter is prepared by dissolving 100 g of sodium acetate in glacial acetic acid (1 d m 3 ) and adding 8-10 drops of bromine to 10 c m 3 of this solution before use.

Procedure. The apparatus is cleaned carefully before assembly, the ground-glass joints are lubricated with water. The furnace is brought to 900°C and the appropriate absorption solution (9 c m 3 ) is pipetted into the U-tube through the funnel of the splash bulb. When the glass beads are completely moistened, the tap at the bot tom of the absorber is opened and 7 c m 3 of the solution are drained out using a measuring cylinder.

The solid sample is weighed in a platinum boat, the sample size being 4-7 mg for chlorine-containing materials and 7-10 mg for iodo compounds.

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Viscous liquids are weighed into porcelain boats of the same size as the platinum boats. Volatile liquids are drawn into capillary tubes, and a crystal of ammonium nitrate is introduced into the capillary tube and melted at the bottom. The capillary is usually placed in the boat with the tipped end backwards, that is, facing the gas stream. The boat is placed in the combustion tube 4-5 cm in front of the quartz-wool plug, the combustion tube is closed with a rubber bung and the oxygen flow is started. About 2 c m 3 of water are added to the rinsing tube. Combustion of the sample is effected by slowly advancing the roll of nickel gauze together with the flame or furnace so as to avoid formation of smoke or mist, as these are swept through the combustion tube without complete decomposition.

In the pyrolysis process, the nature of the sample should be taken into account. If it vaporizes rather than sublimes, very slow, cautious heating must be applied. Substances that decompose are heated first cautiously, then strongly when only carbon remains. If the sample sublimes or vaporizes and condenses in the cooler parts of the tube, it is followed slowly with the flame and the wire roll until the drop of condensate reaches the vicinity of the hot quartz-wool plug. The drop, which cannot migrate further, is approached cautiously with the heat source and evaporated slowly. With iodo compounds, iodine sublimed from the sample can condense on the cool walls of the tube in the section before the quartz-wool plug. This, as with the sample drop, should be evaporated slowly. Finally, in order to combust the residue completely, the portion of the tube containing the boat is heated strongly and the flame is moved along the tube up to the quartz-wool plug. Any carbonaceous deposits should be burnt away completely at this point. The stream of oxygen is maintained for 2-3 min, then heating and passage of oxygen are stopped.

In order to rinse out the absorbed combustion products, the stopper of the rinsing tube is lifted slightly to drain 1-2 c m 3 of water from the tube, thus rinsing down the combustion products adhering to the walls of the tube attached to the absorber. The stopper of the rinsing tube is removed and rinsing is repeated with 1-2 c m 3 of water. The absorber and the combustion tube are detached and the ground-glass joint is rinsed with a few drops of water. The absorber is filled with water until the beads are covered in both limbs and allowed to stand for a few minutes. Then the outlet tap of the vessel is opened and the liquid is drained out dropwise into a suitable receiver. The tap is closed and about half the amount of water that was used for the first washing is added. The absorber is tilted repeatedly so as to rinse the glass beads in both limbs of the U-tube and the surface of the ground-glass joints. After removing the solution through the tap, the treatment is repeated once or

25 355

twice; the total volume of water used for rinsing should not be more than 2 5 -30 cm 3 . Finally, the tip of the tap is rinsed with a few drops of water.

All of the solution obtained is used in the gravimetric determination of halides.

When the combustion products are absorbed in alkaline hydrogen peroxide solution, chloride and bromide ions can be titrated argentimetrically in the presence of dichlorofluorescein as adsorption indicator [129].

Reagents and solutions Nitric acid, 1 N Standard silver nitrate solution, O.OIN Standard sodium chloride or potassium bromide solutions, 0.01 N Saturated sodium acetate solution Acetone, reagent-grade, free from halogens Dichlorofluorescein indicator solution, prepared as follows: 10 mg of the

substance are dissolved in ethanol (100 c m 3 ) (ethanol was distilled from sodium hydroxide), then 2.5 c m 3 of 0.01 N sodium hydroxide solution are added to it.

Saturated solution of hydrazinium sulphate Phenolphthalein, 1% solution in ethanol

Procedure. The liquid drained from the absorber is collected in a ground-glass stoppered 100 c m 3 conical flask, the alkaline solution is neutralized to phenolphthalein with 1 N nitric acid and then heated cautiously to concentrate it to about 10 c m 3 without boiling. After cooling, a few drops of sodium acetate solution and 1-2 drops of hydrazinium sulphate solution are added (to prevent bromate ion formation), then 0.5 c m 3 of indicator solution and 10 c m 3 of acetone are added. The solution is titrated with standard 0.01 N silver nitrate solution in diffuse light. Only a few c m 3 of titrant are usually consumed, so that a 10 c m 3 microburette can be used. The solution is stirred vigorously during the titration by means of a magnetic stirrer. When the pink colour of the indicator first appears in the milky solution, the titrant is added more slowly, with agitation after the addition of each drop, until the precipitate suddenly turns red. This colour may disappear on further agitation; when it persists, the equivalence point has been reached. This is also indicated by the coagulation of the silver halide. The smaller the volume of the solution titrated, the sharper is the end-point.

When the solution contains very small amounts of halide ions (less than 0.2-0.5 mg), the amount of silver halide precipitated is insufficient for the operation of the adsorption indicator. In such cases, a precisely measured

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2-3 c m 3 aliquot of standard 0.01 N alkali metal halide solution is added before starting the titration, and this amount is then subtracted from the result.

Under the conditions given for combustion and absorption, usually no bromate ions are formed. If such formation is still suspected, 1-2 drops of hydrazinium sulphate solution are added after evaporation (concentration).

Conversion numbers: 1 c m 3 of 0.01 N silver nitrate solution is equivalent to 0.3546 mg of chlorine and 0.7992 mg of bromine.

Microburettes cannot be used for the titration, because the tip of the burette becomes clogged by the precipitate.

(4) Determination of chlorine, bromine and iodine after oxygen flask combustion, on the micro-scale [71, 72, 133]

A solid sample is wrapped in a piece of filter-paper or a foil of other suitable material and burnt in a closed flask filled with oxygen. The combustion products are absorbed in a suitable solution in the flask and are usually determined titrimetrically.

Apparatus. A 300 c m 3 conical flask was suggested by Schoniger in his first paper [71], with a platinum wire, a few millimetres thick, sealed into its ground-glass stopper (Fig. 46a). The filter-paper package was fixed to this so that the extending tip of the paper served as a fuse. In a later paper [72] he suggested the use of a small platinum gauze fixed to the end of the platinum wire for holding the filter-paper package. In the combustion procedure, the

Fig. 46. Various kinds of oxygen (Schoniger) flask

(a) External igni t ion; (b) electric ignit ion with a p la t inum spira l ; (c) electric ignition in the p la t inum b o a t ; (d) u l t r amic ro flask

(a) (b) (c)

25* 357

paper tip is ignited outside the flask, holding the stopper in the hand and quickly inserting the stopper.

Several modifications have since been suggested, the most important being those that accomplish the ignition of paper and the sample within the flask.

In the author's laboratory, the flask shown in Fig. 46b is used. The neck of the 300 c m 3 quartz flask has a collar and the advantage of the quartz flask over that made from glass is that it can be used for the combustion of fluorine compounds. A ground-glass stopper with a small coefficient of thermal expansion is inserted into the mouth of the flask; two glass tubes are sealed to its bot tom; a platinum wire, 1 mm thick and 20 mm long, is fixed to the end of each glass tube. A copper wire, 1 mm thick, is soldered to the platinum wires, which pass through the glass tubes and are attached to electrical connections situated near the top of the stopper. One of the platinum wires fixed to the glass tube carries a platinum boat (or dish), 16 mm long, 6-7 mm wide and 5 mm deep. The other end of the boat is 2- 3 mm from the other platinum wire. There is a platinum wire, 0.3 mm thick, between the two platinum wires situated so that its central part bends downwards and almost reaches the bottom of the platinum boat (the ends of the wire are simply coiled on to the thick platinum wires). The boat is 2 cm from the bottom of the flask when the stopper is inserted into the ground-glass joint.

The sample is wrapped in filter-paper or polyethylene foil and placed in the boat over the platinum wire, just lying on it. The sample is ignited by slowly heating the platinum wire by a low-voltage current regulated by a toroidal transformer or a sliding resistor, then the current is switched off. In the author's experience, the platinum wire melts only on overheating; the sample burns rather slowly in this way, so that unburnt particles do not fall into the absorption liquid.

According to the original method of Schoniger, 10 c m 3 of water, 1 c m 3 of 2 N potassium hydroxide solution and 3 drops of concentrated hydrogen peroxide are placed in the flask as the absorption solution. When iodine compounds are burnt, hydrogen peroxide can be omitted.

Combustion. The absorption liquid is added to the carefully cleaned flask. The sample (5-10 mg) is weighed accurately on to the middle of the wide part of the L-shaped piece of filter-paper (25 x 25 mm, with a fuse 5 mm wide and 4 cm long) shown in Fig. 41 (p. 334). When necessary, a substance facilitating combustion, usually glucose, is added in an amount twice that of the sample. This substance is necessary for compounds with high halogen contents. The filter-paper is folded (along the imaginary dashed lines), the two ends are overlapped and the parcel is placed in the holder so that the tip of the fuse protrudes from the side of the package. When the flask suggested by the author is used, a 20 x 20 mm filter-paper is sufficient; this makes a parcel of

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5 x 10 mm, which is bent in a V-shape and placed in the boat so that the platinum wire is situated between the two sides of the V, apex upwards. The ground-glass joint of the flask is lubricated with water and a fast stream of oxygen is passed into the flask for about 1 min through a glass tube reaching almost to the bottom of the flask. For micro determinations, the gas is usually not purified. The stopper is then inserted in the flask, some water is poured into the collar and the electric current is switched on. After ignition of the paper the current is switched off immediately. The stopper of the flask is pressed in during combustion, because the increased pressure, owing to the heat evolved, may otherwise force it out. It is advisable to place the flask in a wire safety container, particularly when large samples or large amounts of substances facilitating combustion are used. In the safety container the stopper need not be pressed in manually.

When using a flask with external ignition, the flask is filled with oxygen, the ground-glass joint is lubricated with water and the flask is covered with a wire safety container of a truncated cone-shape so that the neck of the flask extends from the container. The tip of the paper fuse is ignited in a flame and the stopper is inserted quickly into the neck of the flask. The stopper should be pressed in until combustion is complete.

Complete combustion of hydrogen halides and halogens by diffusion alone requires 1-2 h. This period can be reduced by shaking the flask, but it usually still requires 20min. The absorption solution should be perfectly clear; floating black particles or pieces of filter-paper indicate incomplete combustion.

When the gases have been absorbed, some water is added to the collar of the flask and the stopper is slowly removed. The slight vacuum may give rise to some resistance. The water in the collar will rinse down the ground-glass joint during the removal of the stopper. The bot tom of the stopper and the fittings attached to it are washed 2-3 times with a few millilitres of water and the stopper is removed completely and put aside. The volume of the wash water should not be more than 15-20 cm 3 .

Determination of halide ions in the absorption solution. Almost all of the methods suitable for the micro determination of halide ions have been suggested for this purpose, including several electrometric techniques. In the author 's experience, water containing a few drops of ammonia solution can also be used as an absorption liquid for chlorine compounds, when prolonged shaking is applied to ensure complete absorption.

The mercury (I )oxycyanide titration is described for the determination of chloride ions, the Viebock-Kolthoff amplification titration is suggested for the determination of bromide ions, and the Leipert amplification titration procedure is described for the determination of iodide ions.

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1. Determination of chloride ions [71]

Reagents 2 N sulphuric acid Standard 0.01 N sulphuric acid 2 N potassium hydroxide solution, carbonate-free Methyl Red-Methylene Blue mixed indicator solution Cold, saturated mercury(II)oxycyanide solution. (For the preparation of

the latter two solutions, see page 348) Absorption solution: 10 c m 3 of water, 2 c m 3 of 2 N potassium hydroxide

solution and 3 drops of concentrated hydrogen peroxide

Procedure. The absorption solution is boiled for 2-3 min to decompose the excess of hydrogen peroxide, then 3 c m 3 of 2 N sulphuric acid solution are added, and the solution is boiled again for 1-2 min and cooled. Two drops of indicator solution are added to the contents of the flask, followed by dropwise addition of 2 N potassium hydroxide solution until the colour changes from violet-red to green. The walls of the flask are rinsed with a few millilitres of water and the solution is neutralized with 0.01 N sulphuric acid, added dropwise to achieve the neutral grey colour of the indicator. This step of the procedure should be carried out with particular care, as it is the crucial point of the determination. Then 10 c m 3 of neutral mercury(II) oxycyanide solution are added, and the solution becomes green. The mixture is then titrated with 0.01 N sulphuric acid until the colour disappears. A 1 c m 3 volume of 0.01 N sulphuric acid is equivalent to 0.3546 mg of chlorine.

2. Determination of bromide ions iodimetrically by the Kolthoff amplification method [135]

Reagents 20% sodium dihydrogen orthophosphate solution 30% sodium chloride solution 1 N sodium hypochlorite solution (about 3.7%) 50% sodium formate solution 6 N sulphuric acid solution Standard 0.02 N sodium thiosulphate solution Potassium iodide, crystalline Starch indicator solution Absorption solution: a mixture of 5 c m 3 of 20% sodium dihydrogen

orthophosphate solution, 20 c m 3 of 30% sodium chloride solution and 10 c m 3 of sodium hypochlorite solution.

Procedure. After combustion and absorption, the contents of the flask are brought to boiling, 5 c m 3 of 50% sodium formate solution are added and the

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solution is brought to boiling again. Any chlorine liberated is blown out and the solution is cooled, acidified with 20 c m 3 of 6 N sulphuric acid and 0.2 g of potassium iodide is added. The solution is diluted to 100 c m 3 and the iodine liberated is titrated with 0.02 N sodium thiosulphate solution in the presence of starch as indicator.

A 1 c m 3 volume of 0.02 N sodium thiosulphate solution is equivalent to 0.2664 mg of bromine.

As relatively large amounts of the reagents are used, blank tests should be carried out and the measured values corrected.

3. Simultaneous determination of chloride and bromide ions [136]. As mentioned previously, argentimetric micro determination of halide ions can be accomplished in the presence of redox indicators (e.g., Variamine Blue 6B) [137]. This method is especially favourable when solutions of low ionic concentration are treated, such as those obtained by oxygen flask combustion [75]. In order to absorb the combustion products completely, 1% ammonia solution is used as absorption liquid; the excess of ammonia can be removed by boiling after absorption [59].

About 20 mg samples of the compound containing both chlorine and bromine are burnt and the absorption solution is made up to volume in a 2 5 c m 3 volumetric flask; this is the stock solution.

The total amount of chloride and bromide ions is titrated argentimetrically in a 10 ml aliquot of the stock solution; in another 10 ml aliquot the bromide ions are determined by a modification of the Kolthoff method suggested by Belcher et al. [138].

Reagents 0.02 N silver nitrate solution 0.02 N standard sodium thiosulphate solution Acetic acid-sodium acetate buffer solution, pH 3.6 1 N sodium hypochlorite solution (about 3.7%) Variamine Blue acetate indicator solution, 0.2% Sodium dihydrogen orthophosphate solution, 20% Sodium formate solution, 50% 12 N sulphuric acid solution (about 60%) Ammonium molybdate solution, 3% Potassium iodide, crystalline Starch indicator solution

Procedure. A 10 c m 3 volume of 1% ammonia solution is used as absorbent in the flask. After combustion and absorption, the solution is boiled until the smell of ammonia can no longer be detected. The contents of the flask are cooled and transferred into a 25 c m 3 volumetric flask and made up to volume.

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In order to determine the total amount of chloride and bromide ions, exactly 10 c m 3 of the stock solution are pipetted into the titration flask and 5 c m 3 of acetic acid-sodium acetate buffer solution and 3 drops of Variamine Blue solution are added. The solution is titrated with standard 0.02 N silver nitrate solution while stirring vigorously, until the solution acquires a persistent pale violet colour.

