Three series of alkali vanado-selenites are known, the members of which are all prepared by treating solutions of alkali vanadates with selenous acid in varying proportions, or by acidifying mixed solutions of vanadates and selenites with acetic acid. The members of the first series are yellow, and have the composition R•2O.V2O5.2SeO2.xH2O. The second series of salts is orange, and includes types of varying composition: R•2O.3V2O5.4SeO2.xH2O, R•2O.6V2O5.8SeO2.xH2O, as well as other types which contain greater proportions of SeO2. The salts belonging to the third series are red, and have the composition 4R•2O.6V2O5.5SeO2.xH2O. All these vanado-selenites lose selenous oxide on being boiled with water. It has been shown that when the same amount of ammonium metavanadate is boiled with selenous acid solutions of gradually increasing concentration, the yellow crystals obtained contain gradually increasing proportions of selenous oxide. It appears, therefore, that the large number of vanado-selenites known, and their complexity, are due to their semi-colloidal character, in consequence of which they adsorb constituents from solutions. As might be expected, only a few of them can be represented by co-ordinative formulae. According to Prandtl, the second and third series above are derivatives of hexavanadic acid, H4V6O17.
By the action of vanadium pentoxide on selenous acid solutions, or by reduction of a solution of vanadium pentoxide in selenic acid, red crystals of a free vanado-selenous acid have been obtained, the composition of which is 3V2O5.4SeO2.4H2O.xaq. It contains four molecules of water of constitution, and, according to Prandtl, should be formulated H4V6O17.4H2SeO3.(x – 2)aq. |