Tellurium Tetrafluoride, TeF₄

Tellurium Tetrafluoride, TeF₄, is formed by the action of fluorine on tellurium, the heat of the reaction causing the mass to become incandescent.

By dissolving tellurium dioxide in hydrofluoric acid and concentrating the solution, a tellurium oxyfluoride separates, which, according to Berzelius, yields on heating first water and then a sublimate of the tetrafluoride. Metzner, by cooling the solution to about -50° C., obtained crystals to which he assigned the formula TeF₄.TeO₂.2H₂O, and on treating the mother-liquor with anhydrous hydrofluoric acid and cooling to -70° C., it solidified. On allowing the temperature to rise once more, the portion which remained unfused at -28° C. was regarded as tellurium tetrafluoride. Prideaux and Millott, however, using anhydrous hydrogen fluoride throughout, have not been able to obtain the tetrafluoride by such means. These investigators obtained a liquid which, in a vacuum desiccator, yielded a mass of white crystals of composition TeF₄.TeO₂.H₂O.

Tellurium tetrafluoride is a deliquescent solid which is decomposed by water, forming tellurous acid. The foregoing method of preparation is therefore dependent on the reversibility of the reaction

TeF₄ + 3H₂O ⇔ H₂TeO₃ + 4HF.

The presence of tellurite makes a special method necessary for the estimation of the fluoride, and it is found that after hydrolysis the hydrogen fluoride may be titrated with alkali, using p-nitrophenol as indicator, the end-point not being affected by the presence of tellurous acid.

Tellurium tetrafluoride combines with the fluorides of the alkali metals (including ammonium) and barium fluoride, producing colourless crystalline salts of the general formula XF.TeF₄, in which X represents the equivalent weight of the metal.