To determine the amount of bromide ions, exactly 10 c m 3 of the stock solution are pipetted into a ground-glass stoppered 200 c m 3 conical flask and 3 c m 3 of sodium hypochlorite solution and 2 c m 3 of sodium hydrogen orthophosphate solution are added. The flask is kept in a water-bath at 9 4 + 1°C for 15 min, then 2 c m 3 of 50% sodium formate solution are added and the flask is allowed to cool. The walls of the flask are washed with a few c m 3 of water, the solution is acidified with 2 c m 3 of sulphuric acid, 0.5 c m 3 of ammonium molybdate solution is added and the flask is stoppered. The contents of the flask are mixed by careful swirling. The stopper is then removed and 0.5 g of potassium iodide is rapidly added. The stopper is inserted into the flask immediately and the solution is allowed to stand for 5 min. The iodine liberated in the process is titrated with standard 0.02 N sodium thiosulphate solution first to pale yellow colour, then about 2 c m 3 of the starch indicator solution are added and the titration is continued to the disappearance of the blue colour.

It is advisable to carry out blank determinations and to correct the results; the correction is usually not more than 0.2-0.3 cm 3 .

A 1 c m 3 volume of 0.02 N silver nitrate solution is equivalent to 0.7092 mg of chlorine or 1.5984 mg of bromine. A 1 c m 3 volume of 0.02 N sodium thiosulphate solution is equivalent to 0.2664 mg of bromine.

4. Determination of iodide ions [71] . Iodine vapour is dissolved as hypoiodite ions in an alkaline absorption solution. Hypoiodite ions are oxidized to iodate ions by bromine and the six-fold amount of iodine liberated on addition of iodide ions is titrated with standard sodium thiosulphate solution.

Reagents 2 N potassium hydroxide solution Bromine (in glacial acetic acid), 10 g of potassium acetate dissolved in

100 c m 3 of glacial acetic acid to which 0.5 c m 3 of bromine is added Concentrated formic acid 2 N sulphuric acid Standard 0.02 N sodium thiosulphate solution Potassium iodide, crystalline Starch indicator solution

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Procedure. A 1 0 c m 3 volume of the water and 1 c m 3 of 2 N potassium hydroxide are used in the flask for absorption.

After combustion and absorption, 10 c m 3 of bromine (in glacial acetic acid) are added to the mixture in the flask, which is allowed to stand for 1-2 min while stirring, then diluted to about 50 cm 3 and 2-3 drops of formic acid are added. The mixture is allowed to stand until the yellow colour of bromine disappears. If this does not occur within 2-3 min, 2-3 g of potassium or sodium acetate must be added. The bromine vapour is removed by blowing it from the flask and the solution is acidified with 10 c m 3 of 2 N sulphuric acid. About 0.2 g of potassium iodide is added and the liberated iodine is titrated with 0.02 N sodium thiosulphate first to a pale yellow colour. Then about 2 c m 3 of starch solution are added and the titration is continued to the disappearance of the blue colour.

A 1 c m 3 volume of 0.02 N sodium thiosulphate solution is equivalent to 0.4231 mg of iodine.

(D) ANALYSIS O F ORGANIC F L U O R I N E C O M P O U N D S

Problems that occur in the analysis of organic fluorine compounds are discussed in a separate chapter of Volume IB of "Comprehensive Analytical Chemistry" (pp. 551-561) and here only recent advances and newly published procedures are presented.

The analytical properties of fluoride ion are so different from those of chloride, bromide and iodide ions that different methods must be employed for their quantitative determination. Further, the liberation of fluorine atoms from covalent bonds and their conversion into fluoride ions, that is, mineralization of samples, requires more vigorous conditions. This is due primarily to the considerable energy of the carbon-fluorine bond (434.9 kJ/mole). Of course, the carbon-fluorine bond can also be weakened by electron-attractive substituents, and there are sulphur-fluorine, phosphorus-fluorine and silicon-fluorine compounds with much weaker bonds, but these occur much more rarely.

Of the decomposition methods suitable for organic fluorine compounds, alkaline hydrolysis methods can be used for compounds with weakened carbon-fluorine bonds. Thus, for example, the organic phosphorofluorides and phosphonium fluorides were decomposed with sodium ethoxide by Sass et al. [139]. Fluoroacetates can be treated with metallic sodium dissolved in isoamyl alcohol [140]. Several organic fluorine compounds can be reduced and decomposed with sodium dissolved in liquid ammonia [141, 142]. Sodium biphenyl has also proved to be suitable for the reduction of organic

363

fluorine compounds in ethylene glycol-diethyl ether or diisopropyl ether solutions [143, 144].

However, the only procedures that are applicable in general to all kinds of fluorine compounds are those which employ powerful oxidizing or reducing agents at relatively high temperatures (~1000 C).

Complete destruction of some involatile, solid organic fluoro compounds can be achieved in a metal bomb (Parr bomb) as described for chloro compounds, particularly when additives that facilitate combustion (sugar, potasssium chlorate and sugar, starch, etc.) and elevated temperatures are employed. However, organic fluoro compounds that are volatile at the temperature of the metal bomb, which thus enters the atmosphere of the bomb and escapes the action of the oxidizing agent, cannot be decomposed in this way without loss.

These compounds can be decomposed in a metal bomb only with alkali metals at temperatures high enough to ensure the appearance of alkali metal vapour in the atmosphere of the bomb. The temperature of decompositon should be at least 600°C and takes 1-2 h, while at 800°C the reaction period is only 10 min. The capacity of the bomb is about 2.5 cm 3 , and it is made from nickel or stainless steel, with a copper sealing ring [145, 146].

Complete decomposition of organic fluorine compounds in a quartz combustion tube in a stream of oxygen requires temperatures of 900-1250°C [147, 25]. According to the experience of the author, the pyrohydrolysis procedure with water vapour in a nitrogen atmosphere is suitable for the decomposition of almost all fluorine compounds at about 1000°C. A detailed description of the procedure is given on pp. 332. The method has proved particularly advantageous in the analysis of volatile and gaseous fluorine compounds.

Wickbold modified his method developed for the analysis of chlorine compounds to make it suitable for the analysis of fluorine compounds [148]. In this instance, the auxiliary flame was fed with hydrogen instead of town gas (this is reflected in the name of the technique, the oxy-hydrogen flame method). The high temperature of the flame (minimum 2000°C) is sufficient to ensure combustion of any organic fluorine compounds. An advantage of the method is that the amount of the condensate is small, and it is therefore not necessary to concentrate the solution, e.g., before precipitation of the mixed halide lead chloride fluoride. The apparatus can be equipped with a suitable nebulizer to allow liquids to be analyzed. Sweetser [149] suggested absorption of hydrogen fluoride in sodium hydroxide solution and titrimetric determination of fluoride. The Wickbold semimicro method was adapted to the micro-scale by Lewy and Debel [150] by using a mixture of oxygen and

364

hydrogen in the flame. A similar procedure was developed by Ehrenberger [25], which differs from the former methods in the combustion process: the hydrogen and oxygen streams are not mixed before burning, but are led through nozzles placed opposite each other into a quartz ball, where they are burned. The pyrolysis products of the sample are carried into the flame in the hydrogen gas stream. In this way, a higher temperature can be achieved and combustion is faster; a mixture of 6 0 0 c m 3 of oxygen and 1200cm 3 of hydrogen can be burned per minute. Ehrenberger titrated the solution collected in the quartz absorber with thorium nitrate solution.

The application of the oxygen flask method to the analysis of organic fluorine compounds was first suggested by Schoniger [37]. In this procedure, the hydrogen fluoride formed was absorbed in water and titrated with 0.01 N cerium(IV) solution in the presence of murexide. Analytical data only for some solid organic fluorine compounds were given [37]. Since that time, several papers [151-154] have been published dealing with combustion flasks of various construction designed for the decomposition of different organic fluorine compounds. Several workers have investigated the problem of the complete combustion of such compounds, which, in certain instances, can be ensured only by the addition of substances that facilitate combustion. However, Ferrari et al. [155] reported the combustion of compounds containing even the trifluoromethyl group, without the use of additives. Others suggested the addition of sodium peroxide or potassium chlorate to the sample.

Olson and Shaw [156] observed the adsorption of fluorine on the walls of the glass flask used for combustion, and suggested the use of a quartz flask washed with hydrofluoric acid and then with water before the combustion procedure. Since that time, the use of quartz flasks has become widely accepted. When a glass flask is used, it must first be washed several times with dilute hydrogen fluoride solution and then with water. The combustion products should be absorbed in water and the use of alkali solutions should be avoided [157].

Organic fluorine compounds were burned in a 300-cm 3 horizontal quartz tube in the micro apparatus designed by Francis et al. [158] for the combustion of 5-10 mg-samples; this apparatus is similar to that developed by Kirsten [159] for ultramicro work. The sample was moistened with dodecanol, which facilitated the conversion of organically bound fluorine into hydrogen fluoride, owing to its high hydrogen content. After completion of the combustion, the apparatus was kept at 1000°C for several minutes to ensure complete reaction. Thus, no residual carbon tetrafluoride was observed even after the combustion of PTFE. The method is described in detail later in this chapter.

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Later, the use of polypropylene flasks was suggested because hydrogen fluoride can be recovered completely from them after combustion [160,161].

The absorption solutions first used in the oxygen flask method were sodium hydroxide, ammonia or buffer solutions. More recent observations indicate that hydrogen fluoride can be completely absorbed in pure water when shaken for 20-30 min.

In the absorption solution, the fluoride ions are usually titrated with titrants containing thorium or lanthanum ions and fluoride-selective electrodes have been increasingly employed [162, 163].

In 1970, the International Union of Pure and Applied Chemistry, Division of Analytical Chemistry, Commission on Microchemical Techniques and Trace Analysis started a thorough international cooperative research project, headed by Macdonald, to decide the most suitable micro methods for the routine determination of the fluorine content of organic fluorine compounds. It has been suggested that the combustion of samples containing various elements, including those with interfering actions, should be effected in quartz or polypropylene flasks filled with oxygen. Thorium nitrate and lanthanum nitrate solutions have been suggested as titrants for fluoride ions in the absorption solution. Potentiometric titration, using fluoride-selective electrodes, or the use of haematoxylin as indicator, has also been recommended. Titration with thorium nitrate in the presence of methyl thymol blue as indicator and the spectrophotometric method using lanthanum-alizarin fluorine blue can also be utilized [157].

As mentioned earlier, the analytical properties of the fluoride ion are peculiar and differ from those of chloride, iodide and bromide ions. Although fluoride ions can form hardly soluble precipitates with certain metal ions, e.g.:

Barium fluoride, B a F 2 L = 1.7 x 10 " 6

Lead fluoride, P b F 2 L = 3.2 x 10 8

Magnesium fluoride, M g F 2 L = 7.1 x 1 0 " 9

Strontium fluoride, S r F 2 L = 2.8 x 10 " 9

Calcium fluoride, C a F 2 L = 3.3 x 1 0 " 1 1

the precipitates appear in a colloidal state, with a strong tendency for adsorption, and calcium fluoride can be filtered only when precipitated together with calcium carbonate. Only the mixed halide of lead, that is, lead chloride fluoride (PbCIF, L = 2.8 x 10~ 9 ) has proved to be an appropriate compound for use in gravimetric analysis, owing to its good filtration properties and the favourable conversion factor with respect to fluorine (F/PbCIF = 0.0726). The lead or chloride ion content of the precipitate can also be titrated.

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In the semimicro or micro determination of fluoride ions, titration procedures are most frequently employed. When in the decomposition procedure only hydrogen fluoride or hexafluorosilicic acid is formed in the absorption liquid, it can be titrated acidimetrically in the presence of, e.g., phenolphthalein indicator, with 0.01 N sodium hydroxide solution. However, the end-point is not very sharp and back-titration may prove to be more advantageous, when hydrogen fluoride or hexafluorosilicic acid are absorbed in standard alkali solutions.

In the determination of fluoride ions, precipitation titration procedures are much superior to acidimetric methods.

Fluoride ions form stable complexes with some highly charged metal ions:

U 6 + F e 3 + A l 3 + T h 4 + Zr 4 +

l o g * , 4.48 5.25 6.13 8.72 9.82 \ogK2 3.32 3.95 5.02 5.74 7.23 log /C 3

2.63 2.70 3.85 4.46 5.76

It can be seen that the stability of the fluoride complexes of zirconium and thorium are particularly high, and this property can be utilized in the titrimetric and spectrophotometric determination of fluoride ions. In practice, thorium nitrate standard solutions are frequently employed. Thorium and fluoride ions form a series of complex compounds in a slightly acidic medium, of which the hexafluoro complex anion:

T h 4 + + 6 F - = [ ( T h F 6 ) ] 2 -

is the most stable. The compound, which initially appears as a colloid, separates as a geletinous precipitate on standing. In the titration procedure, first sodium alizarin sulphonate was used as indicator; its aqueous solution is yellow and a red complex is formed with thorium ions. In the course of the titration, thorium ions combine with fluoride ions and the solution remains yellow, owing to the presence of free alizarinate ions. When all fluoride ions have been consumed, that is, at the equivalence point, on further addition of thorium ions the solution turns red because of the formation of a thorium alizarinate complex. Alizarin sulphonic acid also acts as an acid-base indicator, being yellow and violet in acidic and basic solutions, respectively. Thus the colour appearing at the equivalence point also depends on the pH of the solution. The colour change at the end-point can be made sharper by the use of a neutral dye; when methylene blue is used, the colour changes from greyish blue to clear blue.

The reaction between thorium and fluoride ions is stoichiometric only when at least 10 mg of fluoride ions are present in the solution at pH 3 and a

367

not too dilute (0.05-0.1 N ) thorium nitrate titrant solution is used. When more dilute standard solutions are used, a calibration graph is necessary or the concentration of the standard solution must be checked.

A disadvantage of the method is the insufficient sharpness of the end-point. This can be attributed to the adsorption of the indicator by the thorium fluoride precipitate so that the colour change takes place only slowly. This problem can be eliminated by the addition of a protective colloid (e.g., starch) to the solution, which prevents the coagulation of the precipitate. When the titration is effected in a spectrophotometer cell, the end-point is sharper.

Several workers have investigated the optimal pH of the titration, which was found to be 3.0-3.3. A glycine-perchloric acid-sodium perchlorate buffer solution was the most suitable for this purpose [164].

The other significant disadvantage of the procedure is its sensitivity towards interfering ions. A sharp end-point can be achieved only in the absence of other ions. The effect of interfering ions was dealt with by Reynolds and Hill [165], while Belcher et al. [166] stated that the interfering action of other ions can be completely eliminated only by the use of reference solutions containing the same interfering ions in amounts identical with those present in the sample solution.

Willard and Horton [167] started the search for other indicators, but none that is better than sodium alizarin sulphonate has been found. Of indicators suggested later, methyl Thymol Blue [168] and haematoxylin [169] are useful in practical applications.

The IUPAC survey [157] also dealt with methods for the determination of fluoride ions. It was stated that the electrometric, titrimetric or spectrophotometric techniques examined seemed to be equivalent in usefulness, when the amount of fluoride ions to be determined was maintained in the relatively narrow concentration ranges found to be most favourable for the individual procedures. Potentiometry was not superior to visual titration methods in either accuracy or reliability. Direct potentiometric measurement with a fluoride-selective electrode was advantageous, but the number of laboratories that have expressed an opinion is not yet sufficiently high.

In the last 20 years or so, several other procedures have been suggested for the determination of fluorine in organic compounds.

Awad et al. [170] published an amplification method based on the following

reaction: ,

C a ( I 0 3 ) 2 + 2 F~ = 2 I O 3 + C a F 2

Calcium fluoride is almost insoluble in aqueous isopropanol solutions, and the procedure is stated to be suitable for the measurement of 0.4-8 mg of fluoride ions. Iodate ions can be titrated in the filtered solution by iodimetry.

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Hems et al. [171] reported a catalytic method which is based on the fact that methyl thymol blue reacts very slowly with zirconium ions in aqueous solutions, but this reaction is catalyzed by fluoride ions. Spectrophotometric measurement was applied, and the technique was suitable for the determination of 0.5-4.75 [ig of fluoride ions.

Much information is available in the literature on various titrimetric determinations of fluoride ions using electrometric end-point indication.

O'Donnell and Stewart [172] employed zero-point potentiometry, making use of the decreased potential of the cerium(IV)-cerium(III) redox system in the presence of fluoride ions, as fluoride ions react with cerium(IV) ions to form a complex. The method has also been employed in micro titrations. Johannesson [173] proposed amperometric end-point indication in the titration of fluoride ions with 0.1 N thorium nitrate solution, using a rotating aluminium electrode.

