Hydrogen Telluride, H₂Te

In 1808 the observation was made by Ritter that in the electrolysis of water using a tellurium cathode, an unstable tellurium-hydrogen compound was produced, and in repeating this experiment with potassium hydroxide solution as electrolyte, Sir Humphry Davy two years later further observed the formation of a deep red solution of Hydrogen Telluride, H₂Te. Berthelot and Fabre in 1887 first prepared the hydrogen compound in a state approaching purity.

1. Tellurium can be reduced to hydrogen telluride by strongly heating in an atmosphere of hydrogen, but the yield is very poor, and the purer the tellurium the greater is the resistance against the action of hydrogen. The reduction can also be effected by zinc and dilute sulphuric acid, the tellurium being conveniently added as the dioxide, but the most satisfactory method of reduction is to make tellurium the cathode in an electrolysis, at 0° C. or lower, of 50 per cent, sulphuric acid or preferably phosphoric acid; the evolved gas may contain 5 to 10 per cent, of free hydrogen.
2. Hydrogen telluride is also obtainable from the metallic tellurides, for example, from magnesium, aluminium, zinc and iron tellurides. These are decomposed by water or by a non-oxidising acid such as dilute hydrochloric or phosphoric acid. It is necessary to displace the air from the apparatus previously by means of a current of nitrogen and to collect over mercury. The hydrogen telluride may be purified by passing it through a freezing mixture of ether and solid carbon dioxide. Aluminium telluride is the most suitable telluride to use and hydrogen chloride the best acid. Under the most favourable conditions a yield of more than 80 per cent, of the theoretical may be obtained.

Hydrogen telluride is a colourless gas, the odour of which is notably different from that of its selenium and sulphur analogues, being less pronounced and faintly recalling that of arseniuretted hydrogen. The gas is poisonous; a bubble inhaled is sufficient to cause a severe attack of vertigo. The gas can be solidified to a colourless crystalline mass which at -57° C. melts to a very pale yellow liquid of boiling-point -1.8° C. at 760 mm. and density 2.57 at -20° C. The critical temperature lies in the region of 200° C. When freshly distilled, liquid hydrogen telluride is almost colourless, but darkens gradually on keeping, owing to the formation of tellurium, which remains dissolved in the liquid. This decomposition is greatly accelerated by daylight and by ultra-violet light.

Conductivity measurements in N/10 aqueous solution show the dissolved gas to be ionised to the extent of 50 per cent., whilst hydrogen selenide in N/10 solution is only 4.1 per cent, ionised. The acidity of the hydrides of the elements oxygen, sulphur, selenium and tellurium therefore falls into the regular series H₂Te > H₂Se > H₂S > H₂O, in inverse order to the stability.

The gas is fairly soluble in water and also in ether, the latter solvent giving a relatively stable solution. The vapour density accords very closely with that expected from the formula H₂Te.

Hydrogen telluride is an unstable gas. It is an endothermic compound, the heat of formation being as follows:

H₂ (gas) + Te (cryst.) = H₂Te (gas)—35,000 calories.

When the gas is kept in sealed tubes a deposit of tellurium gradually forms on the walls. This dissociation does not appear to be accelerated by light, as is the case with liquid hydrogen telluride (see before), since it occurs just as rapidly in the dark.

Hydrogen telluride burns in air with a blue flame, producing water and tellurium dioxide. Moist air decomposes the gas immediately even at the ordinary temperature with liberation of black tellurium. The dry gas is immediately oxidised by oxygen.

Solutions of the alkalis dissolve the gas with formation of the corresponding telluride, but on account of the presence of more or less free oxygen, some free tellurium is formed and dissolves in the telluride solution, so that the solution is generally deep red in colour. If oxygen is entirely excluded the solution is colourless. The solution of alkali telluride can be used for the precipitation of some of the heavier metals. Hydrogen telluride itself also precipitates many of the heavy metals as tellurides.

Hydrogen telluride is very sensitive towards the halogen elements. It not only readily reduces chlorine, bromine and iodine to the corresponding hydracids with simultaneous liberation of tellurium (which in the case of chlorine can further pass easily into the tetrachloride), but it also reduces solutions of such salts as ferric chloride and mercuric chloride to the lower chlorides, tellurium being precipitated. It also reduces tellurium chlorides, the only products being hydrogen chloride and tellurium.

The composition of the gas is demonstrated by the action of heated tin, when the volume of hydrogen obtained is equal to that of the original gas.

Organic tellurides are known and clearly show the tendency of tellurium to pass from the bivalent condition to one of higher valency; thus the dialkyl tellurides act in an unsaturated manner and readily form dihalides, oxides and hydroxides, for example, (C₂H₅)₂TeCl₂, (CH₃)₂TeO.