9-Molybdo-arsenates

In the series of the 9-Molybdo-arsenates the complex anion is more stable than in the two preceding series. The salts are derived from the acid,



which is formed by agitation of a solution of sodium arsenate saturated with molybdic acid, with hydrochloric acid and ether. It is very soluble in water and yields two hydrates: H₁₂[As₂O₂(Mo₂O₇)₉].24H₂O, red crystals, stable at ordinary temperatures; and H₁₂[As₂O₂(Mo₂O₇)₉].34H₂O, yellow crystals, stable below 8° C. On treating the solution with alkali hydroxide or carbonate, the alkali salts are obtained. The sodium salt, Na₆H₆[As₂O₂(Mo₂O₇)₉].20H₂O, is yellow, and is an acid salt, indicating that the basicity of the acid is higher than 6. On standing, the crystals of this salt crumble and change into a colourless 25-hydrate. The corresponding yellow potassium salt, containing 11H₂O, changes similarly into a colourless 25-hydrate. Normal salts, of composition R^•₁₂[As₂O₂(Mo₂O₇)₉].xH₂O (R^• = Cs, Ag, Tl, CH₆N₃), have been obtained by adding the metallic chloride to a solution of the yellow acid sodium salt, thus indicating that the basicity of the acid is 12.

Arsenic compounds corresponding to the 17-molybdo-2-phosphates have not been prepared, but by mixing solutions of paramolybdates and alkali dihydrogen arsenates, white precipitates of composition 5R^•₂O.As₂O₅.16MoO₃.xH₂O (R^• = NH₄.K) have been obtained, which may be regarded as 8-molybdo-arsenates, R^•₅[As(OH)₂(Mo₂O₇)₄].(x-2)H₂O.