The coulometric method developed by Megregian [174] is based on the introduction of zirconium ions into the solution from a zirconium electrode, which combine with the fluoride ions. The amount of current consumed is pro-portional to the concentration of fluoride ions in the 2.5-20 mg /dm 3 range. In the coulometric titration suggested by Mather and Anson [175], mercury(II) ions produced at a mercury anode in an acetic anhydride-perchloric acid medium were made to react with fluoride ions. In both procedures, electrometric end-point detection techniques were applied. Szantho [176] described an oscillometric technique for the determination of fluoride ions in organic substances after oxygen flask combustion, 0.1 N lanthanum nitrate solution being used as titrant.

All polarographic methods described for the determination of fluoride ions are indirect techniques based on the liberation of polarographically active compounds from their aluminium or thorium complexes by fluoride ions. An indirect polarographic method was described by Gawargious et al. [177], in which the solution obtained on combustion in an oxygen flask was allowed to react with lead(II) nitrate or calcium iodate and the excess of lead or iodate ions was measured in the form of a cathodic wave.

Fluoride ions can be determined very precisely, even on the micro-scale, by means of fluoride-selective electrodes. In these electrodes, a crystal of lanthanum fluoride serves as the detector; one side of this crystal is brought into contact with a suitable supporting electrode and the other side with the fluoride solution to be measured. Its operation is similar to that of a glass electrode: its potential is a function of the fluoride ion activity in the solution. Using a suitable reference electrode (e.g., a calomel electrode) the fluoride ion activity and thus the concentration can be measured directly in the solution, provided that a calibration graph constructed on the basis of a series of

369

solutions with known fluoride ion concentration is available. In this way, fluoride ion concentrations in the 10~ 5 mole/dm 3 range (0.19 mg/dm 3 ) can be determined. The electrode can also be used in potentiometric titrations with, for example, thorium nitrate. The method using the fluoride-selective electrode is superior to all other methods, because most other ions do not interfere. In the titration with thorium nitrate, the only ions that interfere are those which form complexes more stable than those of fluoride ions with thorium ions.

Light and Mannion [178] used an Orion Model 94-05 electrode for potentiometric titration of fluoride ions with 0.005 mole /dm 3 thorium nitrate in 80% ethanol. The organic material was burnt in a polycarbonate flask (Nalgene No. 4103), and dodecanol was used to facilitate combustion. The accuracy of the results was ± 0 . 3 % (absolute) when 1-10mg samples were combusted. Phosphate and sulphate ions interfere and should be removed before measurement.

Francis et al. [179] also used fluoride selective electrodes for the determination of fluoride ions; the electrode was a lanthanum single crystal doped with europium and the electrolytes were 0.1 mole /dm 3 potassium chloride and 0.1 mole /dm 3 sodium fluoride solutions with a silver-silver chloride electrode immersed in them. A calomel reference electrode was used in the titration. Combustion of the organic matter was effected in a horizontal vessel filled with oxygen; dodecanol was used to promote combustion. The procedure is given in detail later.

Baumann [180] investigated the sensitivity of the Orion Model 94-04 electrode and the effect of ions forming complexes with fluoride ions. Selig [181] decomposed phosphorus-containing fluorine compounds by the oxygen flask combustion method; the phosphate ions formed were bound by addition of zinc oxide and the titration was carried out in the presence of a fluoride-selective electrode with 0.02 N lanthanum nitrate solution. Anfalt and Jagner [182] used a lanthanum fluoride electrode and lanthanum nitrate solution as titrant. They stated that buffer solutions prepared from organic acids interfere with the precipitation reaction and contaminate the electrode. Turner [183] suggested the use of a single-crystal silicon electrode for the titrimetric determination of fluoride ions. Selig [184] reported on the determination of fluoride and phosphate or sulphate ions in the presence of each other. Poisier [185] separated the constituents from phosphorus-containing solution of fluoride ions on Dowex 2-X10 ion-exchange resin.

370

(1) Colorimetric and spectrophotometric methods

Colorimetric methods have long been used for the determination of small amounts of fluoride ions in drinking water, foods and biological materials. They are based on the decreased colour intensity of certain metal complexes on addition of fluoride ions, when the stability of the metal-fluoride complex is greater than that of the coloured metal complex, for example:

[ F e ( S C N ) 6 ]3 - + 6 F" = [ F e F 6 ] 3 " + 6 SCN "

#Ct = 1.96 K , = 5 . 2 1

The fluoride ion concentration can be determined by measuring the decrease in the colour intensity, using a calibration graph constructed under identical conditions, covering a relatively narrow concentration range.

Earlier methods based on the decoloration of iron(III) thiocyanate, iron(III) salicylate, iron(III) sulphosalicylate and several other coloured metal complexes are not sufficiently sensitive and can be used only in a very limited concentration range. Some more recent reagents have proved to be more suitable and of higher sensitivity, e.g., complexes of zirconium with eriochrome cyanine [186] and alizarin sulphonic acid [187], aluminium with eriochrome cyanine [188] and thorium with alizarin sulphonic acid [189]. Metal complexes of SPADNS [sodium-3-(sulphophenylazo)-4,5-dihydroxy-naphthalene-2,7-disulphonate] have been examined [190] and the advantages of, for example, the zirconium complex over other zirconium-organic complexes have been established. Investigations have also been carried out on the complexes of xylenol orange [3,3'-bis-N,N-di(carboxy-methyl)aminomethyl-o-cresolsulphophthalein] formed with zirconium ions [191]. Tan Lay Har and West [192] suggested the use of the zirconium-calcein blue complex for the determination of 1 0 " 5 mole /dm 3 fluoride ions. Dixon [193] recommended the Solochrome Cyanine R complex of zirconium for the determination of up to 2.5 \ig of fluoride ions.

All of the above reagents provide only an indirect determination of fluoride ions, that is, the intensity of the colour of the solution is inversely proportional to the concentration of fluoride ions. Apart from other problems, the main disadvantage of this technique is that the sensitivity is lowest at small concentrations of fluoride ions. Some improvement can be achieved by applying differential spectrophotometry, but the difficulty cannot be eliminated completely.

The first reagents giving an absorbance linearly proportional to the fluoride ion concentration were chloranilic acid and its metal complexes. Chloranilic acid (2,5-dichloro-3,6-dihydroxy-l,4-benzoquinone) is violet-red in aqueous solutions and forms complexes with alkaline earth and other

26 371

metal ions (e.g., La, Th, Zr) that are only poorly soluble in water. The reagent was first utilized in the determination of various metal ions, such as calcium ions, by measuring the decreased absorbance caused by them. The complex reagent was used first for the determination of anions by shaking the solution to be examined (containing, e.g., sulphate ions) with finely powdered strontium chloranilate and filtering. The colour of the solution, which was initially pale pink, became more intense, owing to the liberation of sulphanilic acid, accompanied by the formation of strontium sulphate. Later, the reagent was employed in the determination of fluoride ions, chloranilic acid was liberated from strontium chloranilate by fluoride ions in 50% isopropanol.

Hensley and Barney [194] found an increase in the intensity of the colour of chloranilic acid and, through this, achieved an increase in the sensitivity of fluoride determination on the addition of methyl Cellosolve to the solution. However, detailed investigation of the method showed that the increased colour intensity was due to the presence of iron(III) ions in methyl Cellosolve, which produce a vivid red, water-soluble complex with chloranilic acid [195].

Belcher et al. [196] discovered alizarin complexone, a product of alizarin(I) and methyl-N,N-diacetic acid(II) which acts as a compleximetric indicator. It gives a red complex with cerium(IV) (Ce 4 + ) ions (III).

Leonard and West [197] observed the formation of a blue solution when equimolar amounts of this complex and fluoride ions are mixed at pH 4.3; the absorption maximum of this blue solution was at the same wavelength (567 nm) as that of the alizarin complexone (II) at pH 12.4. At the same time, the absorbance maxima of cerium alizarin complexone (II) at pH 4.3 are at much lower wavelengths (490 and 430 nm). Leonard and West studied the characteristics of the new blue compound (IV) and stated that it is composed of cerium alizarin complexone and fluoride ions in a 1:1 molar ratio. An increase in the molar ratio of fluoride ions neither increased nor decreased the colour intensity. A large excess of fluoride ions decomposed the ternary complex and the initial yellow colour of the reagent reappeared in the solution. On the basis of these investigations, it was concluded that the fluoride ion is incorporated in the cerium complex substituting one co-ordinated water molecule on the cerium ion [198]. The colour change is probably due to the electron shift following the rupture of a hydrogen bond in the alizarin molecule.

Belcher and West [199] carried out numerous studies with this complex and found that pH 5.2 was the most suitable for spectrophotometric measurement. They later described a submicro method suitable for the determination of 50 |ig of fluoride ions [200]. Lanthanum alizarin fluorine blue is an even better reagent than the cerium complex, as the concentrat ion-absorption relationship is linear up to 400 jig of fluoride ions, the reagent can

372

OH

SOJ O

OH

/ C H 2 C O O H

CH 2 —NH N CH 2 COOH

be prepared more easily and it is more stable [201]. The procedure is described in detail below. Belcher and West's method [199] was employed by Kirsten and Shah [202] in the determination of fluoride ions. The organic matter was decomposed in a stream of hydrogen at 950°C in the presence of an additive containing orthophosphoric acid and phosphate ions. Phosphorus and sulphur do not interfere, and the latter can also be determined in the same solution. Leonard and Murray [203] and Leonard [204] used the sulphonated derivatives of alizarin fluorine blue as a positive absorption reagent for fluoride ions.

OH OH

/ C H 2 - N

C H 2 C O O H

\ C H 2 C O O H

11 alizarin complexone

(alizarin-3-methylamine-N,N-diacetic acid)

O O H 2 0 OH 2

O -Ce

III IV

cerium complex of alizarin complexone alizarin complexone cerium fluoride ternary complex

As a large number of fluorine compounds exist in a gaseous state or are volatile, gas chromatography is suitable for their determination, although gas chromatographic detectors are not specific for fluorine. Janak [205], in a fundamental paper, described a method designed for the separation of Freon

3 6 * 373

alizarin

compounds, in addition to other gaseous mixtures. Percival [206] also suggested a method for the separation of Freons, with Celite as the stationary phase and dioctyl phthalate was used as the mobile phase. The column temperature was 56°C and hydrogen was used as the carrier gas. The results obtained were accurate to within ±0 .5%.

Ellis et al. [207] studied the gas chromatographic separation of corrosive halogen compounds, such as halogen fluorides and hydrogen fluoride mixtures. Ming-Ho-Yu and Miller [208] described a procedure for the determination of fluoroacetates and citrates in vegetable and animal tissues. The method involved the conversion of these compounds into their methyl esters and subsequent gas chromatographic determination using helium as the carrier gas and flame-ionization detectors.

The infrared spectra of numerous organic fluorine compounds are available. Ayscough [209] published a method suitable for the determination of small amounts of trifluoromethane, carbon tetrafluoride and hexafluoro-ethane. The intensity of the absorption bands appearing in the 1000-1500 c m " 1 range was proportional to the concentration of trifluoromethane and hexafluoroethane. Samples of 5-10| imole could be measured with a precision of ± 5 % . Raman spectra of certain fluoroalkanes and fluorobenzenes were reviewed by Nielsen [210].

Neutron-activation analysis has been utilized for the determination of fluorine. Anders [211] made use of the 1 9 F ( n , a ) - > 1 6 N reaction; the sensitivity was 0.1 mg of fluorine, and 100 ppm of fluorine could be measured in 1 g of sample. Hislop et al. [212] applied the 1 9 F (y , n ) - » 1 8 F reaction, the 1 8 F being distilled off and the activity of the condensate or of the calcium fluoride precipitated from it being measured. The procedure was suitable for the determination of as little as 2 ng of fluorine. Carmichael and Whitley [213] decribed a sub-stoichiometric solvent extraction method for the determination of 25-150 \ig of fluorine, in which fluorine was extracted with tetraphenylstibonium sulphate into chloroform. Sulphate and phosphate ions did not interfere, but halogens did. Kosten and Slunecko [214] determined fluorine in organic and inorganic materials irradiated at 18.7 MeV in a betatron by the photon activation technique.

Nuclear magnetic resonance spectrometry was applied first by Shoolery [215] in the analysis or fluorohydrocarbons. Gutsche et al. [216] described an atomic-absorption spectroscopic method for the analysis of fluorine-containing gas samples (e.g., from gas chromatographs). The gas was mixed with argon saturated with sodium vapour and the decrease in the emission of sodium was measured and related to the formation of sodium fluoride.

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(2) Detailed description of some methods suitable for the determination of fluorine in organic compounds

1. Determination of fluoride ions as lead chloride fluoride after fusion with metallic sodium or potassium. The only suitable method for the gravimetric determination of fluoride ions on the macro- or semimicro-scale is by precipitation as lead chloride fluoride. The advantage of this method is that few ions interfere and it can be carried out in the presence of large amounts of alkali metal ions.

The gravimetric determination of fluoride ions as lead chloride fluoride Was investigated in detail by Belcher and co-workers [217, 218] in the 1950s. It was stated that the method gave accurate and reliable results only when the fluoride ion content of the solution treated was at least 10 mg, and preferably 25-30 mg. When precipitation was carried out in very dilute acetic acid solution with lead chloride nitrate solution, precipitates of stoichiometric composition were obtained. Of the ions occurring after decomposition of organic fluorine compounds, cyanide ions and metal ions do not interfere, provided that their chlorides and fluorides are not stable complexes. Bromide and iodide ions, particularly when present in large amounts, interfere owing to co-precipitation of lead bromide and iodide, respectively. Anions that form sparingly soluble lead salts, such as sulphate, sulphide and phosphate ions, also interfere, but arsenic has no effect because lead arsenate is not precipitated under the conditions used. Belcher and co-workers suggested a procedure for the removal of interfering ions and for the precipitation of lead chloride fluoride.

As fusion with alkali metals is suitable for the decomposition of all organic fluorine compounds, except gases and very volatile substances, this method can be applied advantageously in routine semimicro work.

Reductive decomposition of organic fluorine compounds is usually effected in a 25-ml Parr bomb (Fig. 47) made from nickel as described by Belcher and Tatlow (214).

Most organic fluorine compounds can be decomposed with sodium; the use of potassium is necessary only for perfluoro compounds.

Reagents 5N nitric acid 30% acetic acid Ethanol Acetene Lead chloride nitrate prepared as follows: lead chloride (10.5 g) and lead

nitrate (13 g) are dissolved in hot water (1 dm 3 ) . If the solution becomes too

375

cold and lead chloride separates, heat may be applied or the clear solution is transferred into another dish.

Wash liquid: saturated aqueous solution of lead chloride fluoride

Decomposition procedure. The size of the sample weighed into the carefully cleaned cup of the dismantled bomb is chosen so as to contain 10-30 mg of fluorine. Liquids are weighed into a gelatin capsule and placed in the vessel. Clean sodium or potassium (300-500 mg) is added in the form of small slices. The copper sealing ring of the bomb is heated to redness, annealed, cooled by immersing it in ethanol and placed in position. A new sealing ring is necessary for each fusion. The bomb is closed and kept in a muffle furnace at 600-650°C for 60-75 min. Heating at higher temperatures is unnecessary and shortens the life of the bomb. The bomb is then allowed to cool in air.

After dismantling the bomb, the melt adhering to the lid is washed into a 250-cm 3 beaker with less than 10 c m 3 of water. The cup of the bomb is placed in the beaker and cautiously half filled with absolute ethanol, added dropwise. A few minutes later, after having destroyed the excess of sodium, the contents of the vessel are stirred with a thin nickel rod and some water is cautiously added dropwise. If potassium was used, water is not added to the contents of the bomb. After complete destruction of the alkali metal, the bomb is removed from the beaker and the contents are washed into the beaker with not more

376

Fig. 47. The Belcher micro metal bomb for decomposition of organic fluorine compounds with alkali metals

/ -Nickel cup; 2 nickel l id ; j steel sealing rings; 4 brass gaskets

than 20 c m 3 of water. The alkaline solution is filtered through a large-pore glass filter in order to remove the floating carbonaceous particles and the filter is washed with water. The filtrate is made neutral with 5 N nitric acid in the presence of methyl red. The volume of the solution should not exceed 80 c m 3

and smaller volumes are to be preferred. The solution is brought to boiling and acidified with 1 c m 3 of 30% acetic acid.

Precipitation oflead chloride fluoride. Lead chloride nitrate solution (50 cm 3 ) is added to the hot solution with continuous stirring. The contents of the beaker are brought to boiling and allowed to stand overnight.

