Determination of Antimony and Tellurium in an Antimony–Tellurium Master Alloy

V. M. Aksenenko, I. P. Onufrienok

(Presented by Prof. A. G. Stromberg, Doctor of Chemical Sciences)

It is known that antimony and tellurium, when present together, are separated before determination. Tellurium is separated from antimony and other metals by reduction in an acidic medium with sulfurous gas, hydrazine hydrochloride, hydrazine sulfate, etc. [1, 2]. This complicates the analysis, especially in the determination of antimony. We therefore set ourselves the task of developing titrimetric methods for their determination without preliminary separation.

From a comparison of the normal oxidation potentials of antimony(V)/antimony(III) \(+0.64\ \text{V}\) and tellurium(VI)/tellurium(IV) \(+1.026\ \text{V}\), it is evident that, when antimony(III) and tellurium(IV) are present together, they can be titrated separately.

As the basis for determining antimony and tellurium, we used titration of a mixture of antimony(III) and tellurium(IV) in a hydrochloric–sulfuric acid medium with strong oxidizing agents such as potassium permanganate, bromate, and dichromate. To fix the equivalence point we used methyl red, which is ordinarily employed in bromatometry.

Our experiments showed that, for titrating antimony(III) in the presence of tellurium(IV), potassium permanganate proved the least suitable, although in practice it is successfully used for determining antimony in babbitts, typographic metals, etc. In the presence of tellurium(IV), when the antimony has already been titrated, the addition of an excess of permanganate does not cause coloration of the solution because permanganate is consumed in oxidizing tellurous acid. In this case the end of titration of tellurous acid also cannot be fixed accurately, since near the equivalence point no sharp coloration of the solution occurs. The coloration appears when a portion of tellurous acid is still present in the solution and then gradually intensifies, giving no sharp transition. When an excess of permanganate is introduced and then back-titrated with some reducing agent, concordant results are likewise not obtained.

Potassium bromate differs advantageously from permanganate in that, in the presence of methyl red, it distinctly oxidizes antimony and does not react with tellurous acid; this was used by us for determining antimony in the presence of tellurium.

Potassium dichromate acts universally: it makes it possible to determine both antimony and tellurium from one sample portion and, moreover, to titrate their sum accurately. First, antimony is titrated until methyl red is decolorized; then an excess of dichromate, in an amount of 5–6 ml, is added, which oxidizes tellurous acid to telluric acid. The excess of remaining dichromate is determined in the usual way, i.e., by adding an excess of titrated Mohr’s salt solution and titrating with potassium dichromate in the presence of diphenylamine indicator. Direct titration of tellurous acid with potassium dichromate does not give a positive result.

because of the insufficient rate of their interaction. At low tellurium contents in samples, its direct determination from a single portion together with antimony is of little use; the use of large portions in sulfuric-acid decomposition, for example of lead alloys, is very inconvenient. In such cases one may proceed in various ways: in one portion determine their sum, and in another portion determine one of them and calculate the other by difference. When it is necessary to determine antimony alone, preference should be given to the bromate method.

Experimental Part

The determination of antimony in the presence of tellurium, as well as their separate titration, carried out on artificial mixtures of antimony (III) and tellurium (IV), showed complete agreement of the results obtained by the bromate and dichromate methods. Solutions of antimony (III) and tellurium (IV) were prepared by dissolving accurately weighed portions of the corresponding elements in sulfuric acid. After dilution with water and addition of hydrochloric acid (1:1), they were titrated either with potassium bromate or potassium dichromate in the presence of methyl red as indicator. The experimental data are summarized in Table 1.

Table 1

We used this method for the determination of antimony and tellurium in an antimony–tellurium master alloy and in an alloy containing lead, antimony, and tellurium. For these objects we give the course of the analysis.

Course of analysis. A portion of the alloy, 0.1–0.5 g, is decomposed with 8–20 ml of concentrated sulfuric acid in a conical flask of 250–500 ml capacity, with heating (without allowing boiling) and stirring. After decomposition, the solution is cooled, 30–100 ml of water, 10–15 ml of concentrated hydrochloric acid, and a further 30–100 ml of water are added. The mixture is boiled for 10–15 minutes to remove sulfurous gas. After cooling, 2 drops of methyl red are added, and the solution is titrated slowly, dropwise, with 0.1 N potassium dichromate solution, with vigorous stirring, until the indicator is decolorized. The completeness of oxidation of antimony is checked by adding 1–2 drops of methyl red.

The consumption of potassium dichromate used for titrating antimony is recorded, and enough more is added to be sufficient for the oxidation of tellurous acid, with an excess of 5–6 ml; the solution is allowed to stand for 30 minutes. After standing, 0.1 N Mohr’s salt solution is added with an excess of 5–10 ml, and this excess is back-titrated with 0.1 N potassium dichromate solution to a blue-violet coloration, after first adding 5 ml of hydrochloric acid (1:1), 10 ml of phosphoric acid (1:1), and 2 drops of diphenylamine indicator. From the consumption of 0.1 N potassium dichromate solution, the antimony and tellurium in the alloy are calculated.

If only antimony is to be determined, the described course of analysis remains unchanged, except that decomposition of the alloy by gentle boiling with sulfuric acid may be allowed. The titration is carried out either with potassium dichromate or with potassium bromate at a temperature of 70°. The results of the determination of antimony and tellurium in an antimony–tellurium master alloy are given in Table 2.

Table 2

¹ The consumption of 0.1 N solution was recalculated to 0.1000 g of alloy.

In the analysis of lead–antimony–tellurium alloys, the course of the analysis remains basically the same as that adopted for the determination of antimony in lead alloys [3], but with a bromatometric endpoint.

If it is necessary to determine tellurium together with antimony, the following should be borne in mind.

Tellurous acid, after decomposition of the weighed sample and subsequent treatment with water, hydrochloric acid, and again with water, is then brought completely into solution by boiling. Unlike antimony(III), it is not precipitated with lead sulfate. Therefore, after boiling and cooling to 80°, the solution is decanted from the precipitate; the precipitate is washed by decantation 3–4 times with 20 ml portions of 1 N hydrochloric acid, and the solutions after decantation are combined with the first decanted solution; in this combined solution antimony and tellurium are determined by the dichromate method. The antimony residue coprecipitated with lead sulfate is dissolved in 4 N hydrochloric acid, and the antimony is titrated with the bromate method.

Conclusions

1. A dichromate method has been developed for determining antimony and tellurium from a single weighed sample in an antimony–tellurium master alloy.

2. It has been established that the presence of tellurous acid does not interfere with the determination of antimony by the bromate method. The permanganate method under analogous conditions is unsuitable for the determination of antimony.

Literature

1. Hildebrand W. F., Lundell G. E., Bright G. A., Hoffman D. I. Practical Guide to Inorganic Analysis. Goskhimizdat, Moscow, p. 352, 1957.

2. Analysis of Mineral Raw Materials. Collection edited by Yu. N. Knipovich, Yu. V. Morachevsky. Goskhimizdat, Leningrad, p. 515, 1956.

3. Dymov A. M. Technical Analysis of Ores and Metals. Metallurgizdat, Moscow, p. 452, 1949.