In Section 8B, we considered the statistics of sampling and sample handling. In this chapter, we discuss some of the details of preparing laboratory samples. In addition, the influence of

moisture on samples and the determination of water in samples are explored.

36A PrePArIngLAborAtorySAmPLeSIn Section 8B-4, we presented the statistical considerations for reducing the particle size of the gross sample so as to obtain a laboratory sample. In the following section, some of the specific techniques used are described.

36A-1 Crushing and Grinding SamplesA certain amount of crushing and grinding is usually required to decrease the particle size of solid samples. Because these operations tend to alter the composition of the sample, the particle size should be reduced no more than is required for homogeneity (see Section 8B-4) and ready attack by reagents.

Several factors can cause significant changes in sample composition as a result of grinding. The heat inevitably generated can cause losses of volatile components. In addition, grinding increases the surface area of the solid and thus increases its suscep-tibility to reaction with the atmosphere. For example, it has been observed that the iron(II) content of a rock may be decreased by as much as 40% during grinding—apparently a direct result of the iron being oxidized to the 13 state.

Frequently, the water content of a sample is altered substantially during grinding. Increases are observed as a consequence of the increased specific surface area that accompanies a decrease in particle size (page 287). The increased surface area leads to greater amounts of adsorbed water. For example, the water content of a piece of porcelain changed from 0 to 0.6% when it was ground to a fine powder.

In contrast, decreases in the water content of hydrates often take place during grinding as a result of localized frictional heating. For example, the water content of

Crushing and grinding the sample often change its composition.

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The particle size of laboratory samples is often reduced prior to analysis by crushing and grinding operations. The techniques used in the laboratory are similar to those used in large-scale operations, such as the V-mixer/grinder used in a uranium plant shown in the photo. A V-mixer for laboratory use is described in Section 36A. In addition, this chapter considers several other methods of preparing samples for analysis, including various pulverizing and mixing methods. The chapter also considers the forms that moisture takes in solid samples and the methods of drying these samples.

PreparingSamplesforAnalysisChApTer 36

Carl Iwasaki/Time Life Pictures/Getty Images

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36A Preparing Laboratory Samples  971

gypsum (CaSO4 · 2H2O) decreased from about 21 to 5% when the compound was ground to a fine powder.

Differences in hardness of the component can also introduce errors during crushing and grinding. Softer materials are ground to fine particles more rapidly than are hard ones and may be lost as dust as the grinding proceeds. In addition, flying fragments tend to contain a higher fraction of the harder components.

Intermittent screening often increases the efficiency of grinding. Screening involves shaking the ground sample on a wire or cloth sieve that will pass particles of a desired size. The residual particles are then returned for further grinding; the operation is repeated until the entire sample passes through the screen. The hardest materials, which often differ in composition from the bulk of the sample, are last to be reduced in particle size and are thus last through the screen. Therefore, grinding must be continued until every particle has been passed if the screened sample is to have the same composition as it had before grinding and screening.

A serious contamination error can arise during grinding and crushing due to mechanical wear and abrasion of the grinding surfaces. Even though these surfaces are fabricated from hardened steel, agate, or boron carbide, contamination of the sample is nevertheless occasionally encountered. The problem is particularly acute in analyses for minor constituents.

Tools for Reducing SizeSeveral different tools can be used for reducing the particle size of solids, including jaw crushers and disk pulverizers for large samples containing large lumps, ball mills for medium-sized samples and particles, and various types of mortars for small amounts of material.

The ball mill is a useful device for grinding solids that are not too hard. It consists of a porcelain crock with a capacity of perhaps two liters that can be sealed and rotated mechanically. The container is charged with approximately equal volumes of the sam-ple and flint or porcelain balls with a diameter of 20 to 50 mm. Grinding and crushing occur as the balls tumble in the rotating container. A finely ground and well-mixed powder can be produced in this way.

A commercial laboratory mixer/mill is shown in Figure 36-1 along with several stainless steel mixing vials. The unit has motion along three axes for vigorous grinding of samples. Two or three vials can be accommodated simultaneously. The Plattner diamond mortar, shown in Figure 36-2, is used for crushing hard, brittle materials. It is constructed of hardened tool steel and consists of a base plate, a removable collar, and a pestle. The sample is placed on the base plate inside the collar. The pestle is then fitted into place and struck several blows with a hammer, therefore reducing the solid to a fine powder that is collected on glazed paper after the apparatus has been disassembled.

