Tellurites

The alkali tellurites are obtainable from the dioxide by treatment with an aqueous solution of an alkali hydroxide or carbonate or by fusion with an alkali carbonate. They are soluble in water and the other tellurites can be prepared from them by precipitation, those of the alkaline earths being sparingly soluble, the remainder even more sparingly soluble.

In composition, the salts are not all closely related to the monohydrated acid, H₂TeO₃, although many are derived from this acid. Other salts are known which appear to be derived from hypothetical condensed acids of composition H₂Te₂O₅, ditellurous acid, and H₂Te₄O₉, tetratellurous acid, which may be regarded as products of the partial dehydration of the normal acid, H₂TeO₃, or as intermediate products in the hydration of tellurium dioxide.

The tellurites in aqueous solution are not very stable, being decomposed by hydrogen sulphide or atmospheric carbon dioxide and undergoing oxidation when treated with oxidising agents such as the permanganates, the halogens or hydrogen peroxide, with formation of tellurates.

They are also reducible to tellurium by dextrose, by sodium hydrosulphite and by electrolysis.

On heating to a temperature of 440° to 470° C. in air, tellurites undergo oxidation, although under these conditions tellurium dioxide is not oxidised nor do the tetratellurites undergo oxidation. In the case of potassium ditellurite the oxidation results in the formation of a compound having the composition K₂O.TeO₃.TeO₂.

With the exception of magnesium tellurite the precipitated tellurites are flocculent and show no tendency to crystallise. The following tellurites have been prepared:

Barium Tellurite, obtained by precipitation from barium chloride solution; the precipitate appears to occlude large quantities of the chloride.
Cadmium Tellurite, 3CdTeO₃.2H₂O.
Cobalt Tellurite, CoTeO₃.H₂O.
Lead Tellurite, 3PbTeO₃.2H₂O.
Magnesium Tellurite, 5MgTeO₃.9H₂O and 10MgTeO₃.9H₂O.
Manganous Tellurite.—This salt is readily oxidised by the air at ordinary temperatures to the manganic state.
Nickel Tellurite, NiTeO₃.2H₂O.
Potassium Tellurite, K₂TeO₃.3H₂O; Potassium Ditellurite; Potassium Tetratellurite, K₂Te₄O₉.4H₂O.
Silver Tellurite, obtainable in several varieties, differing in colour.
Sodium Tellurite, Na₂TeO₃.5H₂O; Sodium Ditellurite; Sodium Tetratellurite, Na₂Te₄O₉.4H₂O.
The ammonium salt could not be isolated. A copper ammonium tellurite of composition TeO₂ = 83.84, CuO = 4.63, NH₃ = 5.22, H₂O = 6.10 per cent, has been obtained.