Next day, the solution is filtered by decantation and the precipitate is transferred on to a weighed semi-micro G4 sintered glass crucible. The precipitate is washed twice with 10 c m 3 portions of wash liquid and twice with 10 c m 3 portions of acetone. The substance is dried at 100°C for 30 min, allowed to stand for 30 min to cool, then weighed.

The amount of fluorine can be calculated by multiplying the weight of the precipitate by 0.07263. The procedure is also described, for semimicro- and micro-scale on pages 551-555 of Volume IB of Comprehensive Analytical Chemistry.

2. Micro titration of fluoride ions with thorium nitrate solution (methyl thymol blue indicator). This titration can be applied successfully when the concentration of foreign ions in the solution is small. Thus, it is applicable to solutions obtained by oxygen flask combustion, by combustion in an oxy-hydrogen flame or by pyrohydrolysis. Solutions obtained from sodium peroxide fusion should first be rendered free from sodium ions by ion exchange or distillation.

The micro titration of fluoride ions is nowadays achieved almost exclusively with thorium or lanthanum ions. Thorium ions form very stable complexes with fluoride ions, [ T h F 6 ] 2 _ , and the change in ion concentration at the equivalence point is therefore large. The appearance of thorium ions after the equivalence point had been reached was successfully indicated in the solution with sodium alizarin sulphonate. The colour change from yellow (alizarin sulphonate ions) to violet-red (thorium alizarin sulphonate complex) is, in principle, very sharp. In practice, hower, it involves some errors, which were discussed at the beginning of this chapter.

In 1968, Selig [168] suggested the use of methyl thymol blue as indicator which was applied previously for the titration of thorium ions with EDTA at pH 3. The colour of the indicator changes sharply from blue to yellow. When fluoride ions are titrated with thorium nitrate, the colour change is reversed, but remains sharp, and is far superior to that of sodium alizarin sulphonate.

Up to 10 mg of fluoride ions can be titrated in solutions, preferably with a volume smaller than 15 cm 3 .

377

Reagents . Thorium nitrate solution, 0.02 M. Preparat ion: thorium nitrate (11.044 g of T h [ N 0 3 ] 4 . 4 H 2 0 ) is dissolved in 0.001 N nitric acid and made up to 1 d m 3 with water.

Buffer solution, pH 3.35. Preparat ion: glycine (6.7 g) and sodium perchlorate (11 g) are dissolved in water, 1 N perchloric acid (11 c m 3 ) is added and the volume is made up to 100 c m 3 with water.

Methyl Thymol Blue solution, 0.2% [sodium salt of 3,3'-bis-N,N-di(carboxymethyl)aminomethylthymol-sulphophthalein] aqueous solution or 1 + 9 9 solid mixture with potassium nitrate.

4. Sodium fluoride solution, 0.04 mole /dm 3 .

Titration. The p H of the solution to be titrated is adjusted to 3.35+0.1 with IN perchloric acid, then 2 c m 3 of the buffer solution are added. Three drops of indicator solution are added to give a pale yellow solution, which is titrated with thorium nitrate solution to a vivid blue colour. The titrant is standardized with 0.04 mole /dm 3 sodium fluoride solution under identical conditions.

3. Determination of fluorine content by potentiometric titration after combustion in an oxygen atmosphere. In 1969, Francis et al. [179] suggested a method for the determination of the fluorine content of organic materials, which is, with respect to the relatively simple and efficient accomplishment of the combustion and titration steps, one of the most suitable methods for the micro determination of the fluorine content of solid, involatile organic fluorine compounds.

The sample is combusted in a horizontal quartz tube (Fig. 48) filled with oxygen. The combustion tube is a quartz tube, about 20 cm long and of 2.5 cm i.d. (capacity about 300 c m 3 ) which becomes wider two thirds along its length and is equipped with a ca. 10 c m 3 side-vessel to hold the absorption liquid.

Fig. 48. The Kirsten apparatus for the determination of the halogen content of organic compounds

/ — Q u a r t z combus t i on t u b e ; 2—oven at 1000 C ; 5—heat insulat ing packings ; 4—quartz r o d ; 5—glass gr indings

378

The end of the tube has a ground-glass joint, into which a quartz stopper can be inserted. A quartz rod or tube is attached to the stopper and a coil made from 1mm thick platinum wire is fixed to its end. The coil almost reaches the end of the quartz tube when the stopper is inserted in the tube. The inner diameter of the coil is large enough to surround the usual micro-sized platinum boat. The tube is mounted on a stand which permits horizontal displacement of the apparatus into the combustion chamber of a tube furnace, 20 cm long and of 3 cm i.d.

Combustion of the sample. The tube furnace is pre-heated to about 1000°C. A suitable amount of the sample is weighed into the platinum boat, so as to contain about 2 mg (not more than 10 mg) of fluorine. The sample is moistened with a small drop of dodecanol in the boat (to aid combustion). According to Francis et al. [176], when this substance, with a high hydrogen content is used, all kinds of organic fluorine compounds can be combusted completely, without any formation of carbon tetrafluoride.

In the side-vessel of the combustion tube, are placed 5 c m 3 of 2% sodium hydroxide solution ( = 2.5 mequiv. of N a O H ) , the tube is moved into the furnace so as to avoid too strong heating of the alkaline solution and the tube is flushed with a rapid stream of oxygen for 2 min through the quartz tube reaching to the bot tom of the tube. When this operation is completed, the stopper of the tube carrying the sample at its end is quickly inserted into the hot tube and the ground-glass joint is secured with a spring. The sample is ignited immediately and burns rapidly. The tube is left in the furnace for a further 2 min, then it is removed and allowed to cool. The quartz tube dismounted from the stand is raised to a vertical position and the alkaline solution in it is shaken for a few minutes. The stopper and the rod are then removed and the boat and the platinum coil are rinsed carefully with water into the quartz tube. The contents of the quartz tube are washed with water into a 100 c m 3 beaker and rinsed thoroughly. The volume of the liquid in the beaker should not exceed 30-35 c m 3 . Hydrochloric acid is added to the alkaline solution until the pH is 4, then the solution is boiled for 5 min to expel carbon dioxide and cooled to 25°C.

A polyethylene-coated magnetic stirrer is placed in the solution, together with the electrodes (lanthanum fluoride single-crystal indicator electrode and calomel reference electrode) coupled to a pH meter with a wide measuring range (100-250 mV) or to a recording pH meter.

Reagents

Lanthanum nitrate solution, 0.01 mole /dm 3 . Preparat ion: lanthanum nitrate [ L a ( N 0 3 ) 3 . 6 H 2 0 ] 4.33 g in 1 d m 3 of water

379

Sodium fluoride standard solution, 0.005 mole /dm 3 prepared by dissolving 210 mg of analytical grade, dry sodium fluoride in 1 d m 3 of water. The solution is stored in a polyethylene flask.

Titration procedure. The sample solution is titrated with standard lanthanum nitrate solution by adding the titrant in 0.5 c m 3 increments with continuous, rapid stirring. A waiting period of 1 min is necessary before each potential reading. A titration curve is constructed from the potential-volume data.

The standard solution (20 c m 3 = 2 mg of fluorine) of fluoride ions is transferred into a similar beaker, the pH is adjusted to 4 and the solution is diluted with water so as to make its volume nearly equal to that of the sample solution. The temperature is adjusted to 25°C and the same rate of stirring as before is applied. Standard lanthanum nitrate solution is added in 0.5 a p -portions and a titration curve is constructed from the potential-volume data. In the titration, 3-3.5 c m 3 of titrant are consumed by a solution containing 2 mg of fluoride ions.

The two titration curves are correlated and the fluorine content of the sample solution and therefore of the organic substance is calculated. If an electrode of suitable sensitivity is used, a potential change of 50-100 mV can be observed at the equivalence point.

The method is also suitable for the titration of fluoride solutions obtained by other combustion procedures.

4. Spectrophotometric methods using lanthanum alizarin fluorine blue reagent. The ternary lanthanum-alizarin fluorine blue complex is, at present, the most suitable reagent for the spectrophotometric determination of fluoride ions, on both the micro- and ultramicro-scales, as the absorbance measured is linearly proportional to the amount of fluoride ions and as little as 0.6 | ig/cm 3 of fluoride can be determined.

The method given is based on the work of Fernandopulle and Macdonald [198], who investigated the problem very thoroughly.

Reagents Reagent solution: alizarin fluorine blue, 5 x l 0 ~ 3 mole /dm 3 (Alizarin

Complexan, Hopkin and Williams Ltd., England). Preparat ion: 1.9264 g of the reagent is dissolved in 750 c m 3 of water. Sodium hydroxide solution (2 N ) is added in small portions to facilitate dissolution, then sodium acetate (0.5 g, crystalline) followed by 2 N hydrochloric acid are added until the colour of the solution becomes red (pH 5-6). After the addition of 50 c m 3 of acetone, the volume of the solution is adjusted to 1 d m 3 with water. The solution is stable for at least 1 month when stored in the dark.

380

Lanthanum nitrate solution, 5.0 mole/dm 3 . Preparation: crystalline lan-thanum nitrate [ L a ( N 0 3 ) 3 . 6 H 2 0 ] 2.166 g is dissolved in 1 d m 3 of water. Acetate buffer solution, pH 5.2. Preparat ion: crystalline sodium acetate (100 g) is dissolved in water, 11 c m 3 of glacial acetic acid are added and the volume is adjusted to 1 d m 3 with water. The pH is checked with a pH meter.

Standard sodium fluoride solution, 50 ng /cm 3 of fluoride. Prepared by dissolving 110.6 mg of dried analytical-reagent grade sodium fluoride in water and diluting to 1 d m 3 with water. The solution is stored in polyethylene flasks.

For the preparation of the solutions and during the subsequent operations, water distilled from glass apparatus is used.

Construction of the calibration graph. A 10 cm 3 -volume of the reagent solution is added to each of eleven 50-cm 3 volumetric flasks and 5 c m 3 of acetate buffer are added to each. Exactly 0, 1, 2, 3,4, 5, 6, 7, 8,9 and 10 c m 3 of standard fluoride solution are added, respectively, to the series of flasks, then 10 c m 3 of lanthanum nitrate solution are added to each flask, with stirring, followed by 5 c m 3 of acetone. The volumes are made up with water and the solutions are mixed and are allowed to stand in diffuse light until colour development is complete (90 min). The absorbance of each solution is measured at 620 nm in 2 mm cells, against the solution containing no added fluoride ions as the reagent blank. Then the calibration graph is constructed.

Procedure. The sample, 3-6 mg, is combusted in an oxygen flask made from quartz or polypropylene. A polypropylene flask should not be used when a liquid sample is burnt in, e.g., a methyl cellulose capsule. Of course, fluoride solutions obtained by any combustion or decomposition method can be used when it does not contain interfering ions in large concentrations.

The absorption solution is diluted to 100 cm 3 , in a volumetric flask, then 20 c m 3 of this stock solution is transferred into a 50 c m 3 volumetric flask similar to those used for calibration. The procedure used in the calibration work is followed.

If the sample solution is strongly acidic or alkaline, it must first be neutralized.

Elimination of the effects of interfering ions. In the above procedure, 0.8 mg amounts of nitrate, chloride, bromide and iodide ions do not interfere, when 0.2 mg of fluoride ions is measured; 0.4 mg of sulphate ions and 0.2 mg of arsenic also do not interfere.

Phosphorus compounds interfere when more than 60 jig are present, particularly when present as reductive compounds (e.g., phosphite ions). In the combustion of phosphorus-containing fluorine compounds, the use of alkaline oxidizing absorption solutions is advisable. For this purpose, 10 c m 3

of 0.005 mole /dm 3 sodium hydroxide solution and 0.5 c m 3 of concentrated

381

hydrogen peroxide solution are added to the combustion flask. After combustion, 2 c m 3 of 0.1 mole /dm 3 zinc nitrate solution and 1 c m 3 of 0.1 mole/dm 3 sodium carbonate solutions are added to precipitate the phos-phate (and arsenate) ions. The contents of the flask are boiled for 3 min, then allowed to cool. The precipitate is filtered off through a small funnel coated with filter-paper pulp directly on to an ion-exchange column, 10 cm long and of 1 cm i.d., packed with Amberlite IR-120 (H + ) resin. The flow rate of the solution is adjusted to 1 cm 3 /min . The column is then washed with three 10 c m 3 volumes of water.

The eluate is neutralized with 0.5 mole /dm 3 sodium hydroxide solution (using a pH meter), transferred into a 100 c m 3 volumetric flask, made up to volume and mixed. Aliquots of 20 c m 3 are used in the spectrophotometric determination.

When a phosphorus compound is present in small amounts and its removal is therefore not necessary, the absorption solution is brought to boiling in order to decompose the hydrogen peroxide, allowed to cool, neutralized and made up to volume in a 10 c m 3 volumetric flask. The spectrophotometric measurement is effected on 20 c m 3 aliquots of this stock solution.

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152. Steyermark, A., Kanp, R. R., Petras, P. A., Bass, E. A.: Microchem. J., 3, 523 (1959). 153. Soep, H.: Nature, 192, 67 (1961). 154. Johnson, C. A., Leonard, M. A.: Analyst, 86, 101 (1961). 155. Ferrari, H. J., Geronimo, F. C , Brancone, L. M.: Microchem. J., 5, 617 (1961). 156. Olson, E. C , Shaw, S. R.: Microchem. J., 5, 101 (1961). 157. Macdonald, A. M. G.: Pure Appl. Chem., 45, 31 (1976). 158. Francis, H. J., Deonarine, J. H., Persing, D. D.: Microchem. J., 14, 580 (1969). 159. Kirsten, W. J.: Microchem. J., 7, 34 (1963). 160. Shearer, D. A , Morris, G. F.: Microchem. J., 15, 199 (1970). 161. Terry, M. B., Kasler, F.: Mikrochimica Acta, 569 (1971). 162. Pavel, J , Knebler, R., Wagner, H.: Microchem. J., 15, 192 (1970). 163. Delig, W.: Z. anal. Chem., 249, 30 (1970). 164. Baumgartel, E., Srecher, F.: Z. Chem., 4, 341 (1964); Ref., Z. Anal. Chem., 220, 53 (1966). 165. Reynolds, D. S., Hill, W. L.: Ind. Eng. Chem. Anal. Ed., 11, 21 (1939). 166. Belcher, R., Caldas, E. F , Clark, S. J., Macdonald, A. M. G.: Mikrochimica Acta, 283 (1953). 167. Willard, H. H., Horton, C. A.: Anal. Chem., 22, 1190 (1950). 168. Selig, E.: Analyst, 93, 118 (1968). 169. Horacek, H., Pechanek, S.: Mikrochimica Acta, 17 (1966). 170. Awad, W. L., Hassan, S. S., Elsayed, M. B.: Mikrochimica Acta, 688 (1969). 171. Hems, R. V., Kirkbright, G. F., West, T. S.: Talanta, 17, 433 (1970). 172. O'Donnel, T. A., Stewart, D. F.: Anal. Chem., 34, 1347 (1962). 173. Johannesson, J. K.: Chem. Ind. (London), 16, 480 (1957). 174. Megregian, S.: Anal. Chem., 29, 1063 (1957). 175. Mather, W. B., Anson, F. C : Anal. Chem., 33, 132 (1961). 176. Szantho, V.: Hungarian Sci. Instruments, 11 (1968). 177. Gawargious, Y. A., Amir Besada, Faltaoos, B. N.: Anal. Chem., 4^, 502 (1975). 178. Light, T. S., Mannion, R. F.: Anal. Chem., 41, 107 (1969). 179. Francis, H. J , Deonarine, J. H., Persing, D. D : Michrochem. J., 14, 580 (1969). 180. Baumann, E. W.: Anal. Chim. Acta, 54, 189 (1971). 181. Selig, W.: Mikrochimica Acta, 337 (1970). 182. Anfalt, T., Jagner, D.: Anal. Chim. Acta, 50, 23 (1970). 183. Turner, D. R.: Anal. Chem., 33, 959 (1961). 184. Selig, W.: Mikrochim. Acta, 515 (1974). 185. Poisier, M.: Talanta, 22, 607 (1975). 186. Ferrari, H. J., Geronimo, F. C , Brancone, L. M.: Microchem. J., 5, 617 (1961). 187. Martin, F. A., Floret, A., Dillier, M.: Bull. Soc, Chim. France, 460 (1961). 188. MacNulty, B. C , Hunter, C , Barrett, D.: Anal. Chim. Acta, 14, 368 (1956). 189. Lothe, J. J.: Anal. Chem., 28, 949 (1956). 190. Peck, L. C , Smith, V. C : Talanta, 11, 1343 (1964). 191. Knapp, G.: Mikrochimica Acta, 467 (1970). 192. Tan Lay Har, West, T. S.: Anal. Chem., 43, 136 (1971). 193. Dixon, E. J.: Analyst, 95, 272 (1970). 194. Hensley, A. L., Barney, J. E.: Anal. Chem., 32, 828 (1960). 195. Papay, M. K., Mazor, L , Takacs, J.: Acta Chim. Acad. Sci. Hung., 66, 13 (1970). 196. Belcher, R., Leonard, M., West, T. S.: J. Chem. Soc, 2390 (1958). 197. Leonard, M. A., West, T: S.: J. Chem. Soc, 4411 (1960). 198. Ma, T. S. in KolthofT, I. M. and Elving, P. S. (Eds.): Treatise on Analytical Chemistry.

