Afialysf, September, 1974, Vol. 99, $9. 551-554 55 1

The Use of Tiron in the Microchemical Analysis of iMimerals

BY W. J. FRENCH AND S. J. ADAMS (Geology Deparfnient, Queen Mary College, University of London,

Mile End Road, London, E . l )

The complexes formed by tiron with aluminium, iron and titanium can be used to determine these elements colorirnetrically on a single aliquot of the solution of a mineral. The tiron also renders these solutions very suitable for analysis in a flame, the sodium from the tiron acting as a flame buffer and the complexing properties inhibiting chemical interferences. A single aliquot of solution can therefore be used to determine several elements and the procedure can form the basis of a scheme €or the microchemical analysis of minerals. Little is known of the use of tiron for the deter- mination of aluminium and this paper describes tests made on the usefulness of this method.

HEY has pointed out that it should be possible to carry out chemical analyses of minerals by using about 1 mg of materia1.l Recently, we have attempted to analyse grains cut from thin sections of rocks to determine a range of elements, the total amount of material available being rather less than 1 mg. In setting up the required methods it was desirable to work on a single solution if possible and two such solution techniques were considered: decomposi- tion with hydrofluoric acid in sealed plastic vessels2 and fusion with lithium metaborate.3 Use of the former technique gives solutions that are suitable for flame-photometric analysis, silicon and iron being determined colorirnetrically. However, the flame routines consume solution and have low sensitivity for aluminium and titanium, while the fluoride limits the application of colorimetric procedures. The fusion technique has fewer limitations but con- siderable errors are introduced because of the large ratio of fusion compound to mineral necessary and the impurities in the borate.

A compromise was found by using two solutions, one prepared by a conventional digestion with hydrofluoric acid - perchloric acid and the other by dissolution with hydrofluoric acid in a sealed vessel. This approach was made possible by the use of tiron (1,2-dihydroxybenzene- 3,5-&sulphonic acid, disodium salt) for the determination of iron, titanium and aluminium on a single aliquot of the solution of a mineral. This tiron - mineral solution was also found to make an ideal medium for the atomic-absorption spectrophotometric determination of mag- nesium and calcium, therefore all five elements could be determined on a single aliquot of the solution of the mineral. Silicon, sodium and potassium were determined on the second solution.

The least well known aspect of this scheme of analysis was the use of tiron in the deter- mination of aluminium. It involves working in the ultraviolet region of the spectrum and close to an organic absorption band. These conditions are not ideal but as Hey1 and Mercy and Saunders4 have pointed out, there are numerous deficiencies in most colorimetric methods for determining aluminium in silicates, those methods requiring considerable concentrations of the element being the most reliable. The possibility of using tiron for this determination was therefore investigated in detail.

DEVELOPMENT OF THE METHOD

Tiron has been widely used for the determination of iron and titanium in a variety of materials. Many of the advantages and drawbacks of the method have been described.”g One source of error is that the reagent is consumed by elements with which it forms complexes that do not absorb in the visible region. Aluminium is one such element, the absorbance of the aluminium - tiron complex being greatest at 315 nm and linear with concentration for concentrations up to at least 3 p g ml-l. Iron(II1) and titanium - tiron complexes also absorb at 315nm but aluminium absorbs more strongly than the other two complexes. The ab- sorptivities are approximately: aluminium, 440; iron(III), 245; and titanium, 192. The

@ SAC and the authors.

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552

absorbance of the iron(II1) - tiron complex is usually measured at 560 nm because at this wavelength absorption by the aluminium and titanium complexes is very low (generally regarded as being negligible). The absorbance of the titanium - tiron complex is usually measured at about 410 nm, where the absorbance due to the iron complex is at a minimum and that due to aluminium is negligible. This wavelength is also preferred when the iron is reduced, as then the absorbance due to titanium is very large compared with that of other compounds. However, there is always a small contribution from all three element - tiron complexes at each wavelength, and for high titanium to iron ratios, even the determination of iron is impaired. I t is therefore preferable to measure standards of all three elements at the three wavelengths.

Different buffer mixtures have different absorptivities. At 315 nm the reagent blank shows an increase in absorbance as the acetate molarity is increased and the effect is greater if ammonium acetate is used in place of sodium acetate. When the total acetate molarity is about 0-6, however, the absorbance is insensitive to acetate concentration so that satisfactory results can be obtained if the acetate molarity is fixed at about this level and a pH of 5.65 is maintained. The metal - tiron complex absorbance increases with the concentration of tiron but the absorbance increases only very slowly with excess of reagent. The concentration of tiron is therefore selected to give a slight excess over metal ions so that blank readings are kept to a minimum. Full colour development for all three complexes occurs within a few minutes and the absorbances have been found to be unchanged after several hours.

These conditions were applied to solutions containing aluminium equivalent to 200 pg of aluminium oxide and a range of elements that could possibly interfere in the determination (Table I). Measurements were made on a Pye Unicam SP500 spectrophotometer. Chromium, tungsten, vanadium, molybdenum and boron all showed an increase in absorbance over that due to the aluminium while the presence of fluorine reduced the absorbance. Only fluorine and chromium showed differences that were likely to be significant in rock analysis. Chrom- ium will increase the apparent amount of aluminium found and a correction would be required if samples containing substantial amounts of this element were to be determined.

