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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.

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