Crushing and grinding must be continued until the entire sample passes through a screen of the desired mesh size.

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Mechanical abrasion of the surfaces of the grinding device can contaminate the sample.

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Figure36-1 A commercial mixer/mill for pulverizing and blending samples. The unit in the back can hold two 0.75sor three 0.5s sample vials. Plastic vials or the stainless steel vials in front can be used along with plastic or stainless steel mixing balls.Co

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972  ChAPter 36 Preparing Samples for Analysis

Unless otherwise noted, all content on this page is © Cengage Learning.

36A-2 Mixing Solid SamplesIt is essential that solid materials be thoroughly mixed to ensure random distribution of the components in the analytical samples. A common method for mixing pow-ders involves rolling the sample on a sheet of glazed paper. A pile of the substance is placed in the center and mixed by lifting one corner of the paper enough to roll the particles of the sample to the opposite corner. This operation is repeated many times, with the four corners of the sheet being lifted alternately.

Effective mixing of solids is also accomplished by rotating the sample for some time in a ball mill or a twin-shell V-blender. The latter consists of two connected cyl-inders that form a V-shaped container for the sample. As the blender is rotated, the sample is split and recombined with each rotation, leading to highly efficient mixing.

It is worthwhile noting that long-standing, finely ground homogeneous materials may segregate on the basis of particle size and density. For example, analyses of layers of a set of student unknowns that had not been used for several years revealed a regular variation in the analyte concentration from top to bottom of the container. Apparently, segregation occurred as a consequence of vibrations and of density differ-ences in the sample components.

36b moIStureInSAmPLeSLaboratory samples of solids often contain water that is in equilibrium with the at-mosphere. As a consequence, unless special precautions are taken, the composition of the sample depends on the relative humidity and ambient temperature at the time it is analyzed. To cope with this variability in composition, it is common practice to remove moisture from solid samples prior to weighing or, if this removal is not pos-sible, to bring the water content to some reproducible level that can be duplicated later if necessary. Traditionally, drying was accomplished by heating the sample in a conventional oven or a vacuum oven or by storing in a desiccator at a fixed humidity. The drying processes were continued until the material had become constant in mass. These drying treatments were time consuming, often requiring several hours or even several days. In order to speed up sample drying, microwave ovens or infra-red lamps are currently being used for sample preparation.1 Several companies now offer equipment for this type of sample treatment (see Section 37C).

An alternative to drying samples before beginning an analysis involves determining the water content at the time the samples are weighed for analysis so that the results can be corrected to a dry basis. In any event, many analyses are preceded by some sort of preliminary treatment designed to take into account the presence of water.

36B-1 Forms of Water in SolidsWater can be essential or nonessential water in solids.

Essential WaterEssential water forms an integral part of the molecular or crystalline structure of a compound in its solid state. Therefore, the water of crystallization in a stable solid hydrate (for example, CaC2O4 · 2H2O and BaCl2 · 2H2O) qualifies as a type of essential water. Water of constitution is a second type of essential water and is found

Finely ground materials may segregate after standing for a long time.

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Essential water is the water that is an integral part of a solid chemical compound in a stoichiometric amount in a stable solid hydrate, such as BaCl2 · 2H2O.

1For a comparison of the reproducibility of these various methods of drying, see E. S. Berry, Anal. Chem., 1988, 60, 742, DOI: 10.1021/ac00159a003.

Figure36-2 A Plattner diamond mortar.

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36B Moisture in Samples  973

in compounds that yield stoichiometric amounts of water when heated or otherwise decomposed. Examples of this type of water are found in potassium hydrogen sulfate and calcium hydroxide, which when heated come to equilibrium with the moisture in the atmosphere, as shown by the reactions

2KHSO4(s) 8 K2S2O7(s) 1 H2O(g)

Ca(OH)2(s) 8 CaO(s) 1 H2O(g)

Nonessential WaterNonessential water is retained by the solid as a consequence of physical forces. It is not necessary for characterization of the chemical constitution of the sample and, therefore, does not occur in any sort of stoichiometric proportion.

Adsorbed water is a type of nonessential water that is retained on the surface of solids. The amount adsorbed is dependent on humidity, temperature, and the spe-cific surface area of the solid. Adsorption of water occurs to some degree on all solids.