FRENCH AND ADAMS: T H E USE OF TIRON [Andyst, VOl. 99

TABLE I INFLUENCE OF VARIOUS ELEMENTS ON T H E ABSORBANCE GIVEN BY 200 pg OF

ALUMINIUM OXIDE COMPLEXED WITH TIRON AND DILUTED TO loom1 Absorbance of 200pg of aluminium oxide complexed with tiron = 0.467

Element added as

MgO CaO K,O p,o, MnO Li Ba Rb

Amount added1p.g

200 200 200 100 50 200 100 200

Absorbance 0.464 0.466 0.466 0.460 0.470 0.465 0.466 0.467

Element added as

Zr co Sr B Cr w v

Amount added1p.g

100 100 100 100 100 100 100

Absorbance 0.467 0.467 0.466 0.472 0.701 0.515 0.606

Fluorine should, of course, be removed during dissolution of the sample. Large amounts of beryllium (e.g., 1Og1-1) slightly inflate the absorbance of the blank, but not sufficiently to invalidate the method if the element is used in measured amounts to suppress interference from small amounts of fluoride. Interference has not been detected from the usual minor elements in minerals, but it might occur if the substance was unusually rich in, for example, rare earths.

REAGENTS- Standard aluminium solution-Dissolve 0.8894 g of aluminium ammonium sulphate in

water that has been acidified with a few drops of sulphuric acid and dilute the solution to 1000 ml. This dilution gives a concentration equivalent to 100 pg ml-l of aluminium oxide.

Standard titanium solution-Fuse 0.20 g of Specpure titanium(1V) oxide with 1.5 g of potassium pyrosulphate in a platinum crucible. Heat the crucible gently (not above dull red heat) and when dissolution is complete, cool it and dissolve the cake in sulphuric acid

The conditions outlined above were tested on a series of rocks.

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September, 19741 I N T H E MICROCHEMICAL ANALYSIS OF MINERALS 553

(1 + 1). Dilute the solution to 2000 ml to give a concentration equivalent to 100 pg ml-1 of titanium (IV) oxide.

Standard iron solzition-Dissolve 0.4911 g of ammonium iron(I1) sulphate in 100 ml of water, and to this solution add 6 ml of sulphuric acid (1 + 1) and 0.5 ml of 100-volume hydro- gen peroxide solution. Boil the solution for a few minutes in order to expel the excess of hydrogen peroxide, then cool and dilute it to 1000 ml. This procedure gives a concentration equivalent to 100 pg ml-l of iron(II1) oxide.

Bufer solution-Dissolve 250 g of sodium acetate hydrate in water, add 11 ml of glacial acetic acid and dilute the mixture to 1000 ml.

Tiron solution-Dissolve 1-5 g of tiron (1,2-dihydroxybenzene-3,S-disulphonic acid, disodium salt) in water and dilute to 100 ml.

PROCEDURE- Rock solutions v;ith a concentration of 1 g 1-1 of rock were prepared by the method of

Riley.1° Volumes (2 ml) of each rock solution, the blank and the standard were transferred by use of a pipette into 100-ml calibrated flasks. Next, 5 ml of tiron solution were added accurately, followed, after mixing, by 30 ml of buffer solution; each mixture was then diluted to 100 ml. The absorbance of each solution was measured four times at 315 nm (in 5-mm cells), at 560 nm and 410 nm (in 20 or 40-mm cells) and the concentrations of aluminium, iron and titanium were calculated by the method of simultaneous equations.11

The procedure was applied to four standard rocks and the results given in Table I1 were obtained. Each result is the average of fourteen separate determinations, each of which represents four measurements of absorbance at each wavelength. The precisions of the iron and titanium determinations are satisfactory because of the small contributions to the total absorbance made by each at the wavelength of determination of the other. The lower pre- cision of the aluminium determination can be mitigated by increasing the number of measure- ments at 315 nm. However, in view of the difficulty normally encountered in determining aluminium in rocks (Mercy and Saunders4) the present method was regarded as being satis- factory and applicable to a wide range of materials.

TABLE I1 COMPARISON OF RESULTS (PER CENT. m/m) OBTAINED BY USING THE PROPOSED

METHOD WITH PREVIOUS RESULTS

*41,O, Fe,O, Ti0 - -7 a b C a b C a b C

Q.M.C.11 13.66 0.10 13.89 0.50 0.02 0.59 0.04 0.01 0.06 Q. 31. C. I3 13.18 0.09 13-14 16.28 0.04 16.23 2.55 0.02 2-59 Q. 31. C.M2 23.65 0.12 24-00 9-23 0.03 9.13 0.80 0.01 0.72

The standard rocks are obtainable from Dr. A. B. Poole, Geology Department, Queen Mary College, Mile End Road, London, E.l. The rock types are as follows: Q.M.C.11, granite; Q.M.C.13, doleritc; Q.M.C.MZ, pelitic schist; and (?.M.C.MS, calc-silicate rock.

