Potassium pervanadates

Pervanadic acid appears, therefore, to be formed from metavanadic acid, HVOa. 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. 

In an excess of concentrated (30 per cent) hydrogen peroxide to which a little sulfuric acid has been added to insure acidity, dissolve the potassium metavanadate, observing that the color changes to an intense yellow. To this solution, well cooled, add 95 per cent alcohol until the pervanadate has been precipitated as a microcrystalline powder. Filter off the product, wash with alcohol, and dry in a desiccator over calcium chloride. 

Concentrated acid solutions of vanadium pentoxide are reduced to the tetravalent state by hydrogen peroxide, the peroxides of sodium, barium, magnesium, and by persulphates of potassium and ammonium.4 Acid solutions of vanadium pentoxide give rise to pervanadic acid with hydrogen peroxide. 

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