Vanado-arsenates

The salts of this class which have hitherto been prepared are all of low vanadium pentoxide content compared with the amount of arsenic pentoxide present. They correspond, therefore, to the luteo vanado-phosphates. Three series are known. The first has the general formula R^••O.V₂O₅.As₂O₅.xH₂O or , where R^•• may be Mg, Zn, Cu, or Co. They form yellow crystals which are obtained by the addition of arsenic pentoxide to solutions of the respective vanadates, or by the action of the carbonate of the metal on the free vanado-phosphoric acid. Of the second series only the ammonium and potassium salts are known, viz. (NH₄)₂O.2V₂O₅.As₂O₅.5H₂O or , and K₂O.2V₂O₅.As₂O₅.5H₂O or . These also give rise to yellow crystals. The third series has the general formula 2R^•₂O.2V₂O₅.3As₂O₅.xH₂O, where R₂ may be (NH₄)₂, Ca, Sr, Mg, Zn, Mn, Co, Ni. They result from the action of arsenic pentoxide on solutions of vanadates, and form red crystals which, however, are unstable, and are readily converted by water into compounds of the first series.

The free acid of the first series has been obtained in yellow plates by boiling vanadium pentoxide with a solution of arsenic acid. Its composition is V₂O₅.As₂O₅.10H₂O or . Recrystallisation of this hydrate from concentrated nitric acid gives a dihydrate, V₂O₅.As₂O₅.2H₂O, which can be looked upon as the acid . The anhydride, V₂O₅.As₂O₅, is obtained by heating the dihydrate to 440° C.