Oxalo-vanadates

These compounds are obtained in large yellow prisms (side by side with red polyvanadates) by the action of vanadium pentoxide on boiling solutions of alkali oxalates, or by the action of normal alkali vanadates on acid oxalates. They have the general formula 3R^•₂O.V₂O₅.4C₂O₃.xH₂O or , i.e. they can be looked upon as derivatives of orthovanadic acid, H₃VO₄. The ammonium, sodium, potassium and barium salts have been prepared. On being boiled with vanadium pentoxide these salts give rise to the oxalo-molybdovanadates, which also form bright yellow crystals. Those which separate most easily have the compositions:

2R^•₂.V₂O₅.2C₂O₃.2MoO₃.xH₂O (RNH₄ or K.)
and
3R^•₂.V₂O₅.2C₂O₃.6MoO₃.xH₂O. (R = NH₄, Na, K or ½Ba.)

These are most probably derivatives of the oxalo-vanadates in which co-ordinative oxygen atoms have undergone substitution by (MoO₄)'' groups. Hence they are alternatively represented as

and
respectively.