Interscience, New York, 1966, Part II, Vol. 12. 199. Belcher, R., West, T. S.: Talanta, 8, 853 (1961). 200. Belcher, R., Leonard, M. A., West, T. S.: J. Chem. Soc, 3577 (1959).

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201. Fern and opu lie, M. E., Macdonald, A. M. G.: Microchem. J., 11, 41 (1966). 202. Kirsten, W. J., Shah, Z. H.: Anal. Chem., V, 184 (1975). 203. Leonard, M. A., Murray, G. T.: Analyst, 99, 645 (1974). 204. Leonard, M. A.: Analyst, 100, 275 (1975). 205. Janak, J.: Mikrochimica Acta, 1038 (1956). 206. Percival, W. C : Anal. Chem., 29, 20 (1957). 207. Ellis, J. F., Porrest, C. W., Allen, P. L.: Anal. Chim. Acta, 22, 27 (1960). 208. Ming-Ho-Yu, Miller, G. W.: Envir. Sci. Techn., 4, 492 (1970); Ref., Anal. Abstr., 4105

(1971). 209. Ayscough, P. B.: Canadian Chem. J., 33, 1566 (1955). 210. Nielsen, J. R.: J. Chem. Phys., 21, 1416 (1953). 211. Anders, O. V.: Anal. Chem., 32, 1368 (1960). 212. Hislop, J. S., Pratchett, A. G., Williams, D. R.: Analyst, 96, 117 (1971). 213. Carmichael, I. A., Whitley, J. E.: Analyst, 95, 393 (1970). 214. Kosten, L., Slunecko, J.: Anal. Chem., 42, 831 (1970). 215. Shoolery, J. R : Anal. Chem., 26, 1400 (1954). 216. Gutsche, B., Kleinolder, H., Herrmann, R.: Analyst, 100, 192 (1975). 217. Belcher, R., Tatlow, J. C : Analyst, ^6, 593 (1951). 218. Belcher, R., Macdonald, A. M. G.: Mikrochimica Acta, 510 (1957).

8. Determination of sulphur content

The elements treated in Sections 1-7 were usually present in one oxidation state in organic compounds. The possible oxidation states of sulphur are — 2, + 4 and + 6. According to the octet theory, the bond between sulphur and oxygen is not a covalent double bond but is of a semipolar type.

The simplest compound with sp 3 hybridization containing sulphur is hydro-gen sulphide, and very similar are the organic compounds — R t — S — R 2 — , where R is a simple or combined radical (a group of atoms or a functional group). Organic compounds containing the S = R 2 function are thioethers and those containing —R—SH are thioalcohols or mercaptans. In the sulphinic acids and sulphins the oxidation state of the sulphur is + 4 , whereas in sulphonic acids and sulphones it is + 6. Sulphur also occurs in the ring of heterocyclic compounds. In organic compounds the sulphur atom may be bound not only to carbon or oxygen, but also to atoms such as halogens or phosphorus. Therefore, methods for the determination of sulphur are more varied than those of the earlier mentioned elements.

The energy of the bond between carbon and sulphur atoms is 280.5 kJ, which is nearly the same as that of the bond between carbon and bromine atoms. Hence compounds with bonds between sulphur and carbon atoms are easily destroyed. Many organic compounds are able to react in organic or aqueous solutions with inorganic reagents, such as metal ions, with the formation of a precipitate, if the metal suphide obtained has a very low solubility in the solvent used. These types of reactions are described in the chapter on the quantitative analysis of functional groups (Chapter 9).

27 387

The usual way of determining elements in organic compounds is to change the covalent bond of the atom into an ionic bond by transforming the organic into an inorganic compound. This is also the usual method of determining sulphur.

There are two ways of decomposing sulphur compounds. An oxidation process can be used, in which the compounds formed are different types of sulphur oxides, such as sulphur dioxide or sulphur trioxide, from which sulphuric acid or sulphate ions can be obtained. Alternatively, a reduction process can be used, in which the compounds formed are hydrogen sulphide or sulphide ions. Using these two procedures, numerous individual methods can be found, with various advantages and disadvantages. There are many problems when a method has to be chosen for the determination of sulphur in organic compounds in a particular instance.

The oxidation of organic compounds containing sulphur is a very simple, rapid and quantitative process, but there is no method for the determination of the sulphate on the micro-scale that is sensitive and accurate.

The digestion of these compounds by a reduction process is not so simple. These methods are usually hazardous, but they give good results. For the products of reductive digestion (hydrogen sulphide or sulphide ions) there are various oxidizing methods that are sensitive and accurate on the micro-scale [1] .

Fusion with alkali metals was carried out earlier using metal bombs or glass ampoules [2 -4 ] , but it is lengthy and complicated. Mazor et al. published a method [5] for the determination of sulphur in non-volatile organic compounds involving decomposition in an open tube with an alkali metal. The sulphide ions formed in the process are titrated in alkaline media with potassium hexacyanoferrate(III) solution in the presence of luminol (3-aminophthalic acid hydrazide) chemiluminescent indicator.

A variation of this method is as follows. The decomposition of the compound is carried out in a small test-tube (5 x 25 mm) placed in a simple apparatus under vacuum. The test-tube is surrounded with a platinum wire spiral, which is heated by a low-voltage electric current. Under vacuum the reaction is very mild [6] . The temperature of the fusion is 100-300°C. The sulphur in the original compound forms sulphide ions. After the decomposition, the contents of the tube are shaken with water in the apparatus still under vacuum, under which conditions the reaction of the excess of potassium with water is not violent. There is a better method for the decomposition of the excess of potassium: the immersion of the test tube in 5-6 c m 3 diluted alcohol (80-90%), and when the reaction is over we dilute this solution with water to 20-25 cm 3 . The alkaline solution of sulphide ions is titrated with 0.01 N silver nitrate solution or, better, with 0.01 N mercury(II)

388

perchlorate solution. The end-point is detected with silver sulphide (or a sulphide ion-selective membrane electrode) and with a calomel electrode with a saturated potassium nitrate salt bridge.

The method is suitable not only for the determination of the sulphur content of organic compounds, but also for halogens, titrating the halide ions with silver nitrate or mercury(II) perchlorate solution. The decomposition of some nitrogen-containing compounds gives cyanides, which can be titrated by argentimetric or mercurimetric methods.

For the oxidative digestion of organic compounds containing sulphur, earlier methods were useful, such as dry digestion by the method of Piria or the wet method of Carius. Both methods are now out of the date, because they are accurate only on the macro-scale and are lengthy and tedious.

The first method for the determination of sulphur in micro amounts was introduced by Pregl. The instruments and method are very similar to the so-called method of "Perlenrohr", already described for the determination of halogen content. The investigated compounds were burned on the surface of platinum metal (as catalyst) and the sulphur dioxide formed was absorbed in alkaline hydrogen peroxide solution. Sulphate ions in the solution are determined by a gravimetric method. The methods of Grote and Krekeler also originate from the same period. The oxidation of the products of pyrolysis was carried out in oxygen (earlier in air) in a quartz tube between porous quartz plates. The products formed were absorbed in neutral hydrogen peroxide solution and the sulphuric acid formed was titrated with sodium hydroxide solution, or there are other acidic compounds (HC1 from CI) so that a gravimetric method must be used.

Solid or liquid oxidants are seldom used, because it is difficult to remove excess of them and, the high concentration of the salt may interfere with the formation of barium sulphate on the semimicro-scale.

Another way of determining sulphur is as follows. The acidic decomposition is carried out with metaphosphoric acid [7] and the distilled sulphuric acid is titrated. A further possibility is to digest the organic compound in a Kjeldahl flask with a mixture of perchloric acid, nitric acid and bromine. The sulphate ions formed are determined as the barium sulphate precipitate after the removal of excess of acids [8] .

For "digestion of organic compounds using oxygen there is the very useful "empty tube" method, developed for the determination of halogens. The method has various modifications, the most important of which is the following. The sulphur dioxide is captured in an absorber tube containing electrolytic silver [9] . The water-soluble silver sulphate which is ultimately formed in ttoe process is dissolved in hot water and the equivalent silver ions are titrated with a solution of potassium iodide using potentiometric end-

27* 389

point detection. The silver sulphate was reduced to hydrogen sulphide in hydrogen gas by Takeuchi et al. [10] and the sulphur was determined in this form.

The temperature of the combustion of the products of the pyrolysis is about 900°C with the "empty tube" method, although Dokladova and co-workers [11-13] found that 750-800°C is satisfactory. Discherl and Erne proposed 1000°C [14] and Pell et al. [15] 1400°C. Drushel [16] made some useful comments on the method.

The combustion of organic compounds using a flask filled with oxygen was proposed by Schoniger [17]. This method is similar to the determination of halogens. The principle of the method has not changed, but today the flask is equipped with an internal electrical or optical switch which initiates combustion [18] . A special flask was designed by Schnessler [19], from which the products of the pyrolysis were transferred directly into a gas chromato-graph. There are smaller flasks than the original 300-500 c m 3 flasks, in which oxidizing acids or a mixture of acids, fusing with oxidizing reagents such as sodium peroxide or by oxygen flask combustion [1 -4 ] . These methods and sulphur content, as is the case with halogens. It was found that the standard deviation is 0.25% (the sulphur content is 99.73%).

In most laboratories nowadays the determination of sulphur is carried out with a flask filled with oxygen. In this atmosphere, at the high temperature of the combustion (1000-1200°C), sulphur trioxide is formed and absorbed as sulphuric acid. Only with some compounds that contain semipolar sulphur-phosphorus bonds is the combustion not complete. Usually the absorber solution contains dilute hydrogen peroxide, which oxidizes the possible sulphites to sulphuric acid. Iodine compounds have been proposed for the oxidation by Gaux and Le Henaft [22] ; the excess of iodine can be reduced with hydrazine.

A gravimetric method for determination of sulphate ions in the absorber solution is necessary only when the solution contains ions that interfere with titrations [23] . Schoniger titrated the sulphuric acid with 0.01 N sodium hydroxide solution. The acidimetric method is applicable only when the compound contains no other elements which ultimately form acids. From the absorber solution used with nitrogen-containing compounds the sulphate ions can be precipitated by a known excess of 0.01 N barium chloride solution as barium sulphate (Schoniger's method). The excess of barium ions can be determined by complexometric titration. The sources of error of the method lie in the facts that the precipitation of the small amount of barium sulphate is difficult, the precipitate is not compact and it dissolves to some extent in the EDTA solution, so the end-point of the titration is not sharp.

390

A better method, which came into general use, is the titration of sulphate with barium ions (barium chloride, barium acetate, barium perchlorate) in the presence of special metal indicators. Various solutions and indicators have been proposed [24-29] . These indicators display better end-points because the precipitation of barium sulphate is faster and more complete in a solution containing about half the volume of water-miscible organic solvent (such as ethanol, isopropanol or 1,4-dioxane). The most commonly used indicator is Thorin with barium perchlorate solution in the presence of isopropanol, or tetrahydroxyquinone with a solution of barium chloride, sodium or alizarin sulphonate with a solution of barium nitrate. The compositions of the most frequently used indicators are as follows:

A s 0 3 H HO SO3H

sodium salt of alizarine sulfonic acid

Scroggins [30] obtained good results with compounds free of phosphorus using tetrahydroxy-p-benzoquinone.

The use of a solution of lead ions as titrant in water-1,4-dioxane as solvent has also been suggested [31]. In this instance dithizone is a suitable indicator [32]. Conductimetry can also be used for end-point detection [27,31, 33, 34]. Balodis et al. [35] developed a method for successive determination of sulphate and phosphate ions. The phosphate ions were masked by a known excess of iron(III) and sulphate ions were titrated with a solution of barium chloride in the presence of Thorin and Methylene Blue mixed indicator. Then the excess of iron (III) ions were titrated with EDTA solution and the amount of phosphorus was calculated from the difference. Gawargious and Farag [36] proposed an amplification method as follows. From the absorbing solution the barium sulphate is precipitated with an excess of barium

391

bromate, the excess of barium bromate is precipitated with acetone and, after filtration, it is dissolved in hot water. To the solution an acid and iodide are added and the iodine formed is titrated. According to the authors the method is suitable in the presence of a number of other ions. There are methods by which coagulation of the barium sulphate can be prevented by potassium chlorate [37] or an ammoniacal solution of EDTA, and the barium sulphate suspension determined by nephelometry. In the excess of the precipitant barium can be determined by flame photometric [38] or polarographic [39, 40] methods.

The sulphur content of organic compounds can be reduced to hydrogen sulphide by pyrolysis in hydrogen using platinum as catalyst at 950°C [41,42] or 1200°C [43, 44]. The hydrogen sulphide can be absorbed by zinc acetate solution and determined by an iodimetric method. In another method the methylene blue formed from hydrogen sulphide can be determined photometrically [45]. Sulphide ions in the absorbing solution can also be determined with a sulphide-specific electrode using a solution containing lead ions as titrant. The sulphur content of the organic compound can be converted into nickel sulphide using a Raney nickel catalyst and, after treatment with acid, hydrogen sulphide is formed again [44]. The reduction with Raney nickel can be carried out in hydrogen at 220°C [46]. According to Toshiyasu and co-workers [47] 5-10 ng of sulphur can be determined by the following method. The sulphur dioxide obtained from the stream of oxygen is absorbed on a silver gauze at 550°C, then the silver sulphate can be reduced to hydrogen sulphide, which is then determined photometrically in the form of methylene blue. Sulphur dioxide is absorbed on the same silver gauze according to Trutnovsky and Alfy [48], the silver sulphate can be reduced by hydrogen and finally, the sulphide ions are titrated [49]. A method for the analysis of organic compounds containing 3 5 S based on catalytic reduction in hydrogen was developed by Mlinko et al. [50]. Volodina et al. [51] pyrolysed organic compounds containing sulphur in a stream of nitrogen at 700-750°C, and the gas, containing sulphur dioxide, was absorbed in a solution of sodium tetrachloromercurate followed by photometric determination using fuchsin formaldehyde reagent. Volodina and Martynova [52, 53] pyrolysed organic compounds in a stream of ammonia at 700-800°C, and from the liquid obtained in the absorber the sulphide ions were determined by potentiometric titration with 0.01 N ammoniacal silver nitrate solution using calomel and silver sulphide electrodes.

The pyrohydrolysis method, which was described in connection with the analysis of halogen compounds, is also suitable for the determination of sulphur compounds. In nitrogen saturated with water vapour, compounds that contain sulphur in the — 2 oxidation state decompose with the formation

392

of hydrogen sulphide, but compounds that contain sulphur in the -1-4 oxidation state give partly hydrogen sulphide and partly sulphur dioxide. If the sulphur is in the -I-6 oxidation state sulphur trioxide is usually obtained, which decomposes on a platinum catalyst above 8 0 0 ° C to give sulphur dioxide and oxygen:

2 S 0 3 ? ± 2 S 0 2 + 0 2

but the hydrogen sulphide does not undergo any change on the platinum surface.

If the nitrogen containing hydrogen sulphide and sulphur dioxide is bubbled through a solution of potassium triiodide of known concentration, the two sulphur compounds react with iodine in the same stoichiometric ratio:

I 2 + H 2 S = S + 2I ~ + 2 H +

I 2 + S 0 2 + 2 H 2 0 = 2I - + S O j - + 4 H +

The excess of iodine can be titrated with sodium thiosulphate and the sulphur content of the compound calculated from the amount of iodine consumed.

A special method of reductive decomposition was described by Osadchii and Fedoseev [ 5 4 ] . The sulphur compound was treated with magnesium silicide (Mg 2 Si) at 5 5 0 - 6 0 0 ° C . The product was decomposed by water and the hydrogen sulphide evolved was determined iodimetrically or argentimetrically.

