Detection of Tellurium

When heated on charcoal in a reducing flame tellurium compounds give rise to free tellurium, which volatilises and forms a crust of the dioxide around the heated area; the flame also is tinged with green. There is no marked odour. In the hydrogen- air flame, tellurium dioxide, hydrogen telluride and elementary tellurium all produce a lilac luminescence in the middle zone and a green luminescence in the outer zone. In the inner zone hydrogen telluride produces blue luminescence, while the colour imparted by the other two compounds is green in this region. The vapour of tellurium dioxide introduced into a Bunsen flame produces a blue coloration tinged with green. If a cold surface is held in the hottest part of the flame a bright metallic mirror is deposited on it. The green luminescence appears to be due to the change from tellurium to tellurous salt, and the blue to the further oxidation to telluric salt.

A test for tellurium which is sensitive to 10 mgms. per litre, consists in boiling 1 c.c. of solution with 1 c.c. of concentrated ammonia and a few crystals of hydrazine sulphate. If tellurium is present a brown coloration or precipitate appears. If selenium is present, it is first removed by boiling the solution with an equal bulk of concentrated sulphuric acid and a few crystals of hydrazine sulphate; the precipitate is removed and the filtrate made alkaline with ammonia, more hydrazine sulphate added and the solution again boiled; a brown coloration indicates the presence of tellurium.

Another method for the detection of tellurium in the presence of selenium depends on the fact that the former is not precipitated by sulphur dioxide in the presence of concentrated hydrochloric acid. A solution in the concentrated acid (dens. 1.16) is heated to 90° C. and saturated with sulphur dioxide; the precipitated selenium is removed by filtration, the filtrate diluted with an equal volume of water and saturated in the cold with sulphur dioxide. If tellurium is present a black precipitate is formed immediately.

Tellurites and tellurates may be reduced to tellurium by means of titanous chloride in hydrochloric acid solution, and in the absence of selenium, copper, gold and platinum, the reaction may be used as a sensitive test for the element.5 In 5 c.c. of concentrated hydrochloric acid, 0.00005 gram of tellurium gives with 2 c.c. of a 2 per cent, titanous chloride solution a grey cloud which slowly flocculates. In the presence of a little acetic acid and a large quantity of sodium acetate, reduction to hydrogen telluride results; this gas may be detected by its disagreeable odour and by the production of a mirror on passing it through a heated tube.

A microchemical test, in which the tellurium is precipitated by means of a saturated solution of quinol in concentrated sulphuric acid, has been described. The quinol reagent, however, also reacts with selenium and manganese.