Tellurium Trioxide, TeO₃

Tellurium Trioxide, TeO₃, like so many of the tellurium compounds, was discovered by Berzelius. It is formed when telluric acid is heated a little above 360° C. Care is necessary, since overheating induces decomposition of the yellow trioxide into the colourless dioxide. Any dioxide that forms can be removed by treating with concentrated hydrochloric acid, in which the trioxide is only sparingly soluble.

Tellurium trioxide is an orange-yellow solid, of density 5.10 at the ordinary temperature. Its heat of formation per gram-molecular weight is 83.6 Calories, and according to Mixter the decomposition of the trioxide into the dioxide is an exothermic reaction. As might therefore be expected, this decomposition occurs readily and at such a temperature that the resulting dioxide is unfused.

The trioxide is insoluble in water, hot or cold, in nitric acid and in cold hydrochloric acid. Concentrated hydrochloric acid when heated attacks it with liberation of chlorine and formation of tellurium tetrachloride in the solution. The caustic alkalis only dissolve tellurium trioxide in hot concentrated solution, with formation of tellurates.