Tungsto-arsenic Acids

Tungsto-arsenic Acids and Tungsto-arsenates are analogous to the phosphotungstic acids and their salts, but the complex anions are essentially more unstable than those of the phosphorus compounds. They are analogous also to the molybdo-arsenates. The investigation of these acids and salts is as yet very incomplete.

Only the ammonium salt, (NH₄)₃H₄[As(W₂O₇)₆].4H₂O, has been prepared. It separates as a white crystalline precipitate when a solution containing normal sodium tungstate (8 gram-molecules) and arsenic acid (1 gram-molecule) is acidified with hydrochloric acid, treated with solid ammonium chloride, and boiled. The free acid is unstable and undergoes hydrolysis in solution.

These are completely analogous to the 11-tungstophosphates. The barium salt, 7BaO.As₂O₅.22WO₃.54H₂O or Ba_3.5[As(OH)(W₂O₇)₅]-W₂O₇-[(W₂O₇)₅(OH)As]Ba_3.5.53H₂O, is obtained by adding a 5 per cent, solution of ammonium carbonate to a suspension of the ammonium 12-tungsto-arsenate in boiling water until a clear solution results, when, on adding barium chloride, the salt crystallises in double pyramids isomorphous with the corresponding phosphate. Other salts of the series may be obtained from the barium salt by double decomposition. As indicated by the co-ordination formula, the molecule is considered to contain two nuclei joined by an outer bridge.

9-Tungsto-arsenic acid (luteo-arsenotungstic acid) may be obtained by saturation of a boiling solution of an alkali arsenate with tungstic anhydride and allowing the mixture to stand for several weeks until the reactions of the 12-tungsto-arsenate have disappeared; or by acidifying a solution of one of its salts and shaking out with ether. The acid is isomorphous with the corresponding phosphoric acid and has the composition H₅[As(OH)(W₂O₇)₄-W₂O₇-(W₂O₇)₄(OH)As]H₅.36H₂O, the two nuclei being joined by an inner bridge. It melts at 26° C. Only the tribasic ammonium and potassium salts of this series have been prepared. These are obtained by adding arsenic acid to a boiling solution of alkali tungstate until the colour becomes deep yellow, and then treating with ammonium or potassium chloride. On evaporation the solution yields lemon-yellow crystals of composition 3R₂O.As₂O₅.18WO₃.xH₂O (R = NH₄, K).

Solutions of the 9-tungsto-arsenates, when treated with the corresponding carbonates until the yellow colour disappears, yield colourless crystalline precipitates of salts belonging to this series, isomorphous with the corresponding phosphates and of composition 5(NH₄)₂O.As₂O₅.17WO₃.16H₂O and 5K₂O.As₂O₅.17WO₃. 22H₂O.

Alumino-tungsto-arsenates of composition 6(NH₄)₂O.2Al₂O₃.3As₂O₅.18WO₃.14H₂O, 4BaO.2Al₂O₃.3As₂O₅.18WO₃.12H₂O and 4CdO.2Al₂O₃.3As₂O₅.18WO₃.17H₂O, have been described.

According to Jander and his co-workers, the gradual addition of hydrochloric acid to a solution of an alkali tungstate produces first a polymerisation to hexatungstic acid -

6WO₄⁼ + 6H⁺ ⇔ W₆O₂₁^VI + 3H₂O

the salts of which are identical with the paratungstates. With addition of more acid, further polymerisation occurs to metatungstic acid, which is probably a diparatungstic acid, and in the presence of arsenic acid an heteropoly-acid is formed, thus:

H₂WO₄ → H₆[W₆O₂₁] → H₈[As₂(W₂O₇)₉]

The existence of such an acid or of derived salts lacks confirmation, however.