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These salts are made by reducing hot solutions of alkali vanadates with sulphur dioxide; after addition of acetic acid they can be salted out by addition of alkali acetate. The following are known:

(NH4)2O.4VO2.2V2O5.14H2OGreenish-black crystals.
3(NH4)2O.4VO2.4V2O5.6H2OPurple crystals with metallic lustre.
(NH4)2O.2VO2.4V2O5.8H2OGreen crystals.
2K2O.4VO2.V2O5.6H2O.Greenish-black crystals.
5K2O.4VO2.4V2O5.H2OPurple crystals with metallic lustre.
2Na2O.4VO2.V2O5.13H2OBlack, shining, hexagonal prisms.

In addition to the above, several vanado-vanadates have been prepared by fusing alkali carbonates, borates, phosphates, silicates, arsenates, etc., with vanadium pentoxide in air. The fused mass evolves oxygen during cooling, and "spitting" of the material takes place. The phenomenon recalls the behaviour of solidifying silver. On again heating in air, oxygen is re-absorbed and acid vanadates are produced. The evolution of oxygen is accompanied by reduction of a portion of the vanadium pentoxide content with formation of the vanado-vanadate. The reaction is represented:

The oxygen is liberated between the upper limit of stability of the vanado-vanadate and the point of solidification of the fused mass. The proportion of oxygen evolved is independent of the anion with which the metal is combined. The alkali salts can be substituted by thallium compounds but not by alkaline earth oxides. Investigation shows that for each base there is a definite vanadium pentoxide content for which the evolution of oxygen is a maximum. Phase diagrams for the systems V2O5-Na2O, V2O5-K2O, V2O5-Li2O, and V2O5-Tl2O show that in each case three definite compounds are formed, corresponding to the meta-, pyro-, and ortho-vanadates. By the foregoing method the following salts have been prepared: 6Li2O.2VO2.11V2O5; 4Li2O.2VO2.7V2O5; Na2O.2VO2.5V2O5; Na2O.2VO2.4V2O5; Na2O.VO2.2V2O5; 3Na2O.2VO2.5V2O5; K2O.2VO2.8V2O5; K2O. 2VO2.5V2O5; 2K2O.2VO2.9V2O5; K2O.2VO2.4V2O5; K2O.VO2.2V2O5; Ag2O.2VO2.5V2O5.

Vanado-vanadates can also be obtained by fusing alkali vanadites with metavanadates, when no oxidation or loss of oxygen takes place. This process has furnished two salts, Na2O.2VO2.V2O5 and K2O. 2VO2.V2O5.

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