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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 R12[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 5R2O.As2O5.16MoO3.xH2O (R = NH4.K) have been obtained, which may be regarded as 8-molybdo-arsenates, R5[As(OH)2(Mo2O7)4].(x-2)H2O.
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