Tellurates

It is an interesting fact that tellurium dioxide dissolves in a solution of telluric acid; the solution on slow evaporation gives crystals of telluric acid together with granules of a tellurium tellurate, 2TeO₂.TeO₃ or Te₃O₇.

Although it can be completely neutralised by alkali hydroxides, telluric acid is so weak that with cold solutions of the alkali carbonates only acid salts of the type KHTeO₄ are produced, the solutions of which are alkaline in reaction.

The frequent presence of two molecules of water of crystallisation in the normal salts of the "normal" acid, for example Ag₂TeO₄.2H₂O, HgTeO₄.2H₂O and K₂TeO₄.2H₂O, serves as a confirmation of the suggested existence of an orthotelluric acid, H₆TeO₆, of which such compounds can be regarded as acid salts; indeed such salts as Hg₆TeO₆ and Cu₃TeO₆ may be considered to be normal salts of this acid, although on the other hand they may be classed as basic salts of the acid H₂TeO₄.

The fact that the dihydrate in aqueous solution is a much weaker acid than sulphuric acid may be cited as evidence that the two acids are materially different in nature, and this supports the view that the dihydrate in solution is actually orthotelluric acid.

The ordinary tellurates of colourless metallic radicals are colourless substances. The salts of the alkali metals are soluble in water but have no definite solubility. In many respects these salts resemble colloids, many of the basic and so-called acid salts which have been described having been shown to be adsorption compounds. When the alkali tellurates are heated, they decompose with the formation of tellurites, and they are generally more easily reducible than the latter salts.

In addition to the afore-named classes of salts, pyrotellurates, such as NaHTe₂O₇, can be obtained by the action of the calculated quantities of alkali carbonate or hydroxide on ordinary telluric acid. When heated, these pyrotellurates, which are colourless, soluble salts, eliminate the elements of water:

2KHTe₂O₇ = H₂O + K₂Te₄O₁₃.

The resulting tetratellurates or anhydrotellurates are very sparingly soluble yellow solids, and it has been suggested (by Berzelius) that the yellow insoluble tetratellurates are closely related to the yellow insoluble tellurium trioxide, whilst the colourless ordinary tellurates are to be referred to the colourless soluble telluric acid. On boiling with nitric acid, the tetratellurates are converted into the soluble colourless tellurates. Potassium tetratellurate may also be obtained by fusion of tellurous acid or an alkali tellurite with potassium nitrate. On cooling, the mass is extracted with water, and the tetratellurate, K₂Te₄O₁₃, remains behind as an insoluble powder. On reduction of this salt with nascent hydrogen, for example by suspending the powder in dilute hydrochloric acid and adding zinc, elementary tellurium is obtained as a black powder.

That the foregoing classes of salts may be derived from different acids is quite feasible, for the second form of telluric acid described, allotelluric acid, obtained by heating the crystalline acid H₆TeO₆, differs from the orthoacid not only in its greater acidity but in its precipitation reactions.

It is evident that telluric acid is notably different from wfrat might be expected by analogy with sulphuric and selenic acids. One is hardly surprised, therefore, that isomorphism is exceedingly rare between the tellurates and the sulphates or selenates. The tellurates do not form alums, they do not as a rule form mixed crystals with the sulphates or selenates, the only fairly satisfactory case of mixed-crystal formation being with rubidium hydrogen sulphate and rubidium hydrogen tellurate.

This exceptional behaviour of the tellurates is one of the arguments used by those chemists who wish to place tellurium elsewhere than in the sulphur group of elements.

Tellurates of organic bases have been described, for example carbamide tellurate, CO(NH₂)₂.H₂TeO₄.0.5H₂O; thiocarbamide tellurate, CS(NH₂)₂.4H₂TeO₄; hexamethylenetetramine tellurate, (CH₂)₆N₄.H₂TeO₄.4H₂O; and piperazine tellurate, C₄H₁₀N₂.2H₂TeO₄.4H₂O.

Chromotellurates, 2R₂O.4CrO₃.TeO₃, of sodium, potassium and ammonium have been prepared by spontaneous evaporation of an aqueous solution containing the corresponding dichromate (1 mol.), chromium trioxide (2 mols.) and telluric acid (1 mol.). Two ammonium molybdotellurates, 3(NH₄)₂O.6MoO₃.TeO₃.7H₂O and 3(NH₄)₂O.6MoO₃.2TeO₃.10H₂O, have also been obtained. Guanidinium salts of the molybdotelluric acid and of the corresponding tungstotelluric acid have been prepared, and have been formulated in accordance with Rosenheim's views on the constitution of such heteropoly-acids: (CN₃H₆)₆[Te(MoO₄)₆].6H₂O and (CN₃H₆)₆[Te(WO₄)₆].3H₂O.