9-Tungstophosphoric Acid

9-Tungstophosphoric Acid, (P₂O₅:WO₃ = 1:18), luteophospho- tungstic acid, is formed by treating the 12- or 11-tungstophosphoric acid solutions with excess of phosphoric acid. It may best be prepared from its salts, for example, by treating the ammonium salt with aqua regia, or by decomposing the silver salt with hydrochloric acid. A colourless solution is obtained, which on evaporation turns yellow, and lemon-yellow hexagonal crystals separate. These melt at 28° C. and are very soluble in water. The acid probably exists in two isomeric forms. Its composition is represented by the formula P₂O₅.18WO₃.42H₂O, and its basicity appears to be 10 (see below), so that the constitutional formula

H₅[P(OH)(W₂O₇)₄-W₂O₇-(W₂O₇)₄(OH)P]H₅.36H₂O

has been suggested. In this case an inner bridge connects the two nuclei. This is supported by conductivity measurements and a study of the neutralisation curves. The corresponding molybdo-phosphoric acid is 12-basic, but the complex ion in that acid is not so stable as the tungstic ion. The hydrogen-ion concentration in a 0.005 N solution of 9-tungstophosphoric acid is 4.1×10^-3.

Salts of this acid may be prepared by boiling a mixture containing normal sodium tungstate, Na₂WO₄.2H₂O (2 parts), syrupy phosphoric acid (3 parts), and water (1 part). The solution turns yellow, and on evaporation and cooling, greenish-yellow plates consisting of a mixture of sodium salts separate. If, instead of permitting crystallisation, the mixture is oxidised with a few drops of concentrated nitric acid, allowed to cool, and solid potassium or ammonium chloride added, yellow triclinic prisms of the potassium or ammonium salt separate. These may be purified by repeated crystallisation. They are readily soluble in water, and have composition

3K₂O.P₂O₅.18WO₃.14H₂O,
3(NH₄)₂O.P₂O₅.18WO₃.14H₂O.

These salts, as well as the corresponding sodium and barium salts, may also be obtained by treating the free acid with the corresponding carbonate or chloride. The silver salt, 3Ag₂O.P₂O₅.18WO₃.16H₂O, is prepared by treating a solution of the potassium salt with silver nitrate.

These acid salts correspond to the acid salts of 9-molybdo-phosphoric acid. The following salts, which show a basicity of 10, and appear to be the normal salts of the acid, have also been prepared:

5Ag₂O.P₂O₅.18WO₃.34H₂O,
5(CN₃H₆)₂O.P₂O₅.18WO₃.18H₂O.

The silver salt is obtained by treating a 30 per cent, solution of the acid with 5 to 6 molecular proportions of sodium hydroxide, and adding a soluble silver salt. A yellow flaky precipitate results which is almost insoluble in water. The guanidine salt is produced similarly, and yields bright yellow aggregates of small tables which are only slightlv soluble in water.