EXTRACTION-PHOTOMETRIC METHOD FOR THE DETERMINATION OF TRACES OF ARSENIC, PHOSPHORUS, AND SILICON IN HIGH-PURITY IRON

G. A. DETKOVA, Yu. L. LEL’CHUK, R. D. GLUKHOVSKAYA, G. N. DEMBOVSKAYA

(Presented by the scientific seminar of KhTF)

For the determination of traces of arsenic, phosphorus, and silicon, photometric methods are most suitable; these methods make extensive use of the ability of these elements to form heteropoly acids. In a number of methods the intensity of the yellow color of complexes with molybdenum is measured; in others, in order to increase the sensitivity of the determination, the yellow complexes formed are reduced, yielding blue compounds characterized by a more intense color. The greatest sensitivity is exhibited by methods based on photometry of extracts of these complexes.

Methods for the separate determination of these three elements from a single weighed portion of the substance being analyzed are of particular interest.

R. I. Alekseev [1], and later a number of other authors [2—6], proposed methods for the separate determination of arsenic, phosphorus, and silicon based either on selective extraction of each of the components in the form of heteropoly acids with various extractants [2—6], or on the unequal stability of the indicated heterocomplexes with respect to the acidity of the medium [7, 8], or on different behavior toward one or another complexing reagent [9, 10].

However, all the proposed methods for the separate determination of these elements from a single weighed portion have a sensitivity not exceeding \(10^{-4}\%\). The literature contains no information on the existence of sufficiently sensitive methods for monitoring the trace content of these elements in high-purity iron from a single weighed portion.

The arsenomolybdenum, phosphomolybdenum, and silicomolybdenum blues, in the form of which arsenic, phosphorus, and silicon are usually photometered, are assigned the formulas:

\[ \mathrm{H}_7[\mathrm{As}(\mathrm{Mo}_2\mathrm{O}_7)_5\mathrm{OMo}_3\mathrm{O}_5], \qquad \mathrm{H}_7[\mathrm{P}(\mathrm{Mo}_2\mathrm{O}_7)_5(\mathrm{Mo}_2\mathrm{O}_5)]; \]

\[ \mathrm{H}_4[\mathrm{Si}(\mathrm{Mo}_2\mathrm{O}_7)_5(\mathrm{Mo}_2\mathrm{O}_5)]. \]

From these formulas it is evident that the weight content of molybdenum exceeds the weight content of arsenic by a factor of 17, phosphorus by a factor of 37, and silicon by a factor of 41.

By decomposing these heteropoly acids and determining the molybdenum in them in the form of a complex with rhodanide, it is possible to increase manyfold the sensitivity of the determination of arsenic, phosphorus, and silicon. Such an approach was previously applied in developing procedures for determining \(10^{-5}\)—\(10^{-6}\%\) arsenic and phosphorus in high-purity tin [11] and is used by us for determining traces of arsenic, phosphorus, and silicon from a single weighed portion in high-purity iron.

The method is based on decomposition of a weighed portion of iron with concentrated hydrochloric acid in the presence of hydrogen peroxide, extraction of arsenic(III) chloride with benzene, and extraction from the solution of phosphorus with ethylo-

with ether in the form of its molybdenum complex, further extraction of silicon from the remaining solution with butanol in the form of its heteropoly acid, and subsequent determination of all three components. The procedure was first tested on solutions of pure salts. The data are given in Table 1.

Hydrochloric acid and ammonia were purified by isothermal distillation; pure hydrobromic and nitric acids were obtained by double distillation in a fluoroplastic apparatus; ammonium molybdate was purified by recrystallization from an ammoniacal-alcoholic solution; benzene, ether, and butanol by distillation.

Copper sulfate, potassium rhodanide, potassium hydroxide, and stannous chloride were used in “chemically pure” grade.

The initial standard solutions were prepared from sodium hydrogen phosphate, sodium silicate, arsenious anhydride, and ammonium molybdate of “analytically pure” and “chemically pure” grades. The titers of the solutions were established gravimetrically. Working standard solutions with contents of 10, 1, and 0.1 µg/ml were prepared on the day of use.

