11-Tungstophosphates

11-Tungstophosphates (P₂O₅:WO₃ = 1:22) - The free acid corresponding to this series of salts has not been isolated. The salts are very stable and are obtained by careful hydrolysis of the 12-tungstophosphates. If barium carbonate is added to a 30 per cent, solution of 12-tungstophosphoric acid until a faint turbidity appears, on concentration a white barium salt separates which may be recrystallised from solution in dilute acetic acid. The crystals obtained are of composition 7BaO.P₂O₅.22WO₃.54H₂O.

The potassium salt, 7K₂O.P₂O₅.22WO₃.3H₂O, may be obtained in a similar manner by adding potassium carbonate to a suspension of the potassium 12-tungstophosphate until a clear solution is obtained.

The sodium salt, 7Na₂O.P₂O₅.22WO₃.33H₂O, is best prepared by adding sodium sulphate to a solution of the barium salt. On evaporating the filtered solution a mass of needle-shaped crystals is obtained.

These salts are all soluble in water, and the solutions react neutral to litmus. Acid salts of this series have not been obtained. The addition of strong mineral acids to 11-tungstophosphates causes decomposition into the 12-tungstophosphate and the 21:2-tungstophosphate; the following formula has therefore been suggested as an expression of their constitution:

R^•₇[P(OH)(W₂O₇)₅]-W₂O₇-[(W₂O₇)₅(OH)P]R^•₇.xH₂O.

The molecule is thus considered to contain two nuclei joined by an outer bridge.

The following well-crystallised double salts have been prepared by addition of simple metallic salts to solutions of 11-tungstophosphates:

4BaO.3Ag₂O.P₂O₅.22WO₃.34H₂O,
5BaO.2K₂O.P₂O₅.22WO₃.48H₂O.

A series of salts obtained by the addition of hydrochloric acid to mixtures of alkali tungstates and alkali phosphates have been described by Gibbs, who formulates them, 3R₂O.P₂O₅.22WO₃.xH₂O. From their mode of preparation and reactions these salts appear to be more related to the 12-tungstophosphates than to the salts just described. Their composition, however, is not established.