Estimation of Tellurium

Tellurium is generally precipitated as the element and weighed as such. It may be liberated from its compounds by various reducing agents such as sulphurous acid and its salts (sometimes with the addition of an iodide), hypophosphorous acid, hydrazine salts, glucose, or by electrolysis. The precipitate should be washed with alcohol and dried at 110° C. At this temperature oxidation of the element is extremely slight, but if the utmost accuracy is required the drying should be carried out in an atmosphere of nitrogen.

The use of sulphur dioxide as precipitant was first proposed by Berzelius, but accurate results by this method are only obtainable under special conditions. Complete precipitation does not take place from a strongly acid solution, and in the presence of other metals small amounts of these are liable to be carried down. In the presence of heavy metals such as copper, bismuth and antimony, the following procedure has been recommended: The tellurium is oxidised to telluric acid by the addition of ammonium perdisulphate in the presence of potassium hydroxide, excess of perdisulphate being subsequently removed by boiling. The heavy metals present are next removed by means of hydrogen sulphide. The tellurium may then readily be estimated by reduction with hydrogen chloride and precipitation with sulphurous acid.

Tellurous acid is rapidly reduced to the element by sulphurous acid in the presence of potassium iodide. Owing, however, to the affinity which tellurium has for iodine and the consequent formation of varying proportions of tellurium tetra-iodide, the trustworthiness of the quantitative method based on this reaction is questionable.

Solutions of tellurium in alkali sulphides when boiled with sodium sulphite yield a quantitative precipitate of the element.

An aqueous solution of telluric acid gives with hypophosphorous acid a colloidal solution of tellurium which is precipitated completely on boiling and can then be filtered and weighed. This method is only available if there are no other salts present which might be reduced.

Tellurium in the sexavalentx condition cannot be estimated using phosphorous acid as reducing agent since complete reduction does not take place. Tellurium in the tellurous condition is reduced readily to the elementary condition when its concentrated solution in hydrochloric acid is boiled with phosphorous acid.

Of the various methods of estimation which are based on the use of hydrazine salts as reducing agents the following appears to be one of the most satisfactory: The tellurium, present either as a derivative of the dioxide or as a tellurate, is dissolved in hydrochloric acid and boiled. Sulphurous acid and hydrazine hydrochloride are added, and on continued boiling the tellurium is precipitated as such and may be collected, dried and weighed. If selenium is present it may be separated first by passing sulphur dioxide into the solution in concentrated hydrochloric acid.

According to Muller's experiments, tellurium is not deposited by electrolysis from solutions in which it is present in the sexavalent condition, so that telluric acid must first be reduced to tellurous acid. A weight of tellurous acid not exceeding 0.25 gram is dissolved in 175 c.c. of 2N H₂SO₄ and electrolysed for two and a half hours between platinum electrodes. The grey deposit of tellurium is washed with water and alcohol and dried in a desiccator over sulphuric acid. In this method the average error is estimated to be ±0.1 per cent. By this process tellurous acid may be estimated in the presence of telluric acid. After the tellurium from the tellurous acid has been removed, the solution is boiled with hydrochloric acid to reduce the telluric acid to tellurous acid and the estimation of the tellurium carried out as before.

A method for the estimation of tellurium in tetradymite is given by Hulot. The mineral is first treated with dilute hydrochloric acid to remove calcareous material. The residue is powdered and dissolved in hot concentrated nitric acid. After evaporation of the solution the solid is fused with potassium nitrate, the resulting mass containing potassium sulphate and selenate, bismuth oxide and potassium anhydrotellurate, K₂Te₄O₁₃. The sulphate and selenate are removed by digestion with boiling water and the bismuth oxide is dissolved out with dilute hydrochloric acid. The residue, potassium anhydrotellurate, is suspended in dilute hydrochloric acid and zinc added. A black pulverulent powder is deposited, which is pure tellurium. After the whole of the zinc has dissolved, the tellurium may be collected, dried and weighed.

In the processes described in which the tellurium is precipitated in the elementary form, it is generally assumed that the error due to oxidation of the precipitate is practically negligible under the conditions of the experiment. Browning and Flint, however, maintain that the results are liable to be inaccurate owing to this oxidation. Tellurium dioxide, on the other hand, is unaffected by the air, is anhydrous, non-hygroscopic and easily obtained in the pure condition, and Browning and Flint base a method for the estimation of tellurium on precipitation as dioxide. The tellurium compound is precipitated from a faintly acid solution by means of ammonia, the acidity being restored by the cautious addition of acetic acid. The mixture is heated for some time to render the precipitate crystalline. The method is applicable to the separation of tellurium from selenium.

According to Perkins electrolytic silver may be used in the quantitative estimation of tellurium. Tellurium dioxide, like selenium dioxide, liberates iodine from an acidified solution of potassium iodide according to the equation:

TeO₂ + 4KI + 4HCl = Te + 4KCl + 2H₂O + 2I₂.

The liberated iodine may be estimated by shaking with specially prepared electrolytic silver in an atmosphere of hydrogen and measuring the increase in weight of the silver. The increase in weight represents the iodine liberated plus the tellurium (or selenium).

Tellurium may be determined by oxidation from the tellurous to the telluric condition, using an excess of potassium dichromate or permanganate and subsequently titrating the excess of oxidising agent with a standard solution of a suitable reducing agent. In order to obtain accurate results with the potassium dichromate titration, certain very definite steps in the procedure are essential, and it is necessary to control the course of the reaction, since hydrochloric and telluric acids interact with production of chlorine.

3TeO₂ + K₂Cr₂O₇ + 8HCl = 3H₂TeO₄ + 2CrCl₃ + 2KCl + H₂O.

The following procedure has been found very satisfactory. The hydrochloric acid solution of the dioxide is treated with an excess of standard dichromate and allowed to react for at least half an hour. A measured excess of ferrous ammonium sulphate is added and the excess of ferrous salt titrated with standard dichromate, using potassium ferricyanide as outside indicator. The weight of tellurium dioxide should be less than 0.3 gram, and the solution, the volume of which should be approximately 200 c.c., should contain 2 per cent, of free hydrogen chloride for successful working.

Tellurous acid cannot be determined by oxidation with potassium permanganate in acidified solution, but in alkaline solution accurate results may be obtained by cooling to 8°-10° C. after the oxidation and slowly acidifying with dilute sulphuric acid, with continual stirring. Excess of standard oxalic acid is then added and after warming to 50° C. the remaining excess is titrated with permanganate.

Telluric acid may be determined iodometrically by reduction with either hydrogen bromide or hydrogen chloride. The substance is heated in a distillation flask with four times the theoretical quantity of potassium iodide in the presence of the acid, air being excluded from the apparatus by passing a stream of carbon dioxide. The liberated iodine is titrated in the receiver and in the residue.

Other methods have been described which depend on reduction of the tellurium compound by means of titanous chloride, but these generally are not trustworthy owing to the formation of hydrogen telluride. Potentiometric titration with titanous chloride in the presence of hydrochloric acid has been recommended.

Telluric acid may also be estimated alkalimetrically by the addition of a large excess of standard barium hydroxide, or sodium hydroxide containing barium chloride, when barium tellurate is quantitatively precipitated. The excess of the hydroxide is determined by titration with oxalic acid, phenolphthalein being used as indicator.