These compounds are of interest in that they are the only heteropoly-acid compounds containing tetravalent tellurium. They are prepared by mixing hydrochloric acid solutions of tellurium dioxide with solutions of vanadium pentoxide in caustic soda and then neutralising with more hydrochloric acid, or by mixing strongly alkaline solutions of tellurium dioxide (in excess) and vanadium pentoxide. The former method gives rise to yellow powders which readily hydrolyse and become colloidal with water. Their composition is Na2O.nTeO2.2V2O5.xH2O, where n = 5, 6 or 10, according to the composition of the precipitating medium. The latter method is remarkable because all other heteropoly-acid salts are prepared either in acid or neutral solution; it gives rise to white needles, the composition of which again varies with the relative proportions of oxides used. The products lose some of their tellurium dioxide content on being recrystallised, and are probably not true compounds.
No vanado-tellurates have been prepared, although molybdo-, tungsto-, phospho-, and other heteropoly-tellurates are known. |