Sequential Isolation of Elemental Selenium and Tellurium from a Solution of Their Salts

I. G. Onufrienok, E. S. Novikova

(Presented by the seminar of the Department of Inorganic Chemistry, KhTF)

As studies of the action of various reducing agents (sucrose, raffinose, galactose, glucose, etc.) on salts of selenium(IV) and tellurium(IV) have shown, galactose and glucose reduce quantitatively only tellurium from hot alkaline solutions. The addition of small amounts of sodium sulfite prevents coprecipitation of selenium with tellurium. In addition, complete separation of selenium from tellurium during reduction can be achieved if the hot solution with the tellurium precipitate, after reduction with galactose, is filtered through a hot-filtration funnel and washed with a hot 1% sodium sulfite solution. We used this feature for the sequential isolation of selenium and tellurium for the purpose of their quantitative determination.

Experimental Part

20 ml of a standard solution containing 0.01901 g of selenium and 0.02250 g of tellurium is alkalized with a sodium hydroxide solution; about 0.2 g of sodium sulfite and 5 ml of a 0.5 M galactose solution are added. The resulting solution is heated and boiled for 7–10 min. For better formation of the precipitate, the solution is mixed by rotating the flask. The precipitate formed is washed 3–4 times by decantation, 3–4 times on the filter with a hot 1% sodium sulfite solution, and 3–4 times with hot water. The precipitate of elemental tellurium is dissolved in a mixture of potassium dichromate and sulfuric acid for its further quantitative determination by a volumetric method [1]. Selenium is determined in the filtrate after separation of tellurium. For this purpose, the selenium-containing solution is acidified with concentrated hydrochloric acid so that the solution becomes 6 N. The acidic solution is heated to 50–60°C and, in several portions over 10 min, while heating is continued, solid sodium sulfite is added until the solution is saturated. Under these conditions elemental selenium precipitates in the form of the red modification; nevertheless, it is readily filtered through a glass filter 4. The flask in which the selenium was precipitated and the precipitate on the filter are washed 3–4 times with a cold 2% sodium sulfite solution and 3–4 times with water.

The washed precipitate is dissolved on the filter with hot concentrated nitric acid, adding a little solid NaNO₂ to the filter. The filtrate is collected in the flask in which the reduction of selenium was carried out.

The filtrate is washed several times with 2 N nitric acid. To accelerate the dissolution of selenium, the flask with the solution and precipitate is carefully heated. After the selenium has dissolved, the contents of the flask are cooled under the tap, 5–6 ml of concentrated sulfuric acid is added, and, with careful heating, the solution is evaporated until $SO_2$ vapors appear. The solution is again cooled, diluted with water to 40–50 ml, and about 1 g of potassium iodide is added; the flask is then covered with a watch glass and left in a dark place for 5–10 min. In accordance with the selenium content in the solution, an equivalent amount of free iodine is liberated, which is titrated with a sodium thiosulfate solution.

Conclusions

A method is proposed for the quantitative determination of selenium and tellurium, based on their sequential isolation from a solution of salts.

Literature

1. I. P. Onufrienko, V. M. Aksenenko. Journal of Analytical Chemistry, XIV, 5, p. 638, 1959.