Some sulphur compounds are reducible in phosphoric acid by tin(II) phosphate [ 5 5 ] . Sulphur compounds were decomposed in metal bombs in the presence of metallic potassium by Floret [ 5 6 ] , and the hydrogen sulphide obtained from the metal sulphide was titrated iodimetrically or by using amperometric end-point detection.

Wronski [ 5 7 , 5 8 ] suggested o-hydroxymercurybenzoic acid as a titrant for sulphide ions, the reaction being as follows:

For end-point detection dithizone (in 0 . 1 % alcoholic solution) is used. With O.OIN solutions the end-point is very sharp, the colour change of the indicator being from yellow to red. If the compounds contain nitrogen, the cyanide ions formed can be masked with formaldehyde [ 5 9 ] . Binkovski and Wronski [ 6 0 ] decomposed sulphur compounds in closed ampoules in the presence of

3 9 3

potassium. The excess of potassium can be destroyed by ethanol and in the alkaline solution sulphide ions can be titrated in the same way.

Several workers have suggested instrumental methods for the determination of sulphur in organic compounds, including gas chromatography [61-64] , polarography [65], coulometric titration [66-68], manometric measurement [69] and neutron activation analysis [70]. Solutions containing 30 mg /dm 3 of sulphur can be analyzed with the Auto Analyzer, the sulphate ions being reduced to hydrogen sulphide and determined colorimetrically [71].

A good review of the determination of sulphur in organic compounds was published by Debal and Levy [72]. The book by Karchmer [73] treats the complete analytical chemistry of sulphur.

Determination of the sulphur content of organic compounds by titration after oxygen flask combustion [74, 75]. The procedure adopted by most organic analytical laboratories is as follows.

Depending on the sulphur content, 5-15 mg of the sample is combusted, after wrapping in a filter-paper, in a flask filled with oxygen, as was described for the analysis of halogen compounds. The absorbing solution in the flask is about 20 ml of water containing 2-3 drops of 30% hydrogen peroxide.

After the combustion the flask is shaken for 10-15 min and then set aside for about 20-30 min to facilitate perfect absorption. The stopper is then removed and rinsed. The contents of the flask are heated for 4-5 min to decompose hydrogen peroxide, and the solution is transferred into a titration flask and evaporated to about 10 c m 3 . After cooling, 40 c m 3 of propanol-2 are added.

The titrant is prepared as follows. About 2 g of barium perchlorate [ B a ( C 1 0 4 ) 2 . 3 H26] is weighed and dissolved in 200 c m 3 of water and diluted to 1 d m 3 with propanol-2. The pH of the solution is adjusted to 3.5 with perchloric acid. The normality of the solution obtained (about 0.005) is determined by titrating a known amount of 0.005 N sulphuric acid, under the same conditions as were used in the main procedure.

Thorin is used as indicator (20 mg in 10 c m 3 of water) for the titration. A drop of Thorin indicator is added to the solution, which is titrated with the barium perchlorate solution with vigorous stirring by a magnetic stirrer. At the end-point the colour changes from yellow or orange to pink. For the observation of the change of colour some practice is needed; with more concentrated solutions the end-point is sharper. So, if the sulphate ion content is higher, one can use a more concentrated solution for the titration; 1 c m 3 of 0.005 N barium perchlorate solution is equivalent to 0.16 mg of sulphur.

394

If the sample contains phosphorus, phosphate ions are also absorbed and interfere with the titration. In this instance a small amount of magnesium carbonate is added to the solution (0.1-0.2 g of magnesium carbonate in 2-3 c m 3 of water); the magnesium phosphate formed is filtered and washed and the filtrate is evaporated and titrated.

References to Section 8.8.

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10. Takeuchi, T., Fujishima, I., Wakayama, Y.: Mikrochimica Acta, 635 (1965). 11. Dokladova, J., Korbel, E., Vecera, M.: Coll. Czech. Chem, Comm., 29, 1962 (1964); Ref., Z.

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39. Bishara, S. W.: Microchem. J., 15, 211 (1970). 40. Gawargious, Y. A., Besada, A., Faltaoos, B. N.: Mikrochimica Acta, I, 75 (1976). 41. Kato, M , Fujishima, I.: Japan Analyst, 11, 178 (1962); Ref., Anal. Abstr., 1, 185 (1964). 42. Volodina, M. A , Abdukarimova, M., Terentev, A. P.: Zhur. Anal. Khim. 23, 1420 (1968). 43. Slanina, J., Agderdenbos, J., Griepink, B.: Mikrochimica Acta, 1225 (1970); 607 (1973). 44. Wronski, M., Bald, E : Chem. Anal. (Warsaw), 14, 173 (1969); Ref., Anal. Abstr., 18, 4072

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131 (1968). 47. Toshiyasu, K., Ikuko, A., Sachico, T.: Bull. Chem. Soc. Japan, 30,482 (1957) Ref., Anal. Abstr.,

5, 1874 (1958). 48. Trutnovsky, H., Sahla Alfy, B.: Anal. Chim. Acta, 65, 147 (1973). 49. Takeuchi, T., Fujishima, I., Wakayama, Y.: Mikrochimica Acta, 635 (1965). 50. Mlinko, S., Gacs, I., Szarvas, T.: Intern. Appl. Radiation Isotopes. 18,457 (1967); Ref., Z. anal.

Chem., 235, 286 (1968). 51. Volodina, M. A., Abdukarimova, M., Gorshkova, T. A., Borodina, V. G., Zhardetskaya,

V. N.: Vest. Mosk. Gos. Univ., Ser. Khim. 114 (1968), Ref., Anal. Abstr., 23, 1012 (1970). 52. Volodina, M. A., Martynova, G. A.: Zhur. Anal. Khim., 27 (9) 1856 (1972); Ref., Anal.

Abstr., 26, 2103 (1974). 53. Volodina, M. A , Martynova, G. A.: Zhur. Anal. Khim., 26, 1002 (1971); Ref., Anal.

Abstr., 23, 3210 (1972). 54. Osadchii, V. D., Fedoseev, P. N.: Ukr. Khim. Zh., 37, 363 (1971); Ref., Anal. Abstr., 23, 830

(1972). 55. Griepink, B., Slanina, J., Schoohman, J.: Mikrochimica Acta, 984 (1967). 56. Floret, A.: Chim. Anal., 53, 739 (1971). Ref., Z. anal. Chem., 261, 47 (1972). 57. Wronski, M.: Talanta, 21, 776 (1974). 58. Wronski, M.: Analyst, 83, 314 (1958). 59. Wronski, M.: Analyst, 84, 668 (1959). 60. Binkovski, J., Wronski, M.: Mikrochimica Acta, 429 (1971). 61. Chumacenko, M. N., Alekseeva, N. N.: Izv. Akad. Nauk. SSR., Ser., Khim., 964 (1969). 62. Beuerman, D . R, Meloan, C. E.: Anal. Chem., 34, 319 (1962). 63. Okuno, I., Morris, J. C , Haines, W. E.: Anal. Chem., 34, 1427 (1962). 64. Uhdeova, J., Janak, J.: J. Chromatography, 65, 179 (1972); Ref., Z. anal. Chem., 263, 226

(1973). 65. Bishara, S. W.: Microchem. J., 15, 211 (1970). 66. Fraisse, D., Raveau, S.: Talanta, 21, (6) 629 (1974). 67. Cedergren, A.: Talanta, 20, 621 (1973). 68. Dixon, J. P.: Analyst, 97, 612 (1972). 69. Frazer, J. W , Stump, R. K.: Mikrochimica Acta, 651 (1967). 70. Heslop, R. B., Tay, S. K.: Anal. Chim. Acta, 47, 183 (1969). 71. Keay, J., Menage, P. M. A., Dean, G. A.: Analyst, 97, 897 (1972). 72. Debal, E., Levy, R. Mikrochimica Acta, 202 (1966). 73. Karchmer, J. H.: Analytical Chemistry of Sulphur and its Compounds. Wiley, Chichester,

Sussex, 1971, Part 1-3. 74. Fritz, J. S., Yamamura, S. S.: Anal. Chem., 27, 1461 (1955). 75. Ingram, G.: Methods of Organic Elemental Microanalysis, Chapman and Hall, London 1962,

pp. 258-259.

396

9. Determination of phosphorus in organic compounds

The first stage of the analysis of organic phosphorus compounds is their decomposition by oxidation, all of the organically bound phosphorus being converted into orthophosphoric acid or or thophosphate ions, which are then suitable for gravimetric or spectrophotometric determination. When selecting a method for the decomposition of organic phosphorus compounds, we must first consider the physical condition and the volatility of the substance under investigation. The mineralization of some compounds is easy but in some instances we must use more drastic methods. The decomposition of involatile compounds can be achieved in a crucible with a 2 : 1 mixture of sodium carbonate and potassium nitrate [1] or in a metal bomb with sodium peroxide [2] . Liquid samples can be mineralized in a gelatin capsule.

Decomposition in a Kjeldahl flask with sulphuric and nitric acid is a well-known method. Digestion of a volatile compound can be achieved in a closed bomb with concentrated nitric acid by the Carius method. The advantage of Kjeldahl decomposition, which is often used even today, is that one can use a larger sample of compounds with a low phosphorus content. However, during the relatively slow digestion process volatile phosphorus compounds may be formed. In the Carius method the walls of the bomb may retain some calcium phosphate. Medzihradszky and Kutassy [3] decomposed 5-10 mg of a sample completely in 2-3 hours with 0.5 c m 3 of fuming nitric acid using a Kjeldahl flask equipped with a reflux condenser. A similar method was published by Erickson and Sanford [4] . They heated tri-m-tolylphosphine with sulphuric, fumic nitric and finally perchloric acid, precipitated phosphate ions as ammonium magnesium phosphate, filtered and, after heating, weighed the precipitate as magnesium pyrophosphate. Saliman [5] decomposed compounds which contain phosphorus in micro amounts [0.2 \ig P ] by heating them with an aqueous mixture of hydriodic acid, potassium iodide, phenol and acetic acid. Lindner and Edmundsen [6] found that alkyl phosphorus compounds form complexes with 1-2 mole of aluminium chloride, which are hydrolysed by water to form alkylphosphoric acid, which can be selectively converted into phosphoric acid. For the determination of the total phosphorus content, the compound is converted into phosphate with sulphuric acid and nitric acid, using sodium molybdate and perchloric acid as catalysts. For Kjeldahl digestion a mixture of concentrated nitric acid and 70% perchloric acid can be used [7] . Sandhu et al. [8] , for the determination of phosphorus (and arsenic), digested the sample in an ethereal solution with aluminium lithium hydride.

For the decomposition of phosphorus compounds, the principally used method nowadays is oxygen flask combustion. As phosphates attack

397

platinum, it is better to use a flask in which the paper containing the compound is placed in a silica holder [9 ,10] . The absorbing solution is 0.4 N sulphuric acid containing peroxodisulphate as the oxidant [11] or a mixture of sodium hydroxide with bromine [12] or simply water. Shanina et al. [13] combusted the sample in a polyethylene flask containing oxygen to avoid interference from silicate dissolved from the glass. Combustion is helped by cotton. Phosphate ions are determined spectrophotometrically using the molybdophosphate method. Hisatake et al. [14] determined phosphorus in polyphosphates by spectrophotometry with oxygen flask combustion and obtained good results only when a mixture of nitric and perchloric acids was used as absorbent.

Many methods are known for the quantitative determination of phosphate ions. The gravimetric method in the form of ammonium magnesium phosphate is not very sensitive, while the method of Lorenz, in which the phosphate ions are weighed in the form of a yellow ammonium molybdophosphate precipitate (with a favourable stoichiometric factor of 0.016 39), is lengthy and tedious. If the products of the oxygen flask combustion are absorbed in a neutral solution in the presence of zinc sulphate and ammonium ions, one can determine the total acid by iodimetry, as from the iodate-iodine system all three hydrogen ions of phosphoric acid liberate iodine [15]. Sass et al. [16] developed a method for the analysis of phosphorus halogen compounds and pyroesters. The absorbing solution was treated with alkaline peroxide solution and, after acidifying, potassium iodide was added and the equivalent amount of iodine titrated.

The method based on titration with cerium(IV) or lanthanum ions in the presence of Eriochrome Black T indicator [17] or which pH-metric end-point detection is useful [18,19] . The latter method [19], according to the authors, is especially suitable for the determination of phosphorus in phospholipids and lecithins even in the presence of barium, cadmium, mercury (II) and silver ions. The phosphate ions are titrated with lead(II) in the presence of Eriochrome Black T indicator [20]. For the determination of small amounts of phosphate ions a spectrophotometric method is applicable, based on molybdenum blue [21, 22] or, better, molybdovanadophosphoric acid [23]. The spectrophotometric methods are well suited here, because after the oxygen flask combustion the absorbing solution does not contain interfering ions. A very fast and sensitive method was developed for the analysis of organic phosphorus and carbonyl compounds by Guilbault and Lubrano [24], based on the Schoenemann reaction. The fluorescence of the solution is measured in the 0.02-100 ng /cm 3 range. Using this method, phosphorus can be determined in phosphorus-fluorene, chlorine phosphate and chlorine triphosphate compounds with an error of 1.5%. A method described for the

398

determination of organic phosphorus compounds by Guilbault et al. [25] is based on the fact that these compounds hinder the hydrolysis of the buthyrylthiocholine iodide with choline esterase. The potential of two platinum electrodes dipped into the solution is monitored. The decrease in the potential is proportional to the concentration of anticholinesterase.

An indirect method was suggested by Kidani et al. [26] for the determination of phosphorus. From the solution, containing phosphate ions, molybdophosphate was precipitated and then extracted with diisobutyl ketone and the molybdenum content was determined by atomic-absorption spectroscopy.

Spectrophotometric determination of the phosphorus content of organic phosphorus compounds [27,28] . After oxygen flask combustion the phosphate ion content of the absorbing solution is determined by the absorption of the molybdovanadophosphate complex at 430 nm.

Reagents 0.5 N sodium hydroxide solution Saturated bromine water 5% Ammonium molybdate solution 0.25% Ammonium vanadate solution 25%(v/v) Sulphuric acid Standard potassium dihydrogen orthophosphate solution

(Preparation. 0.4390 g of potassium dihydrogen or thophosphate is dissolved in 1000 c m 3 water; 1 c m 3 of the solution is equivalent to 0.1 mg of phosphorus.)

For the preparation of the solutions and the analysis, deionized water should be used.

Weigh a sample, containing a maximum of 2 mg of phosphorus and wrap it in filter-paper. Place 5 c m 3 of 0.5 N sodium hydroxide solution in the flask and add 4 drops of bromine water.

The sample, wrapped in filter-paper, is combusted as described for the determination of halogens. The flask must be set aside for about 15-20 min, then the stopper is removed and rinsed with water. A 6-cm 3 volume of 25% sulphuric acid is added, and the solution is heated until the complete removal of the bromine, when the solution becomes colourless. The cold solution is then transferred into a 100-cm 3 volumetric flask with 20-30 c m 3 of water, 10 c m 3 of ammonium vanadate and 10 c m 3 of ammonium molybdate are added and the flask is filled to the mark with water. It must be set aside for 30 min to develop the colour. A blank experiment is also carried out. The absorbance of the two solutions is measured in a 1-cm cuvette at 430 nm and the value for the

399

reference solution (about 0.03) is subtracted from that for the sample. The results are obtained from a calibration graph prepared in the following way.

Into seven 100-cm 3 volumetric flask 0.0, 2.5, 5.0, 7.5,10.0,15. 0 and 20 c m 3

of standard phosphate solutions are dispensed and 6 c m 3 of 25% sulphuric acid are added to each. The mixtures are diluted to 60 c m 3 with water, 10 c m 3

of ammonium vanadate and 10 c m 3 of ammonium molybdate solutions are added and each flask is filled to the mark with water. After standing for 30 min the absorbance is measured as described above.

The calibration graph is a straight line which does not necessarily pass through the origin.

The method is very sensitive to phosphorus impurities, so meticulous cleaning of the flask is necessary. It is best to use one flask solely for the determination of phosphorus.

The spectrophotometric method is not only suitable after oxygen flask combustion, but can also be used after wet digestion [29] or fusion with sodium peroxide [30]. It is very important that the acidity of the final solution should always be the same (0.05 N sulphuric acid or possibly perchloric acid), as described above.