A second type of nonessential water is called sorbed water and is encountered with many colloidal substances, such as starch, protein, charcoal, zeolite minerals, and silica gel. In contrast to adsorbed water, the quantity of sorbed water is often large, amounting to as much as 20% or more of the total mass of the solid. Solids contain-ing even this amount of water may appear to be perfectly dry powders. Sorbed water is held as a condensed phase in the interstices or capillaries of the colloidal solid. The quantity contained in the solid is greatly dependent on temperature and humidity.

A third type of nonessential moisture is occluded water, liquid water entrapped in microscopic pockets spaced irregularly throughout solid crystals. Such cavities of-ten occur in minerals and rocks (and in gravimetric precipitates).

36B-2 Temperature and Humidity Effects on the Water Content of SolidsIn general, the concentration of water in a solid tends to decrease with increasing temperature and decreasing humidity. The magnitude of these effects and the rate at which they manifest themselves differ considerably according to the manner in which the water is retained.

Compounds Containing Essential WaterThe chemical composition of a compound containing essential water is dependent on temperature and relative humidity. For example, anhydrous barium chloride tends to take up atmospheric moisture to give one of two stable hydrates, depending on temperature and relative humidity. The equilibria involved are

BaCl2(s) 1 H2O(g) 8 BaCl2 · H2O(s)

BaCl2 · H2O(s) 1 H2O(g) 8 BaCl2 · 2H2O(s)

At room temperature and at a relative humidity between 25% and 90%, BaCl2 · 2H2O is the stable species. Since the relative humidity in most laboratories is well within these limits, the essential water content of the dihydrate is ordinarily independent of atmospheric conditions. Exposure of either BaCl2 or BaCl2 · H2O to these conditions causes compositional changes that ultimately lead to formation of the dihydrate. On a very dry winter day (relative humidity , 25%), however, the situation changes; the dihydrate becomes unstable with respect to the atmosphere, and a molecule of water is lost to form the new stable species BaCl2 · H2O. At relative humidities less than

Water of constitution is water that is formed when a pure solid is decomposed by heat or other chemical treatment.

Nonessential water is the water that is physically retained by a solid.

Adsorbed water resides on the surface of the particles of a material.

Sorbed water is contained within the interstices of the molecular structure of a colloidal compound.

Occluded water is trapped in random microscopic pockets of solids, particularly minerals and rocks.

Relative humidity is the ratio of the vapor pressure of water in the atmo-sphere to its vapor pressure in air that is saturated with moisture. At 25°C, the partial pressure of water in satu-rated air is 23.76 torr. Thus, when air contains water at a partial pressure of 6 torr, the relative humidity is6.0023.76

5 0.253 (or the percent relative

humidity is 25.3%).

the essential water content of a compound depends on the temperature and relative humidity of its surroundings.

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974  ChAPter 36 Preparing Samples for Analysis

Unless otherwise noted, all content on this page is © Cengage Learning.

about 8%, both hydrates lose water, and the anhydrous compound is the stable spe-cies. Therefore, we can see that the composition of a sample containing essential water depends greatly on the relative humidity of its environment.

Many hydrated compounds can be converted to the anhydrous condition by oven drying at 100°C to 120°C for an hour or two. Such treatment often precedes an analysis of samples containing hydrated compounds.

Compounds Containing Adsorbed WaterFigure 36-3 shows an adsorption isotherm in which the mass of the water adsorbed on a typical solid is plotted against the partial pressure of water in the surrounding atmosphere. The diagram indicates that the extent of adsorption is particularly sensi-tive to changes in water-vapor pressure at low partial pressures.

The amount of water adsorbed on a solid decreases as the temperature of the solid increases and generally approaches zero when the solid is heated above 100°C. Adsorption or desorption of moisture usually occurs rapidly, with equilibrium often being reached after 5 or 10 min. The speed of the process is often observable during the weighing of finely divided anhydrous solids, where a continuous increase in mass will occur unless the solid is contained in a tightly stoppered vessel.

Compounds Containing Sorbed WaterThe quantity of moisture sorbed by a colloidal solid varies tremendously with atmospheric conditions, as shown in Figure 36-4. In contrast to the behavior of adsorbed water, however, the sorption process may require days or even weeks to attain equilibrium, particularly at room temperature. Also, the amounts of water retained by the two processes are often quite different from each other. Typically, adsorbed moisture amounts to a few tenths of a percent of the mass of the solid, whereas sorbed water can amount to 10 to 20%.

