QUANTITATIVE DETERMINATION OF TELLURIUM BY REDUCTION WITH GALACTOSE

E. S. NOVIKOVA, T. S. GURNITSKAYA

(Presented by the seminar of the Department of General and Inorganic Chemistry)

If tellurium is present in the substance under study in sufficiently large amounts, the most convenient quantitative methods for its determination are gravimetric and volumetric analysis.

The determination of tellurium by volumetric analysis is reduced to the oxidation of the tellurite ion by a suitable oxidizing agent. The literature contains data on the oxidation of the tellurite ion by potassium dichromate [1], potassium permanganate [2], potassium periodate [3], and also on the oxidation of elemental tellurium to the tellurite ion [4].

\[ \mathrm{Te}+\mathrm{Cr_2O_7}^{\prime\prime}+14\mathrm{H}^{\cdot} = \mathrm{TeO_4}^{\prime\prime}+2\mathrm{Cr}^{\cdots}+7\mathrm{H_2O} \]

This last method—oxidation of elemental tellurium by potassium dichromate with subsequent determination of its excess—has a number of advantages, but nevertheless is associated with the difficulties of preliminary separation of elemental tellurium from its mixture with other elements. The separation of tellurium from selenium is especially difficult.

Usually tellurites and selenites can be separated:

a) by the action of ammonia in the presence of sufficient amounts of trivalent iron ions [5, 6];

b) by reducing selenite to free selenium with sulfur dioxide in a concentrated hydrochloric acid solution (8–10 N);

c) by distilling selenium bromide from an acidic solution containing HBr and Br₂;

d) by boiling a weakly alkaline solution with milk sugar in the presence of tartaric acid and a sufficient amount of sodium sulfite [7];

e) by the action of thiourea [8];

f) by ion-exchange resins.

Each of these operations requires a great deal of time, which lengthens an already prolonged determination. In carrying out the present work the aim was, first, to ascertain the effect of certain organic reducing agents on the tellurite ion and, second, to find the possibility of simultaneous separation of selenium during the reduction of tellurite. In this connection, the reducing action on tellurite and selenite ions was tested for invert sucrose, non-invert sucrose, galactose, glucose, raffinose, formaldehyde, acetaldehyde, benzaldehyde, mannitol, and pyrogallol.

Experimental Part

The test solution, containing tellurite or selenite ions, was neutralized against phenolphthalein with a solution of caustic soda, after which an alkaline medium was created by adding a small excess (6–7 drops) of 40% NaOH solution. To the alkaline solution obtained, a sufficient amount (5 ml of 0.5 M) of an aqueous solution of the reducing agent was added. The reaction mixture was diluted with water to a total volume of approximately 30–40 ml, heated to boiling, and then boiled for about 10 minutes.

For better formation of the precipitate during boiling, the solution was mixed by rotating the flask. The solution with the precipitate was cooled and filtered under vacuum through a funnel with a porous bottom (No. 4) or through an ordinary funnel with an asbestos plug.

The filtered precipitate was washed with hot 2% ammonium sulfite solution 3–4 times by decantation and the same number of times on the filter.

Freshly precipitated tellurium dissolves fairly readily in a mixture of potassium dichromate and dilute sulfuric acid; therefore, the washed precipitate together with the asbestos plug was transferred into the flask in which the reduction of tellurium had previously been carried out, and through the emptied funnel, in order to dissolve traces of precipitate on its walls, 20 ml of 0.1 N \(K_2Cr_2O_7\) solution and 20 ml of 2 N \(H_2SO_4\) were poured in.

To accelerate dissolution of the precipitate, the solution was heated to 60–70°C.

After all the tellurium had dissolved, an excess of Mohr’s salt—20 ml of 0.1 N solution—5 ml of orthophosphoric acid 1:1 were added to the cooled solution, and it was diluted with water to a total volume of 100–150 ml. The excess Mohr’s salt was titrated with 0.1 N potassium dichromate solution in the presence of diphenylamine.

The calculations were performed according to the formula:

\[ \mathrm{Te} = \frac{21.27\left(n.\,K_2Cr_2O_7 \cdot V_{K_2Cr_2O_7} - n.\,\text{Mohr’s salt} \cdot V_{\text{Mohr’s salt}}\right)}{1000}. \]

Analysis of the data obtained on the reduction of tellurium and selenium by various reducing agents in an alkaline medium makes it possible to conclude that, under these conditions, elemental tellurium can be quantitatively separated only with galactose and glucose, while small additions (\(0.2\) g \(Na_2SO_3\)) keep selenium in solution. Invert and non-invert sucrose, as well as raffinose, reduce tellurite by 50–60%, and selenite by 10%. Formaldehyde, acetaldehyde, and benzaldehyde reduce tellurite by 10–15%, and selenite only in traces. Mannitol and pyrogallol precipitate only traces of tellurium and selenium.

Conclusions

A method is proposed for the quantitative determination of tellurium by reduction with galactose.

Literature

1. B. Brauner, C. Chem. Soc. 59, 62, London, (1891).
2. J. T. Schrenk and B. L. Browing. G. Am Soc. 48, 2550, New-York, (1926).
3. V. S. Syrokamskii, R. N. Knyazeva. Zavodskaya laboratoriya, 16, 1041–1044, 1950.
4. I. P. Onufrienok, V. M. Aksenenko. Zh. analit. khimii, 14, No. 5, 638, 1959.
5. V. K. Zamel. Zav. lab., 5, 1438, M., 1936.
6. T. L. Pokrovskaya. Zav. lab., 7, 1, M., 1938.
7. V. P. Gladyshev. Zav. lab., 24, No. 3, 375–378, M., 1958.