References to Section 8.9

1. Masao Maruyama, Kazue Hasegawa: Ann. Rep. Takamine Lab., 13, 173 (1961); Ref., Anal. Abstr., 11, 186 (1964).

2. Buss, H., Kohlschutter, H. W., Preiss, M.: Z. anal. Chem., 214, 106 (1966). 3. Medzihradszky, H , Kutassy, S.: Acta Chim. Acad. Sci. Hung., 41, 265 (1964). 4. Erickson, A., Sanford, L.: Talanta, 19, 1457 (1972). 5. Saliman, S.: Anal. Chem., 36, 112 (1964). 6. Lindner, G., Edmundsen, I.: Acta Chem. Scand., 21, 136 (1967). 1. Dinguid, L. I., Johnson, N. C : Microchem. J., 13, 616 (1968). 8. Sandhu, S. S., Sandhu, R. S., Sharma, K. D . : Z. analyt. Chem., 273, (1) 32 (1975). 9. Puschel, R., Wittmann, H.: Mikrochimica Acta, 670 (1960).

10. Hesse, G., Bock el, V.: Mikrochimica Acta, 939 (1962). 11. Hsiang Yung Yu, I Hsien Sha: Chem. Bull Peking, 9, 557 (1965); Ref., Anal. Abstr., 14, 191

(1967). 12. Bishara, S. W., Attia, M. E.: Talanta, 18, 634 (1971). 13. Shanina, T. M., Gel'man, N. E , Mikhajlowskaja, V. S:, Serbrikova, T. S.: Zhur. Anal Khim.,

11 (9) 1853 (1972); Ref., Anal Abstr., 26, 2104 (1974). 14. Narasaki Hisatake, Miyaji Kiyoshi, Unno Akiyuki: Japan Analyst, 11,541 (1973): Ref., Anal.

Abstr., 11, 3330 (1974). 15. Gawargious, Y. A., Farag, A. B.: Microchem. J., 16, 333 and 342 (1971); Ref., Anal Abstr., 11,

4066 (1972) and 4067 (1972). 16. Sass, S., Master,L, Davis, P. M., Beitsch, N.: Anal Chem., 32, 285 (1960). 17. Nuti, V.: Farmaco, Ed. ScL, 11, 179 (1972); Ref., Anal Abstr., 23, 3212 (1972). 18. Griepink, B., Slanina, J.: Mikrochimica Acta, 21 (1967); 607 (1973).

400

19. Griepink, B.: Mikrochimica Acta, 1151 (1964). 20. Meyer, E.: Mikrochimica Acta, 70 (1961). 21. Lebedeva, A. I., Novozhilova, I. V., Aksenova, N. A.: Izv. Akad. Nauk Armyan SSR, Khim.

Nauk, 19, 743 (1966). 22. Pilz, W.: Mikrochimica Acta, 35 (1965). 23. Debal, E.: Chim. Anal, 45, 66 (1963); Ref., Anal. Abstr., 11, 602 (1964). 24. Guilbault, G. G., Lubrano, J. G.: Anal. Chim. Acta, 43, 253 (1968). 25. Guilbault, G. G., Kramer, D. N., Cannon, P. L., Jr.: Anal. Chem., 34, 1437 (1962). 26. Kidani, Y., Takemura, H., Koike, H.: Japan Analyst, 23, (2) 212 (1974); Ref., Anal. Abstr., 28,

4C10 (1975). 27. Dixon, J. P.: Modern Methods in Organic Microanalysis, D. Van Nostrand, London, 1968. pp.

160-162. 28. Liddel, C : J. Inst. Petrol., 48, 221 (1962). 29. Kirsten, W. J., Carlsson, M. E.: Microchem. J , 4, 3 (1960). 30. Fennel, T. R. F. W., Roberts, M. W., Webb, J. R.: Analyst, 82, 639 (1957).

10. Determination of arsenic and antimony content

For decomposing organic arsenic compounds or organic substances which contain arsenic, ashing is not suitable because of the danger of losses, although it may be applied to the analysis of coal sample [1] . One can digest the sample in a metal bomb with sodium peroxide, but the usually preferred method is wet digestion in a Kjeldahl flask with sulphuric and nitric acids, followed by oxidation with hydrogen peroxide and potassium permanganate [2] . The presence of halogen ions in the decomposition can cause the loss of volatile arsenic halides. In recent investigations of wet digestion with sulphuric and nitric acids was found to be unsatisfactory as losses occur [3] . A better procedure involves decomposition in a closed apparatus in the presence of potassium bromate. Arsenic bromide formed during the treatment is distilled and determined spectrophotometrically using the molybdenum blue method.

A widely applied method nowadays is oxygen flask combustion [4 -6 ] . Griepink and Krijgsman [7 ] used a Belcher-type micro-flask and the compound was combusted in quartz-wool. In the combustion arsenic oxides are formed, which are absorbed in alkaline solution, and are oxidised or reduced depending on the method used for the determination.

A method for the determination of arsenic and vanadium was published by Sliepcevic et al. [8 ] . The sample is combusted in a flask containing oxygen and the combustion products are absorbed in 2% sodium acetate solution. This solution is then poured on to a Dowex A ion-exchange column, the vanadium being returned and the arsenic appearing in the eluate. Celon et al. [9] recommended benzene peroxide as a catalyst for the determination of arsenic

401

in organometallic compounds to assist oxygen flask combustion, with dilute iodine solution as absorber. Arsenic is then determined spec-trophotometrically in the form of the molybdate complex.

For the determination of arsenic in the solution a gravimetric method has been described [7] , in which a uranyl arsenate precipitate is obtained and weighed in the form of uranium dioxide after heating. Most often titrimetric methods are used, e.g., titration with lead ions after oxidation to arsenate [6] . PAN or SNAZOXS indicators may be used [ P A N = pyridyl-2-azo-4-sulphonic acid; SNAZOXS = 7-(4-sulpho-l-naphthylazo)-8-hydroxyquino-line-5-sulphonic acid]. Phosphorus interferes in the titration. If the neutral absorbing solution is titrated with lead, the acidity of the solution increases:

3 P b ( O H ) + + 2 H 2 A s 0 4 - P b 3 ( A s 0 4 ) 2 + H 3 0 + + 2 H 2 0

Thus, monitoring the pH is suitable for end-point detection [7, 10]. Stefanac [5] precipitated arsenate ions with silver nitrate. The precipitate is dissolved in a reagent solution containing tetracyanonickelate(II) complex, nickel ions being liberated, and titrated with EDTA in the presence of murexide indicator. Halogens and phosphorus interf:re in the titration. Iodimetric titration was proposed by Sandu et al. [11] after reduction with zinc. The end-point is detected with starch or by potentiometry. Pahil and Krishnan [12] titrated a methanolic solution of some organic compounds with 0.1 N iodine solution in the presence of starch or used potentiometric or conductimetric end-point detection. Hassan and Elsayes [13] dissolved arsenic acids in dimethylformamide and titrated with 0.02-0.04 N benzene methanolic sodium methoxide in a nitrogen atmosphere with bromophenol blue as indicator. Bigois [2] recommended iodine for the coulometric determination of arsenic, antimony and copper from the same solution as oxidant, generated by electrolysis from potassium iodide solution, while sodium thiosulphate was used as an intermediate reagent and the end-point was detected amperometrically. For the determination of small amounts of arsenic, spectrophotometry based on molybdenum blue can also be used, but it is too sensitive and not as accurate as with phosphate ions. Fictchelt et al. [14] and Mamta and Sudok [15] proposed atomic-absorption spectroscopy for the determination of both organically and inorganically bound arsenic.

For the mineralization of organic materials that contain antimony, the best method is wet digestion in a Kjeldahl flask with sulphuric acid and hydrogen peroxide. If the compound contains chlorine it is better to start the digestion with dilute ( 1 : 2) nitric acid then to add concentrated sulphuric acid and hydrogen peroxide d rop by drop until the solution becomes clear. The excess of hydrogen peroxide is decomposed by heating, then antimony(V) is reduced to antimony(III) with sodium sulphite. After the decomposition of

402

the excess of the reducing agent antimony(III) ions are titrated with 0.01 N iodine containing sodium hydrogen carbonate [16, 17]. Volatile antimony compounds may be digested in a Carius bomb with 0.5 c m 3 of concentrated nitric acid.

References to Section 8.10

1. British Standard 1016. Part 10 (1960): Analyst, 86, 360 (1961). 2. Bigois, M : Talanta, 19, 157 (1972). 3. Analytical Methods Committee: Analyst, 100, 54 (1975). 4. Wilson, A. D., Lewis, D. T.: Analyst, 88, 510 (1963). 5. Stefanac, Z.: Mikrochimica Acta, 1115 (1962). 6. Puschel, R., Stefanac, Z.: Mikrochimica Acta, 1108 (1962). 7. Griepink, B., Krijgsman, W.: Mikrochimica Acta, 574 (1968). 8. Sliepcevic, Z., Siroki, M., Stefanac, Z.: Mikrochimica Acta, 945 (1973). 9. Celon, E., Degetto, S., Marangoni, G., Sindellari, L.: Mikrochimica Acta, I, 113 (1976).

10. Griepink, B., Krijgsman, W., Leemaers-Smeets, A. J. M. E., Slanina, J., Cuijpers, H.. Mikrochimica Acta, 1018 (1969).

11. Sarjit Singh Sandu, Sarvinder Singh Pahil, Krishan Dev Sharma: Talanta, 20, 329 (1973). 12. Pahil Sarvinder Singh, Sharma Krishan: Indian J. Chem. 12,1316; Ref., Anal. Abstr., 29, 3C8

(1975). 13. Hassan, S. S. M., Elsayes, M. B.: Mikrochimica Acta, 801 (1973). 14. Fictchelt, A. W., Hunter Daughtrey, E. Jr., Mushak, P.: Anal. Chim. Acta, 79, 93 (1975). 15. Mamta, T., Sudok, G.: Anal Chim. Acta, 11, 37 (1975). 16. Analytical Methods Committee: Analyst, 85, 629 (1960). 17. Ingram, G.: Methods of Organic Elemental Microanalysis. Chapman and Hall, London, 1962,

pp. 293-296.

11. Determination silicon

In the analysis of organic silicon compounds, we have to consider their special properties. Many of them are gases or are volatile. In the digestion we cannot use glass or porcelain dishes because acids and especially alkalis dissolve considerable amounts of silica.

Non-volatile silicon compounds may be digested in a platinum dish by heating with a mixture of concentrated suphuric acid and peroxydisulphate, according to Luskina et al. [1] . The dehydrated silicic acid is filtered and determined gravimetrically. The other elements (P, Ti) can be determined in the filtrate.

The most suitable method is digestion in a nickel bomb with sodium peroxide [2-5] or potassium hydroxide [6] . Volatile compounds are digested in a gelatine capsule.

28 403

Oxygen flask combustion in a glass vessel is not suitable, but Reverchon and Legrand recommend combustion of the sample in a 1 d m 3 nickel flask [7] . For the quantitative determination of silicon the gravimetric method on the micro-scale is useful. The precipitated silicic acid is heated to give silicon dioxide and weighed. Sometimes the silica is removed by volatilization with hydrogen fluoride, and silicon is determined from the difference in weights [1] . A better stoichiometric factor (0.012) is obtained if the silicic acid is precipitated and weighed in the form of quinoline molybdosilicate, ( C 9 H 4 N ) 4 • H4Si(Mo3O10)4 [2] . According to Fritz and Burdt [8] the —SiH and — S i C 6 H 5 groups react quantitatively with bromine dissolved in acetic acid. The excess of bromine is titrated iodimetrically.

For the spectrophotometric determination of silicon compounds two methods are available. In strongly acidic solutions containing hydrochloric acid, a yellow complex is formed with ammonium molybdate. The reaction is not very sensitive but is reliable. Much more sensitive is the method based on the formation of molybdenum blue in a less acidic solution. The first method was recommended by Terentev et al. [5] for the analysis of organic silicon compounds containing fluorine, because fluorine does not interfere in the formation of the yellow complex but it does affect the formation of the colour of molybdenum blue with silicon. If the compound does not contain fluorine, in the presence of other elements (e.g., aluminium) the spectrophotometric method based on the formation of molybdenum blue is applicable [3, 6, 7] . Garzo et al. [9] carried out the analysis of methylchlorosilane compounds in the following way. The products from a gas chromatographic separation process are conducted into a flowing potassium chloride solution and the change in conductivity, caused by hydrogen chloride formed in the hydrolysis process is measured. For the separation it was found that for moistening the Celite column 10% Apiezon is better than dioctyl phthalate [10] . Smith [11] published a review in which problems in the analysis of silanes and siloxanes are detailed.

References to Section 8.11

1. Luskina, B. M., Terent'ev, A. P., Gradskova, N. A.: Zhur. Anal. Khim., 20, 990 (1965); Ref., Anal. Abstr., 14, 2584 (1967).

2. Christopher, A. J., Fennel, T. R. F. W.: Talanta, 12, 1003 (1965). 3. Terent'ev, A. P., Bond are vskaya, E. A., Gradskova, N. A., Kroptova, E. D.: Zhur. Anal.

Khim. 22, 454 (1967); Ref., Z. anal. Chen., 240, 61 (1968). 4. Gradskova, N. A., Bondarevskaya, E. A., Terentev, A. P.: Zhur. Anal. Khim., 28,1846 (1973);

Ref., Anal. Abstr., 28, 106 (1975).

404

5. Terentev, A. P., Gradskova, N. A., Bondarevskaya, P. N., Kulesova, O. D.: Zhur. Anal. Khim., 26, 1850 (1971); Ref.,Z. anal. Chem., 261, 223 (1972).

6. Shanina, T. M., Gelman, N. E., Kipaerenko, L. M.: Zhur. Anal. Khim., 20, 118 (1965); Ref., Anal. Abstr., 13, 3602 (1966).

7. Reverchon, R., Legrand, Y.: Chim. analytique, 4 7 , 134 (1965); Ref, Anal. Abstr., 13, 4179 (1966).

8. Fritz, G , Burdt, H.: Z. anorg. Chem., 317, 35 (1962). Ref., Anal. Abstr., 10, 1840 (1963). 9. Garzo, T., Till, F., Till, I.: Magyar Kern. Folyoirat, 68, 327 (1962).

10. Garzo, G., Till, F.: Talanta, 10, 583 (1963). 11. Smith, J. C. B.: Analyst, 85, 465 (1960).

12. Determination of boron

Some organic boron compounds are very reactive and may even be pyrophoric, but others are stable and difficult to decompose.

Wet digestion in a Kjeldahl flask equipped with a reflux condenser using nitric, perchloric and sulphuric acids is practical if the boron content is low and thus larger amounts have to be used for analysis. This method is unsuitable for the decomposition of very reactive compounds. Shaken and Braman [1] oxidized boron compounds in a Carius bomb with fuming nitric acid, while Strahm and Hawthorne [2] oxidized them with trifluoroper-oxyacetic acid to give boric acid. Non-volatile boron compounds can be fused in a platinum crucible by melting them with sodium carbonate. It may also be carried out in a metal bomb with sodium peroxide, while volatile compounds can be weighed in a gelatine capsule. Kuck and Grim [3] devised a semimicro bomb in which a vigorous reaction between the sample and sodium peroxide may be avoided. The solution obtained after the fusion contains large amounts of sodium, which may interfere in the subsequent analytical reaction. In such a case sodium can be removed by ion exchange.

Corner [4] proposed oxygen flask combustion. Yasuda and Rogers [5] found this method to be suitable for the mineralization of most organic boron compounds. Mazzeo-Far ina [6] applied this method to volatile compounds using a methylcellulose capsule and mixing the sample with sodium carbonate and glucose. Borate was then titrated by the usual alkalimetric method. Schreiber and Frei [7] obtained too low results with oxygen flask combustion for fluorine-containing organic boron compounds, but found Wickbold combustion to be satisfactory.

The best known method for the determination of orthoboric acid formed during the decomposition is titration of the medium strong complex acid formed with mannitol, using an ultramicro burette for the titration with 0.1 N sodium hydroxide solution. The end-point is detected with glass and calomel electrodes, titrating to pH 8.6, the total volume is about 50 c m 3 and the error

28* 405

of the titration is ±0.01 c m 3 , which is equivalent to about 1 \ig of boron. Fluorine interferes in the titration as hydrogen fluoride and boric acid are titrated together.

