Tellurium Tetra-iodide, TeI₄

Tellurium Tetra-iodide, TeI₄ may be prepared in a very pure state by heating together finely divided tellurium and excess of iodine and allowing the product to cool slowly. The excess of iodine may be removed by extracting with carbon tetrachloride.

The tellurium tetra-iodide may also be formed from the dioxide and concentrated hydriodic acid. Telluric acid gives a similar result. The resulting halide may separate as such or in needles of an additive compound, HI.TeI₄.8H₂O, which gives a residue of tetra-iodide when warmed to 50°- 60° C.

The tetra-iodide forms small, brilliant, black crystals, having a density of 5.05 at 15° C. When heated above 100° C. it dissociates into tellurium and iodine. When dissolved in excess of iodine it appears probable that part of the iodide is in a bimolecular condition. It is unattacked by moist air, but is slowly decomposed by cold water, and on warming, hydrolysis occurs with the formation of an oxy-iodide of uncertain composition; on boiling with water complete hydrolysis to hydrogen iodide is effected:

TeI₄ + 3H₂O = H₂TeO₃ + 4HI.

It is sparingly soluble in alcohol and acetone, insoluble in ether, chloroform and carbon disulphide. Alkalis and ammonia in aqueous solution readily dissolve it. It is also soluble in a solution of hydriodic acid.

Analogous with the tetrachloride and tetrabromide, additive compounds are formed by the tetra-iodide with the alkali iodides. The telluri-iodides, of the general formula X₂TeI₆, are black, crystalline substances, which, like the tellurichlorides and telluribromides, generally crystallise in the regular system when anhydrous. They are decomposed by water in the same way as the tetra-iodide, although they are soluble without decomposition in concentrated hydriodic acid. It has not been found possible to obtain the corresponding di-hydrogen tellurium hexa-iodide. By the interaction of tellurium tetra-iodide, iodine and hydriodic acid in a sealed tube, the only product is a substance having the composition TeI₄.HI.8H₂O.