THE EFFECT OF LEAD ON THE REDUCTION OF TELLURIUM

V. B. Sokolovich, I. P. Onufrienok

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

At the Department of Analytical Chemistry of the Tomsk Polytechnic Institute, rapid and accurate methods have been developed for the determination of tellurium in various objects, based on the reduction of tellurite to elemental tellurium by milk sugar in an alkaline medium and the subsequent oxidation of elemental tellurium to tellurate by potassium dichromate in a sulfuric acid medium [1]. In lead alloys, it proved impossible to determine tellurium in an analogous way. After boiling with milk sugar, and sometimes even during boiling, the reaction mixture thickened, forming a jelly-like precipitate of lead salts of organic acids, which hindered the filtration of elemental tellurium. This phenomenon was eliminated by raising the pH with the use of additions of aluminum sulfate or zinc sulfate to improve coagulation of the precipitate of elemental tellurium under alkaline conditions. It was also found that thickening of the reaction mixture is prevented by substantial dilution or by introducing an addition of sodium acetate (10 g of sodium acetate per 1 g of the tellurium–lead alloy sample).

Experiments with pure tellurium, artificial mixtures of tellurium–antimony, tellurium–lead, tellurium–antimony–lead, and alloys of analogous composition showed that, whereas pure tellurite and tellurite in a mixture with antimony compounds is reduced by milk sugar in an alkaline medium completely and precipitates in the form of large flakes, from mixtures and alloys of composition tellurium–lead and tellurium–antimony–lead, when significant amounts of lead are present (approximately 100 times more than tellurium), elemental tellurium separates in the form of a fine powder, being reduced under the same conditions by only 20–40% (Table 1); moreover, residues of tellurium in the filtrate cannot be detected with stannous chloride.

The determinations were carried out by the following procedure. A 2.0–5.0 g sample of alloy was decomposed with a mixture of a 30% solution of tartaric acid and nitric acid of specific gravity 1.38 in a ratio of 2:1. To the resulting solution, boiled to remove nitrogen oxides, 40% sodium hydroxide solution was added until the precipitate of lead hydroxide dissolved, plus another 6–8 drops in excess, as well as 5 ml of a 20% solution of milk sugar. The reaction mixture was heated to boiling; when turbidity appeared from the separating tellurium, 4 ml of 0.1 N aluminum sulfate solution was added, and boiling was continued for about 20 minutes until the precipitate coagulated. The completion of the determination was titrimetric [1].

Changing the reduction conditions without separating lead—replacing the excess amount of sodium hydroxide with potassium hydroxide, sodium acetate, or sodium tartrate, and increasing the boiling time—did not increase the yield of tellurium (Table 1).

Table 1

In order to increase the yield of elemental tellurium in the presence of lead, milk sugar was replaced by other reducing agents capable of reducing tellurium in an alkaline or weakly acidic medium. The following were used: a mixture of milk sugar with stannous chloride, glucose, galactose, sucrose, sulfurous-acid hydrazine, ascorbic acid, and sodium hypophosphite.

A mixture of milk sugar with stannous chloride. The determinations were carried out in an alkaline medium according to the procedure described above, but 1–2 drops of a 10-percent solution of stannous chloride (II) were introduced into the reaction mixture, in some experiments before the addition of milk sugar and in others after it. Tellurium separated in the form of a fine powder. The yield was about 45% (Table 2).

Glucose, in the form of a 20-percent solution in an alkaline medium, completely separated tellurium from solutions containing no lead; in the presence of considerable amounts of lead, however, it separated approximately 30% (Table 2).

Galactose, under the same conditions, in the presence of lead, separated tellurium by approximately 20% (Table 2).

Sucrose, from pure solutions of tellurites in sulfurous-acid and hydrochloric-acid media, noticeably reduced tellurium (complete separation of elemental tellurium could not be obtained). In a weakly acidic medium (acetic- or tartaric-acid), and also in an alkaline medium, reduction proceeded more weakly, and in the presence of lead was almost not observed. Invert sugar (fructose) reduced tellurium in the presence of lead only insignificantly. The precipitate was very fine.

Sulfurous-acid hydrazine, with prolonged boiling, quantitatively reduced tellurium from tellurite solutions in both acidic and alkaline media. In the presence of lead, however, tellurium separated very slowly. The yield was about 10–20% (Table 2).

Ascorbic acid reduced tellurium under various conditions, especially well in a weakly alkaline medium by the following procedure. The weighed portion was decomposed as indicated above. To the resulting solution, boiled to remove nitrogen oxides and neutralized to phenolphthalein with a 40-percent solution of sodium hydroxide, about 2 g of potash was added. Into the heated reaction mixture was introduced a freshly prepared solution of ascorbic acid, previously neutralized with potash until the evolution of carbon dioxide ceased (1,2 g of acid per 0,02 g of tellurium). The reaction mixture was boiled for about 20 minutes until the precipitate coagulated. The determination was completed titrimetrically [1].

From solutions of pure tellurites, and also in the presence of antimony, the tellurium precipitate separated rapidly and coagulated well into large flakes. The yield was 100%. In the presence of significant amounts of lead, however, tellurium was reduced only by 30% (Table 2), and the precipitate had the appearance of a fine powder.

Sodium hypophosphite in an alkaline medium does not precipitate tellurium from tellurite solutions either in the cold or on heating. In hydrochloric-acid and sulfuric-acid media the reduction proceeded quantitatively; in a weakly acidic (tartaric- and acetic-acid) medium, it proceeded to 95–98%.

To the neutralized solution obtained upon decomposition of the weighed sample, 15 ml of 30% tartaric acid solution was added, and water was added to a volume of about 80 ml. After heating, dry sodium hypophosphite was introduced (0.5 g per 0.02 g of tellurium), and boiling was continued for about 30 minutes. The precipitate, coagulated into large flakes, was filtered off; the determination was completed titrimetrically. In the presence of significant amounts of lead, reduction of tellurium by hypophosphite was not observed (Table 2).

Table 2

The experimental data show that, when a number of reducing agents act on tellurite solutions in the presence of significant amounts of lead, the yield of elemental tellurium decreases, and in some cases reduction does not occur at all. We suppose that the reason for this is the formation, under these conditions, of stable complexes of lead compounds with tellurites. The ability of tellurium to form complexes is also confirmed by literature data [2, 3].

Conclusions

1. It has been established that the presence of significant amounts of lead prevents the reduction of tellurium in alkaline and weakly acidic media by a number of reducing agents and worsens the structure of elemental tellurium.

2. In the quantitative determination of tellurium with the indicated reducing agents, lead should first be separated.

LITERATURE

1. Onufrienok I. P. and Aksenenko V. M. Journal of Analytical Chemistry, 13, issue 1, 1958.

2. Treadwell F. P. and Hall V. T. Qualitative Analysis. Goskhimizdat, p. 553, 1946.

3. Issa J. M., Hewaidy J. F. Chemist Analyst., 44, 70, 1955.