Some reagents are available for the spectrophotometric determination of boron, including curcumin [8] , quinalizarin [9] , 5-benzamido-6-chloro-l,l '-bis(anthraquinone)amine [10] and 5-p-toluidino-l,l-bis(anthraquinone)-amine, which are suitable for the determination of 1-30 |*g of boron. If it is necessary to separate the boron from the solution obtained after decomposition or combustion, it can be removed by distillation in the form of methyl borate. Ion-exchange chromatography can also be used [11].

In Volume IB of Comprehensive Analytical Chemistry (pp. 593-594), Sykes has described a procedure for the titration of boric acid, in the presence of mannitol, with 0.01 N sodium hydroxide solution following Kjeldahl digestion.

References to Section 8.12

1. Shaken, D. G., Braman, R. S.: Anal. Chem., 33, 893 (1961). 2. Strahm, R. D., Hawthrone, M. F.: Anal. Chem., 32, 530 (1960). 3. Kuck, J. A., Grim, E. C : Z. anal. Chem., 172, 140 (1960). 4. Corner, M.: Analyst, 84, 41 (1959). 5. Yasuda, S. K., Rogers, N. R.: Microchem. J., 4, 155 (1960). 6. Mazzeo-Farina, A.: Farmaco,Ed.Scient.,28(11)937 (1973);Ref., Anal. Abstr.,27,140 (1974). 7. Schreiber, B., Frei, R. W.: Mikrochimica Acta, 1, 219 (1975). 8. Spicer, G. S., Strickland, J. D. H.: Anal. Chim. Acta, 18, 231 (1958). 9. Johnson, E. A., Toogood, M. J.: Analyst, 79, 493 (1954).

10. Grob, R. L., Joe, J. H.: Anal. Chim. Acta, 14, 253 (1956). 11. Wolszon, J. D., Hayes, R. J.: Anal. Chem., 29, 829 (1957).

13. Determination of selenium and tellurium

Organic selenium compounds can be digested with an oxidizing acid or a mixture of acids. After their treatment selenium can be precipitated with reduction. Selenium dioxide is formed on combustion in an empty tube containing oxygen [1 , 2] . In the oxygen flask combustion bromine water is used for absorption [3, 4 ] . The quantitative determination of selenate ions can be effected by iodimetric titration [1 ,3 ,4 ] , or spectrophotometrically with 3,3'-diaminobenzidine reagent [2, 5]. Stefanac et al. [6] compared the results of four different methods suggested for the determination of selenium. Tellurium is rarely found in organic compounds. It can be determined by using methods similar to those for selenium.

406

Detailed procedures for the determination of selenium and tellurium are given in Vol. IB of Comprehensive Analytical Chemistry on pp. 612-618 and 621-623, respectively.

References to Section 8.13

1. Stefanac, Z , Rakovic, Z.: Mikrochimica Acta, 81 (1965). 2. Noburu Kunimine, Hisakazu Ugajiu: J. Pharm. Soc. Japan, 83, 59 (1963); Ref., Anal. Abstr.,

11, 184 (1964).

3. Ihn, W , Hesse, G , Neuland, P.: Mikrochimica Acta, 628 (1962). 4. Meyer, E , Shaltiel, N.: Mikrochimica Acta, 580 (1960). 5. Kelleher, W. J., Johnson, M. J.: Anal. Chem., 33, 1429 (1961). 6. Stefanac, Z., Tomaskovic, M , Bregovec, I.: Microchem. J., 16, 226 (1971).

14. Determination of metals in organic compounds

Organometallic compounds are being increasingly used in many applications, and so the determination of metals in such samples on the micro-scale is becoming a routine task. Metal ions of some organic compounds may react directly in solution with certain reagents (metals which form insoluble sulphides or stable coloured complexes), but most of them have to be digested first, and the metals can be determined by inorganic microanalytical methods.

We obtain the pure metal from organic silver compounds of gold and platinum, as well as from cobalt and nickel compounds, if after pyrolytic decomposition the residue is heated in a stream of hydrogen to reduce oxides if we heat it in the Pregl's tube furnace (Fig. 49). Heating iron, aluminium, copper, tin, magnesium, chromium and zinc compounds in air gives metal oxides with stoichiometric compositions. Their determination is possible by measuring the decrease in weight of the sample, even on the ultramicro-scale ( < 100 ng). Oxides of non-stoichiometric composition are formed from some metals, and their metal content is determined by the "sulphate ash" method. The sample is weighed into a small platinum or porcelain crucible and evaporated with concentrated sulphuric acid, the excess of the acid is

Fig. 49. Ashing tube furnace according to Pregl

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removed, and the metal content calculated from the weight of the metal sulphate. By this means one can analyze sodium, potassium, lithium, magnesium, calcium, barium, strontium, cadmium, manganese and lead compounds. Lead sulphate must not be heated above 400°C, but the others can be heated to 500-750°C. Some metal oxides ( A 1 2 0 3 ) and metal sulphates ( L i 2 S 0 4 ) are hygroscopic. Organic mercury compounds cannot be miner-alized by dry ashing, as they are volatile.

Organic metal compounds can be digested by other methods, e.g., with oxidizing acids or a mixture of acids, fusing with oxidizing reagents such as sodium peroxide or by oxygen flask combustion [1 -4 ] . These methods are preferable to ashing if the amount which remains is not sufficient for accurate weighing, or its composition is questionable. Claus and Kriiger [5] avoided digestion when using an X-ray fluorescence method with internal standards. Anderson et al. [6] suggested an X-ray spectrometric method for the analysis of organic compounds containing, in addition to the metal, non-metallic elements such as sulphur and phosphorus.

Penic et al. [7] used oxygen flask combustion for organic calcium compounds; calcium ions were determined in the solution spectrophoto-metrically using glyoxal bis(2-hydroxyanil) reagent. Kovac et al. [8] determined calcium by atomic-absorption spectroscopy after oxygen flask combustion. The Analytical Methods Committee [9] proposed an atomic-absorption spectroscopic method for the determination of cadmium compounds. Crompton [10] used an iodimetric method for the analysis of aluminium compounds. The alkyl group of such compounds reacts with iodine in a solution buffered by hydrogen carbonate, and so they can be titrated.

Duda et al. [11] suggested a neutron activation method with fast neutrons for the analysis of organic germanium compounds. Shanina et al. [12] recommended a spectrophotometric method after oxygen flask combustion. The reagent Resarson [5-chloro-3-(2,4-dihydroxyphenylazo)-2-hy-droxybenzenearsonic acid] is used. A spectrophotometric method can be based on the formation of a germanium-molybdenum complex. Watson and Eastham [13] titrated organic lithium compounds such as benzyl- and phenyl-lithium with oscillometry end-point detection directly with acetone solution dissolved in hydrocarbons.

Henderson and Snyder [14] developed a fast spectrophotometric method for the determination of organic lead compounds. Triethyl- and diethyl-lead are determined as their dithizonates formed in chloroform solution. The same authors [15] described a method for the determination of organic lead compounds in air. The air sample is drawn through an absorber containing crystalline iodine and the lead is determined spectrophotometrically in the form of dithizonate. Chromy and Vrestal [16] described a complexometric

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method for the analysis of organic tin compounds. Seidlitz and Geyer [17] digested such compounds with a mixture of chloric and hydrochloric acids in a stream of carbon dioxide or nitrogen prior to complexometric titration. Reverchon [18] used oxygen flask combustion, metallic tin, tin(II) oxide or tin(I V) oxide being formed from the organic tin compound. After dissolution, tin was reduced to the + 2 state with hypophosphoric acid and titrated with potassium iodate solution.

A spectrophotometric method was described by Frankel et al. [19] for the determination of tin, in which an alcoholic solution the organic tin(II) compounds reduce isatin to dihydroxyindole, which is colourless in the presence of azobisisobutyronitrile. Under the same conditions ninhydrin is reduced by organic tin hydride but not by other tin compounds, and in this instance 2-hydroxyindane-l,3-dione is formed, which has a blue-pink colour in the absence of air, and this colour is measured. Mark [20] oxidized organic tin compounds with sulphuric acid and hydrogen peroxide and the tin(IV) ions formed were determined spectrophotometrically after the extraction of the ternary tin(IV)-chloride-oxine complex with chloroform.

Sahla and Taleb [21] published a method for the analysis of organic cobalt, manganese and titanium compounds after oxygen flask combustion. The oxinates of metal ions were precipitated and weighed gravimetrically. After the dissolution of oxinates, selective determination of the elements was carried out. In the analysis of organic titanium and titanium-silicon compounds, Kreskov et al. [22] compared gravimetric, spectrographic and volumetric (complexometric) methods and the results obtained agreed well. Terenteva and Pruslina [23] described a method for the analysis of organic rhenium compounds. The organic compounds (e.g., bromodicarbonylcyclo-pentadienylrhenium) were mixed with potassium chlorate and burned in an oxygen flask. The products were absorbed in an alkaline solution and oxidized with hydrogen peroxide to rhenium(VII), which was determined by a polarographic method.

Strukova et al. [24] described a method for the analysis of organic palladium compounds. The compound was decomposed in a nickel dish with sodium peroxide, then palladium was determined in a hydrochloric acid solution spectrophotometrically with 8-mercaptoquinoline. Macquet et al. [25] published a method for the determination of metal-organic platinum complexes without destruction of the complexes, in solution and in an acetylene-air flame. They used complex platinum salts { ( N H 4 ) 2 [ P t C l 4 ] , K 2 [ P t C l 4 ] , K 2 [ P t C l 6 ] } and L a 2 0 3 to prepare the standard solutions and dry potassium phosphate which was dried at 120°C for 24 hours. The atomic-absorption measurement can be carried out directly with the solution of the complex in an air-acetylene flame. By this means 1-100 ppm of platinum was

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determined. The sensitivity of the method is 1.2 ppm and the detection limit is 0.05 ppm.

Organic mercury compounds require special mineralization and determination methods, as not only the metal but nearly all of its organic and inorganic compounds are volatile. The first method was described by Boetius [26] using the Pregl ignition method. He absorbed the vapour of mercury on a gold foil for the determination of carbon and hydrogen. Mitsui et al. [27] described a similar method, but the vapour of mercury was adsorbed on granulated silver at 40-100°C and the change in weight was measured. Anderson et al. [28] used a thin gold film for the collection of mercury vapour. The film was subsequently heated to 500°C and mercury was determined by absorption spectrometry.

Kinoshita and Hozumi [29] digested the mercury compounds with a mixture of sulphuric, nitric and orthophosphoric acids and determined mercury ions in the solution by complexometric titration. A wet digestion method was proposed by the Analytical Methods Committee [30], mercury ions subsequently being determined by spectrophotometry using the dithizone method. This method is suitable for the determination of less than 0.5 |ig of mercury. Feldman [31] corrected the earlier method of Smith [32], which proposed the digestion of organic mercury compounds (and other metal compounds) with perchloric acid. Feldman's variant is faster.

Gouverneur and Hoedeman [33] burned organic mercury compounds in an oxygen flask and titrated the absorbing solution with a solution of sodium diethyldithiocarbamate. Using this method one can determine mercury and chloride ions in the presence of each other, as the chloride ions can be titrated selectively with silver nitrate. Donner [34] described the gravimetric determination of the mercury content of organic compounds. After oxygen flask combustion, from the nitric acid absorbing solution the compound [ C o ( N H 3 ) 6 ] [ H g ( S 2 O 3 ) 3 ] 3 . 1 0 H 2 O was precipitated with 0.1 mole /dm 3

hexaamminecobalt(III) chloride solution and the precipitate was weighed. Kinoshita [35], after a wet digestion, measured the mercury content of the solution using a complexometrix method. For the determination of very low mercury contents of organic compounds, the method of Fujita et al. [36] is very useful. They burned about 1 g of a sample (e.g., rice) in a 300-cm 3

chrome-nickel bomb. Mercury (II) ions were determined spec-trophotometrically in the nitric acid absorbing solution using dithizone as reagent.

A good summary of mainly classical methods for the digestion and determination of organic mercury compounds can be found in Vol. IB of Comprehensive Analytical Chemistry on pp. 599-606. Dixon [37] published a good review of the analysis of organic mercury compounds.

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References to Section 8.14

1. Satoshi Mizukami, Tadayashi Ieki: Mikrochimica Acta, 147 (1966). 2. Macdonald, A. M. G , Sirichanya, P.: Microchem. J , 14, 199 (1969). 3. Shizuo Fujiwara, Anal. Chem., 40, 2031 (1968). 4. Sahla, A. B , Bishara, S. W , Hassan Ramodan, A.: Anal. Chim. Acta, 73, 209 (1974). 5. Claus, K. H., Kruger, C : Z. anal. Chem., 262, 257 (1972). 6. Anderson, S. J., Brown, D. S , Norbury, A. H.: J. Organometall Chem., 64 (3), 301 (1974); Ref.,

Anal. Abstr., 27, 1965 (1974). 7. Penic, J , Bregovec, I., Stefanac, Z , Slipcievic, Z.: Microchem. J., 18, 596 (1973). 8. Kovac, V , Tonkovic, M., Stefanac, Z.: Microchem. J., 19, 37 (1974). 9. Analytical Methods Committee: Analyst, 100, 761 (1975).

10. Crompton, T. R.: Analyst, 91, 374 (1966). 11. Duda, I , Obtemperanskaya, S. I , Dudova, I. V.: Zhur. Anal. Khim., 27, 373 (1972); Ref, Z.

anal. Chem., 263, 147 (1973). 12. Shanina, T. M , Gelman, N. E , Bychkova, T. V.: Zhur. Anal. Khim., 28, 2424 (1973). 13. Watson, S. C , Eastham, J. F.: Anal. Chem., 39, 171 (1967). 14. Henderson, S. R, Snyder, L. J.: Anal. Chem., 33, 1172 (1961). 15. Snyder, L. J , Henderson, R.: Anal. Chem., 33, 1175 (1961). 16. Chromy, V , Vrestal, J.: Chemicke Listy, 60, 1537 (1966); Ref, Anal. Abstr., 15, 800 (1968). 17. Seidlitz, H. J , Geyer, R.: Z. Chem., 4, 468 (1964); Ref, Z. anal. Chem., 222, 404 (1966). 18. Reverchon, R.: Chim. analytique, 47, 70 (1965); Ref, Anal. Abstr., 13, 4180 (1966). 19. Frankel, M , Wagner, D , Gerstner, D , Zilkha, A.: Israel J. Chem., 4, 183 (1966); Ref, Anal.

Abstr., 15, 1445 (1968). 20. Mark, I. L , Talanta, 22, 387 (1975). 21. Sahla, A. B , Abo Taleb, S. A.: Microchem. J., 18, 502 (1973). 22. Kreskov, A. P , Myalaeva, L. V , Kuckarev, E. A , Satunova, T. G.: Zhur. Anal. Khim., 20,

1325 (1965); Ref, Z. anal. Chem., 233, 278 (1968). 23. Terenteva, E. A . Pruslina, I. M.: Zhur. Anal. Khim., 28 (12) 2352 (1973); Ref, Anal. Abstr.,

29, IC8 (1975). 24. Strukova, M. P , Kaschinischeva, I. I , Druzhinina, V. V.: Zhur. Anal. Khim., 28, 819

(1973); Ref, Anal. Abstr., 26, 90 (1974). 25. Macquet, P. J, Hubert, I , Theophanides, T.: Anal. Chim. Acta, 72, 251 (1974). 26. Boetius, M.: J. Pract. Chem., 151, 279 (1938). 27. Tetsuo Mitsui, Keichiri Yoshikawa, Yosudo Sakai: Microchem. J., 7, 160 (1963). 28. Anderson, D. H , Evans, J. H , Murphy, J. J , White, W. W.: Anal. Chem., 43, 1510 (1971). 29. Kinoshita, S, Hozumi, K.: Microchem. J., 8, 79 (1964). 30. Analytical Methods Committee: Analyst, 90, 515 (1965). 31. Feldman, C : Anal. Chem., 46, 1606 (1974). 32. Smith, G.: Anal. Chim. Acta, 17, 173 (1957). 33. Gouverneur, P , Hoedeman, W.: Anal. Chim. Acta, 30, 519 (1964). 34. Donner. R.: Z. Chem., 5, 466 (1965); Ref, Z. anal. Chem., 233, 277 (1968). 35. Kinoshita, S.: Microchemical J., 8, 79 (1964). 36. Fujita, M , Takeda, Y , Terao, T , Hoshino, O , Whita, T.: Anal. Chem., 40, 2042 (1968). 37. Dixon, J. P.: Modern Methods in Organic Microanalysis. D. Van Nostrand C o , London,

1968. pp. 188-89.

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