Q.M.C.M3 17.54 0.08 17.65 4.58 0.03 4-54 1.00 0.01 0.91

a , This method; b, standard deviation (14 determinations) ; c, mean o f all previous results.

MICRO-ANALYTICAL PROCEDURE-

Weigh out between 250 and 500pg of each mineral as accurately as possible and digest them in 10-ml crucibles (platinum or PTFE) with ten drops of hydrofluoric acid (40 per cent. m/V) and two drops of concentrated perchloric acid. Heat each crucible with an overhead infrared heater until the fluoride has been removed and the crucible is almost dry. Cool, add about 3 ml of water and warm the crucible gently for a few minutes to complete dissolution, then transfer the solution into a 25-ml calibrated flask, keeping the volume of wash water to below 10 ml. Add accurately 1 ml of 2 per cent. m/V tiron solution and mix well. Finally, add 7.5 ml of buffer solution and dilute the mixture to 25 mi. Measure the absorbance of each solution at 315, 410, and 560 nm and calculate the percentages of aluminium oxide, titanium- (IV) oxide and iron(II1) oxide.

Aspirate the residual solution into an air - acetylene flame to determine magnesium oxide and a nitrous oxide - acetylene flame for calcium oxide. Standardise the colorimetric analysis with laboratory reagents or standard rock solutions prepared in an equivalent manner; use

Conserve as much of the solution as possible.

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554 FRENCH AND ADAMS

rock solutions to standardise thc atomic-absorption analysis. Place about 500 mg of each mineral into a weighed polycarbonate autoclave tube, then add five drops of hydrofluoric acid (40 per cent. m/V) and one drop of aqua regia. Digest the mixture in a pressure cooker for about 15 minutes, cool it, and add 3 per cent. m/V boric acid solution to bring the total mass of solution to 20 g. Determine the silica content by the method described by RileylO and sodium and potassium by use of flame photometry. Standardise the procedure with standard rock or mineral solutions.

RESULTS The proposed scheme of analysis was applied to ten portions of an amphibole previously

analysed by use of conventional techniques. In general, blanks were found to be high but the results indicate that, with care, elements can be deter- mined with a relative standard deviation of about 2 per cent. or better. As expected, alu- minium gave the greatest spread of results, but even here the results are adequate for many purposes. The processes of weighing and sampling probably contribute considerably to the errors.

The results are given in Table 111.

TABLE I11 ANALYSES (PER CENT. m/m) OF A HORNBLENDE FROM COUNTY DONEGAL

SIC), TiO, X1,0, Fe,O, MgO CaO Na,O K,O

(average of two determinations) . . . . 43.40 2.05 12-35 15.88 12.62 9-52 1.87 1-09

(average of ten determinations) . . . . 43-10 2.09 12-51 16.00 12.50 9.68 1.90 1-06

present method.. . . . . . . . . . . 0.62 0.03 0.20 0.06 0.08 0.06 0-03 0.02

Normal macrochemical technique

Present method

Standard deviation of results by

CONCLUSION Tiron - metal complexes, because of their sensitivity, can iorm the basis of a system of

partial microchemical analysis applicable to minerals in amounts of about 1 mg. The method may be applicable to a wider range of materials but the relative errors are such that further reduction in the scale of analysis is not likely to be successful. It has not been found possible to determine manganese, phosphorus or water, but iron(1I) could be determined on a further small portion of the mineral.12 The method can be considered rapid in that between six and ten samples can be analysed in two working days.

1 . 2. 3. 4. 6 . 6. 7. 8. 9.

10. 1 1 .

12.

REFERENCES Hey, 31. H., Min. Mug., 1973, No. 301, 4. French, W. J., and Adarns, S. J., Analytica Chinz. Acta, 1973, 62, 324. Medlin, J. H., Suhr, N. €I., and Bodkin, J . B., Atom. ,4bsorption Newsl., 1962, 8, 26. Mercy, E. P., and Saunders, M. J., Earth Planet. Sci. Lett., 1966, 1, 169. Yoe, J. H., and Jones, .%. L., Ind . Engng Chem., Analyt. Edn, 1944, 16, 111. Yoe, J . H., and Armstrong, A. R., Science, N.Y. , 1945, 102, 207. Corey, R., and Jackson, M., Analyt. Chem., 1953, 25, 264. Nichols, P. N. R., Analyst, 1960, 85, 452. Lacourt, A., Sommereyns, G., Degeyndt, E., Ihruh, J., and Gillard, J . , Nature, Lond., 1949, 163,

Riley, J. P., Annlytica Chim. Acta., 1958, 19, 413. Willard, H. H., Merritt, L. L., and Dean, J. A., “Instrumental Methods of Analysis,” Fourth

French, W.J., and Adams, S. J., Analyst, 1972, 97, 828.

999.

Edition, D. van Nostrand Co. Ltd., London, 1965.

Received January 17th, 1974 Accepted March 25th, 1974

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