21:2-Tungstophosphoric Acid

21:2-Tungstophosphoric Acid, (P₂O₅:WO₃ = 1:21), may be obtained by heating the ammonium salt with aqua regia; nitrogen is evolved and a solution is obtained from which the acid separates on cooling. It may also be prepared from barium 11-tungstophosphate by precipitation of the barium with sulphuric acid; on evaporation of the solution, 12-tungstophosphoric acid first separates, and from the mother-liquor hexagonal crystals of 21:2-tungstophosphoric acid can then be obtained. The acid is readily soluble in water. The hydrogen- ion concentration in a 0.005 N solution is 7.0×10^-6. Its composition may be represented by the formula 3H₂O.P₂O₅.2lWO₃.55H₂O, indicating a hexabasic acid, and the salts prepared have the composition 3R^•₂O.P₂O₅.21WO₃.xH₂O. These, however, appear to be acid salts, their solutions containing hydrogen ions, so that the acid may have a higher basicity than six. This view is supported by conductivity measurements; for example, in the case of the potassium salt at 25° C. the following are the equivalent conductivity values for solutions containing 1/6 molecule in v litres:

v =	32	64	128	256	512	1024
Λ =	166.8	184.2	202.2	226.3	254.1	285.2

Normal salts of this series have not been prepared, so that the constitution of the acid cannot yet be expressed.

The potassium salt, 3K₂O.P₂O₅.21WO₃.31H₂O, may be prepared by treating a solution of potassium 11-tungstophosphate with excess of dilute hydrochloric acid and boiling the mixture. Potassium 12-tungstophosphate is precipitated and separated by filtration, and on standing the filtrate slowly deposits transparent prismatic crystals of the 21:2- tungstophosphate. This salt may also be prepared by the addition of potassium chloride to a solution of the free acid. The corresponding ammonium salt is obtained by treating a solution of the potassium salt or of the free acid with ammonium chloride. The addition of silver nitrate to a solution of the potassium salt yields the silver salt.