3-Molybdo-arsenates

The complex anion of this series is the most stable of those derived from [XO₄]'''. The free acid, H₃[AsO(MoO₄)₃], is tribasic, stronger than arsenic acid, and extremely stable, and is obtained on adding fuming nitric acid to a concentrated solution of arsenic acid saturated with molybdic acid. It readily dissolves in water, from which it may be crystallised in the form of white prisms. The trisodium salt crystallises from water with 11½H₂O, and the triguanidinium salt forms pale yellow anhydrous needles. Other salts have been described.

Salt of acids containing less molybdic acid - for example, 2- and 1-molybdo-arsenates of the types R^•₂H[ASO₂(MoO₄)₂].xH₂O and R^•H₂[AsO₃(MoO₄)].xH₂O - have been described, as also have sulphur- containing compounds of the types R^•₄[As₂S₅(MoS₄)₂].xH₂O and R^•[AsS₂(MoS₄)].xH₂O.

Similar compounds with antimony have been mentioned, but it has not yet been established that this element can function as the central atom of such complex anions or give rise to such series of heteropoly- compounds.

Vanadium, the first element in Subgroup 5A, and neighbour to chromium, is found in association with molybdenum in certain complex compounds. For example, when boiling solutions of molybdo-oxalates react with vanadium pentoxide, crystalline products are obtained which are thought to be substituted vanadates containing the complex anion

or ,

while other series of compounds are knowns, in which anions of the types

, ,

are supposed to function. By the interaction of molybdic acid and alkali metavanadates in aqueous solution, a number of crystalline products have been obtained. These appear to be complex molybdo-vanadates of two types which differ considerably in physical properties; those which are richer in vanadium yield large orange-red prisms, whilst those poorer in this element crystallise in yellow silky needles. Complex arsenomolybdovanadates have also been described.