Molybdo-vanadophosphates

These are derived from the hypothetical acid H₇[PO₆], in which the co-ordinated oxygen atoms have been partly replaced by one or more (V₂O₆)'' radicals and partly by (Mo₂O₇)'' radicals. The relative proportions of these radicals present may undergo variation with constant phosphorus content, so that a phosphorus atom must form the central atom of the complex anion. As the phosphorus content also varies, it is obvious that a very large number of acids becomes possible. These are usually heptabasic, hexabasic, or pentabasic, and the ammonium, potassium, and barium salts, as well as the barium-ammonium and barium-potassium double salts of many of them are known. Their general method of preparation consists in acidifying solutions containing molybdates, vanadates, and phosphates. They form isomorphous, beautiful orange-red to brown, octahedral crystals, which undergo hydrolysis very readily, so that it is frequently impossible to state the experimental directions whereby a salt of definite composition can be produced. A few examples only of these compounds are here set out. For the exhaustive list of those hitherto prepared and analysed, the references cited should be consulted.

6(NH₄)₂O.P₂O₅.6V₂O₅.12MoO₃.41H₂O or ; 5K₂O.P₂O₅.2V₂O₆.20MoO₃.53H₂O or ; 6K₂O. (NH₄)₂O.P₂O₅.3V₂O₅.18MoO₃.43H₂O or ; 3BaO.3(NH₄)₂O,P₂O₅.4V₂O₅.14MoO₃.39H₂O.

Many of the compounds in this class cannot be represented by coordinative formulae.

Tungsto-vanadophosphates. - These are analogous to the molybdo-vanadophosphates, only differing from them in that the place of the (Mo₂O₇)'' radical in the complex anion is now taken by the (W₂O₇)'' radical. Thus, an ammonium tungsto-vanadophosphate which, from its analytical data, has the formula 6(NH₄)₂O.P₂O₅.4V₂O₅.16WO₃.21H₂O, is written , and a barium salt which has the composition 18BaO.3P₂O₅.2V₂O₅.60WO₃.144H₂O becomes / A number of ammonium, potassium and barium salts have been prepared. The ammonium salts in some cases approach to the following series:

; ; .

By the action of dilute sulphuric acid on the barium salts several free tungsto-vanado phosphoric acids have been isolated.