Analytical procedure

A 0.5–1 g portion of iron is dissolved in 10–12 N HCl in the presence of hydrogen peroxide until complete dissolution. The solution is boiled on a hot plate to decompose the excess hydrogen peroxide and remove free chlorine. After cooling, it is transferred to a separatory funnel; 20 ml of concentrated HCl, 5 drops of hydrobromic acid, and 0.03 g of solid hydrazine hydrochloride are added to reduce the arsenic, and the mixture is shaken for 1 min. The arsenic(III) chloride formed is extracted with benzene twice, using 5 ml portions, for 3 min. The benzene extracts are washed twice with concentrated hydrochloric acid, 2 ml each time, in the presence of 1–2 mg of hydrazine hydrochloride. From the benzene extract, arsenic chloride is re-extracted three times with water in 2 ml portions. The aqueous extract is transferred to a quartz dish, 6 drops of HNO₃ are added, and the solution is evaporated to dryness. The dry residue is moistened with 5–6 drops of bidistillate, 1 drop of 5 N sulfuric acid, and 1 drop of concentrated hydrochloric acid, and is evaporated until the odor is removed. The residue is transferred with bidistillate into a test tube with a ground-glass stopper so that the total volume of liquid is no more than 6 ml; 0.25 ml of a 1% solution of ammonium molybdate in 5 N sulfuric acid and 0.1 ml of 0.15% hydrazine sulfate are added, the mixture is heated for 10 min in a water bath, and the arsenomolybdenum blue in the test tube is extracted with 0.4 ml of isoamyl alcohol, which is then also transferred to a 15 ml separatory funnel. After 1 min of shaking and settling, the acid layer is drained off; the extract is washed four times with 4 ml portions of 0.5 N H₂SO₄ solution in the presence of ascorbic acid (0.1 g of ascorbic acid per 100 ml of 0.5 N H₂SO₄), and re-extracted with aqueous ammonia solution, 1.5 ml at a time, collecting the re-extracts in 25 ml volumetric flasks. To 6 ml of re-extract in a flask are added 11 ml of 6.0 N hydrochloric acid, 0.5 ml of freshly prepared 0.04% potassium rhodanide solution, and 3 N HCl to the mark; the mixture is stirred. After standing for 10 min, photometry is carried out on an FEK-N-57 with a light filter having maximum transmittance at 453 mµ, using a cuvette layer thickness of 5 cm. The determination may be completed using a standard scale. The arsenic content is obtained by multiplying the amount of molybdenum found by the coefficient 0.06006. After separation of arsenic, the solutions are evaporated under a lamp to a moist residue, which is dissolved in 10 ml of 6 N HCl, neutralized with ammonia solution, brought to pH ∼2, the contents of the dish are transferred to a separatory funnel, 2.5 ml of 6 N HCl are added while rinsing the dish with it, and 3 ml of 5% molyb-

Table 1

Checking the accuracy of the determination of arsenic, phosphorus, and silicon by the molybdenum method

Table 2

Results of the determination of arsenic, phosphorus, and silicon in a factory iron sample

\(n\) is the number of determinations.

ammonium acetate and water to 25 ml. The contents of the funnel are shaken vigorously for 1 minute; after standing for five minutes, extraction is carried out twice with 5–10 ml of ether. The two ether extracts are washed four times with 5 ml portions of 0.6 N hydrochloric acid, transferred to a 25-ml volumetric flask, the funnel is rinsed twice with 3 ml portions of ammoniacal solution, 1 ml of caustic potassium is added, and the mixture is heated on a water bath to remove the ether and decompose the phosphoromolybdenum complex. After cooling, 7 ml of 8.5 N hydrochloric acid is added; all the necessary reagents are then added and the photometry of molybdenum thiocyanate is carried out in the same way as in the determination of arsenic. The phosphorus content is obtained by multiplying the amount of molybdenum found by the coefficient 0.0269.

For the determination of silicon, after separation of phosphorus the solutions and the washing liquid are transferred to a fluoroplastic dish, heated under a lamp to remove the ether and reduce the volume to 1/3. The contents of the dish are quantitatively transferred to a separatory funnel; after cooling, the pH is adjusted to 1.5–2 with ammonia, 4 ml of 10% ammonium molybdate is added, whereby an abundant precipitate forms, the mixture is stirred, and after standing for 10 minutes 3 ml of N sulfuric acid is added (the precipitate dissolves completely). Extraction is carried out 3 times with 3 ml portions of N butanol, collecting the butanol extracts in another separatory funnel, where they are washed 4 times with 5 ml portions of 2 N sulfuric acid. To the washed butanol extract an ammoniacal solution is added and the reextracts are collected in a 25-ml volumetric flask. Add 1 ml of caustic potassium, mix, and after 10–15 minutes add 7 ml of 8.5 N hydrochloric acid; all remaining reagents are added and the photometry of molybdenum thiocyanate is carried out in the same way as in the determination of arsenic.

The silicon content is obtained by multiplying the amount of molybdenum found by the coefficient 0.02439.

Experimental data obtained in the analysis of iron are given in Table 2.

Conclusions

A procedure has been developed for determining traces of arsenic, phosphorus, and silicon from a single sample in high-purity iron. The sensitivity of the method is \(10^{-5}\)—\(10^{-6}\)%. The error of determination does not exceed ±25%. The duration of the analysis is 8–10 hours.

LITERATURE

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2. N. A. Filippova, A. I. Kuznetsova. Journal “Zavodskaya laboratoriya,” pp. 536–546, 1950.

3. Kleibo and Jensen. Journal of Research of National Bureau of Standarts, May 1959, V. 62, No. 5, Waschington 25, D. C. p 201.

4. Paul J. Simultaneous determination of arsenic and phosphorus. “Microchim acta,” No. 5–6, pp. 830–835, 1965.

5. Paul J. Simultaneous determination of silicon and phosphorus in their joint presence. “Analyt chim. acta.” 23, No. 2, pp. 178–182, 1960.

6. Sakanoue. “Bunseki kagaku. Japan Analyst.” 1959, 8, No. 10, pp. 675–677. Cited from Ref. Zh. “Khimiya,” 73115, No. 18, 1960.

7. A. L. Davydov, B. E. Reznik, V. M. Vaisberg. “Zavodskaya laboratoriya,” No. 8, 1033, 1939.

8. L. I. Kokorin, K. D. Vasil’eva. “Zavodskaya laboratoriya,” XII, 123, 1946.

9. I. P. Alimarin, V. S. Zvereva. Transactions of the Institute of Applied Mineralogy, 63, 3, 1934.

10. E. M. Rudnitskaya-Pokrovskaya. Occupational Hygiene and Safety Engineering. 15 (5), 90, 1937.

11. Methods for the Analysis of High-Purity Substances, pp. 349, 355. Publishing House “Nauka,” Moscow, 1965.