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Pervanadic Acid, HVO4

Pervanadic Acid has not been definitely isolated in the solid state. When vanadium pentoxide is dissolved in hydrogen peroxide solution which also contains sulphuric acid, a red solution is formed from which on concentration in vacuo greyish-yellow crystals, which are assumed to have the composition HVO4, pervanadic acid, are deposited. They give the reactions of a per-acid, but are difficult to analyse in consequence of their unstable character. The preparation of the crystals is best carried out at low temperatures, because at ordinary temperatures vigorous evolution of oxygen takes place in consequence of the catalytic decomposition of the hydrogen peroxide by the vanadium pentoxide. A similarly coloured solution with similar properties is also obtained by the action of sulphuric acid on barium pervanadate. According to Meyer and Pawletta, the red solution contains the compound (VO2)2(SO4)3, which undergoes hydrolysis in the following manner:

(VO2)2(SO4)3 + 6H2O ⇔ 2VO2(OH)3 + 3H2SO4.
Reddish-brown ⇔ Yellow

The yellow compound VO2(OH)3 thus formed can be looked upon as a hydrated pervanadic acid, HVO4.H2O, or as peroxyorthovanadic acid, . It decomposes slowly at ordinary temperatures, and more rapidly on being warmed, with evolution of oxygen and reprecipitation of vanadium pentoxide.

It has been shown that in the formation of the red compound in solution each molecule of vanadium pentoxide reacts with two molecules of hydrogen peroxide:

V2O5 + 2H2O2 = 2HVO4 + H2O.

Pervanadic acid appears, therefore, to be formed from metavanadic acid, HVO3. This view of the reaction and the formula for pervanadic acid are further supported by titrating the red solution with caustic soda, when it is found that two molecules of caustic soda are required for each molecule of vanadium pentoxide. The monobasicity of the acid is confirmed by measuring the equivalent conductivities of solutions of potassium pervanadate.

A solution of pervanadic acid evolves a mixture of oxygen and chlorine on the addition of hydrochloric acid; it evolves oxygen slowly on standing, more rapidly on warming, and leaves a residue of vanadium pentoxide. Decomposition of the solution is accelerated by the presence of dilute sulphuric acid. Pervanadates of the alkali metals have been prepared by the action of hydrogen peroxide on the alkali metavanadates; pervanadates of several heavy metals have also been isolated by double decomposition with ammonium pervanadate. These salts vary from bright yellow to deep orange in colour, and are amorphous or micro-crystalline. They undergo hydrolysis in dilute solution, e.g.

KVO4 + H2OKVO3 + H2O2,

and evolve oxygen when platinised asbestos is introduced into their concentrated solutions. The following metapervanadates have been prepared: Ammonium pervanadate, NH4VO4; barium pervanadate, Ba(VO4)2; cadmium pervanadate, Cd(VO4)2; calcium pervanadate, Ca(VO4)2; lead pervanadate, Pb(VO4)2; lithium pervanadate, LiVO4; potassium pervanadate, KVO4; silver pervanadate, AgVO4; sodium pervanadate, NaVO4; and strontium pervanadate, Sr(VO4)2. Scheuer was unable to prepare the pervanadates of aluminium, cobalt, copper, magnesium, manganese, and nickel.

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