Hypovanadates

Hypovanadates also known as Vanadites. In addition to being soluble in acids, hypovanadic oxide dissolves in excess of hot alkalis. Cooling these solutions in the absence of air gives rise to a series of crystalline hypovanadates or vanadites. Insoluble vanadites are formed by double decomposition of a metallic salt and an alkali vanadite; they usually possess the general formula R₂V₄O₉. xH₂O, and are perhaps salts of the hypothetical acid H₂V₄O₉. They are comparable to the salts produced by other weakly acid oxides, e.g. uranates, metatungstates and borates, in that the acid oxide has undergone condensation to a poly-acid.

Vanadites are either black or brown in the solid state. They become green in damp air, and in solution readily undergo oxidation to the corresponding vanadates. Addition of tannin solution produces a blue-black coloration. Lead acetate throws down a curdy, brown precipitate; silver nitrate gives a black, crystalline precipitate. According to Crow the latter has the composition of a silver vanadite, in which there are, however, only two molecules of acid oxide in combination with one molecule of the base, Ag₂O.2VO₂; but according to Koppel and Goldmann the precipitate consists of a mixture of silver vanadate, silver vanadite, and metallic silver. The following vanadites have been prepared:

A solution of a vanadyl salt, e.g. vanadyl dichloride, VOCl₂, is slowly added in calculated quantity to boiling concentrated ammonia solution. On cooling in the absence of air, golden-brown, glistening needles or scales are deposited, readily soluble in water but insoluble in alcohol, ether, or ammonia. The substance gradually loses ammonia on exposure to air.

Barium Vanadite forms two hydrates, BaV₄O₉.4H₂O and BaV₄O₉.5H₂O, which are obtained by the action of excess of baryta water on a solution of vanadyl dichloride. They are brown, amorphous substances, easily soluble in nitric and hydrochloric acids.

Lead Vanadite, PbV₂O₅ or PbO.2VO₂, is obtained as a brown, curdy precipitate by the action of lead acetate on potassium vanadite.

A boiling solution of a vanadyl salt is poured into an excess of a 10 per cent, solution of caustic potash and the product cooled in the absence of air. Alternatively, a solution of ammonium vanadite is warmed with caustic potash. Potassium vanadite forms a brown, pearly, glistening mass, and is similar to the ammonium salt in its general properties. A heptahydrate, K₂V₄O₉.7H₂O, has been reported.

Sodium Vanadite, Na₂V₄O₉.4H₂O, forms golden-brown needles, and is prepared similarly to the corresponding potassium salt. Brown, crystalline scales of a heptahydrate, Na₂V₄O₉.7H₂O, have also been reported.