The amount of water sorbed in a solid also decreases as the solid is heated. Com-plete removal of this type of moisture at 100°C is by no means a certainty, however, as indicated by the drying curves for an organic compound shown in Figure 36-4. After this material was dried for about 70 min at 105°C, its mass apparently became constant. Note, however, that additional moisture was removed by further increasing the temperature. Even at 230°C, dehydration was probably not complete.

Compounds Containing Occluded WaterOccluded water is not in equilibrium with the atmosphere and, therefore, is insensi-tive to changes in humidity. Heating a solid containing occluded water may cause

Sorption

Adsorption

Partial pressure H2O

g H

2O r

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lid

Figure36-3 Typical adsorption and sorption isotherms.

230 C184 C

130 C

105 C

2.0

1.6

1.2

0.8

0.4

00 40 80 120 160

Heating time, min

Wat

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%

Figure36-4 Removal of sorbed water from an organic compound at various temperatures. (Reprinted (adapted) with permission from C. O. Willits, Anal. Chem., 1951, 23, 1058, DOI: 10.1021/ac60056a003. Copyright 1951 American Chemical Society.)

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36C Determining Water in Samples  975

a gradual diffusion of the moisture to the surface, where it evaporates. Frequently, heating is accompanied by decrepitation in which the crystals of the solid are suddenly shattered by the steam pressure created from moisture contained in the internal cavities.

36B-3 Drying the Analytical SampleHow we deal with moisture in solid samples depends on the information desired. When the composition of the material on an as-received basis is needed, the principal concern is that the moisture content not be altered as a result of grinding or other preliminary treatment and storage. If such changes are unavoidable or probable, it is often advantageous to determine the mass loss on drying by some reproducible proce-dure (say, heating to constant mass at 105°C) immediately after the sample is received. Then, when the time arrives for the analysis to be performed, the sample is again dried at this temperature so that the data can be corrected back to the original basis.

We have already noted that the moisture content of some substances is substan-tially changed by variations in humidity and temperature. Colloidal materials con-taining large amounts of sorbed moisture are particularly susceptible to the effects of these variables. For example, the moisture content of a potato starch has been found to vary from 10 to 21% as a result of an increase in relative humidity from 20 to 70%. With substances of this sort, comparable analytical data from one laboratory to an-other or even within the same laboratory can be achieved only by carefully specifying a procedure for taking the moisture content into consideration. For example, samples are frequently dried to constant mass at 105°C or at some other specified tempera-ture. Analyses are then performed and results reported on this dry basis. While such a procedure may not render the solid completely free of water, it usually lowers the moisture content to a reproducible level.

36C DetermInIngWAterInSAmPLeSOften, the only sure way to obtain a result on a dry basis is to determine the moisture in a set of samples taken concurrently with the samples that are to be analyzed. There are several methods of determining water in solid samples. The simplest involves de-termining the mass loss after the sample has been heated at 100°C to 110°C (or some other specified temperature) until the mass of the dried sample becomes constant. Unfortunately, this simple procedure is not at all specific for water, and large positive systematic errors occur in samples that yield volatile decomposition products (other than water) when they are heated. This method can also yield negative errors when applied to samples containing sorbed moisture (for example, see Figure 36-4). Mod-ern thermal analysis methods, such as thermogravimetric analysis, differential ther-mal analysis, and differential scanning calorimetry, are also widely used in studying the loss of water and various decomposition reactions in solid samples.2

Several highly selective methods have been developed for the determination of water in solid and liquid samples. One of these, the Karl Fischer method, is de-scribed in Section 20C-5. Several others are described in monographs devoted to water determination.3

Decrepitation is a process in which a crystalline material containing occluded water suddenly explodes during heating because of a buildup in internal pressure resulting from steam formation.

2See, D. A. Skoog, F. J. Holler, and S. R. Crouch, Principles of Instrumental Analysis, 6th ed., Ch. 31, Belmont, CA: Brooks/Cole, 2007.

3J. J. Mitchell, Jr., and D. M. Smith, Aquametry, 2nd ed., Vols. 1–3, New York: Wiley, 1977–1980; E. Scholz, Karl Fischer Titration: Determination of Water, Berlin: Springer, 1984.

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