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Molybdo-vanadates or Vanado-molybdates. These are obtained in red crystals, rich in vanadium, or yellow powders, poor in vanadium, by dissolving molybdic anhydride, MoO3, in solutions of alkali meta- vanadates, or by acidifying mixed solutions of molybdates and vanadates. A large number of salts have been prepared, which vary considerably in their analytical data. Repetition of the same process of preparation often gives crystals of different composition, so that a definite compound cannot easily be prepared. The salts are usually unstable, and undergo change even on being recrystallised. The same solution may give rise to different salts according to the temperature. Thus, one portion of a solution divided into two parts gave crystals of 2 (NH4)2O.2V2O5.5MoO3.10H2O at 10° C., while at 30° C. the other portion gave crystals of 4(NH4)2O.3V2O5.5MoO3.10H2O. From a study of the system NH4VO3,MoO3; KVO3,MoO3; NaVO3,MoO3, it has been concluded that solutions of alkali molybdo-vanadates of fixed concentration and at a definite temperature contain definite compounds which are in equilibrium with isomorphous mixtures of polyvanadates and poly molybdates. The yellow powders poor in vanadium are presumed to be definite compounds because they resist fractionation, while the red crystals consist of isomorphous mixtures.

Comparison with other heteropoly-acids strongly suggests that the molybdo-vanadates do not contain either vanadium or molybdenum as the central atom of the complex anion. They are therefore written as derivatives of the hypothetical hexa-aquo-acid, H10[H2O6], in which the oxygen atoms in the anion have undergone complete or partial replacement by (V2O6)'' radicals and by (Mo2O7)'' or (MoO4)'' radicals. The following examples are typical of series of alkali (and in some cases barium) salts that have been prepared:

3(NH4)2O.2V2O5.4MoO3.xH2O or ; 5(NH4)2O.2V2O5.12MoO3.xH2O or ; 2(NH4)2O.V2O5.6MoO3.xH2O or ; 3(NH4)2O.V2O5.6MoO3.xH2O or .

The following are also known, as well as many others for which coordinative formulae cannot be written:

4(NH4)2O.3V2O5.5MoO3.10H2O or ; 4K2O.3V2O5.4MoO3.7H2O or .

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