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Double Vanadates

Vanadium pentoxide exhibits a strong tendency to combine with two or more bases to produce well-crystallised double salts. Their general method of preparation consists in adding a solution of a salt of the required metal to a solution of an alkali vanadate; concentration of the mother-liquor after precipitation of the simple vanadate gives rise to a vanadate which is found to contain both the alkali base and the metallic base. Frequently from the same two bases and vanadic acid a series of such double vanadates can be prepared in which the relative proportions of basic oxides and vanadic acid vary. The decomposition of ammonium metavanadate with acetic acid in the presence of sodium silicate yields glistening red crystals of the composition 2(NH4)2O.Na2O.5V2O5.15H2O. The molecular proportions in this compound are simple, but a large number of double vanadates have been prepared in which the molecular proportions are by no means simple. Thus, by the action of two molecular proportions of ammonium chloride on a boiling, concentrated solution of one molecular proportion of sodium metavanadate, yellowish-red prisms of composition 4[(NH4)12/11.Na10/11]O.7V2O5.17½H2O are obtained. It is a matter of some doubt whether these complex double salts should be looked upon as definite compounds, each having independent existence, or whether each series is not more correctly regarded as composed of isomorphous mixtures of variable composition. The problem is similar to that of the constitution of the salts of the heteropoly-acids, in which vanadium pentoxide is combined with other acid anhydrides.

Other double vanadates which have been prepared are:

2(NH4)2O.2K2O.4V2O5.3H2O or 4NH4VO3.4KVO3.3H2O;
8Na2O.2K2O.18V2O5.35H2O or 4Na4V6O17.K4V6O17.35H2O;
6Na2O.4K2O.15V2O5.30H2O or 3Na4V6O17.2K4V6O17.30H2O;
K2O.3SrO.7V2O5.20H2O and 30H2O;
2(4K2O.7V2O5.22½H2O) + 3(4ZnO.7V2O5-22½H2O) or 16K2O.24ZnO.70V2O5.225H2O;
K2O.7MnO.8V2O5.25H2O or 2KVO3.7Mn(VO3)2.25H2O;
K2O.11MnO.12V2O5.48H2O or 2KVO3.11Mn(VO3)2.48H2O;
K2O.2MnO.5V2O5.15H2O and 16H2O;
K2O.5NiO.6V2O5.27H2O or 2KVO3.5Ni(VO3)2.27H2O;
K2O.2CoO.5V2O5.16H2O and 16½H2O;

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