9-Tungsto-arsenic Acid

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 according to Drechsel. The acid is isomorphous with the corresponding phosphoric acid and has composition

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

It melts at 26° C. Only the 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

R₂O.As₂O₅.18WO₃.14H₂O (R = NH₄, K).