Hypovanadous Sulphate, VSO₄,7H₂O

Hypovanadous Sulphate, VSO₄.7H₂O, is obtained in solution when vanadium pentoxide is dissolved in sulphuric acid and reduced in the absence of air with sodium-amalgam, zinc, or cadmium. The separation of the hydrate is effected by first reducing the pentoxide in acid solution with sulphur dioxide to the blue tetravalent state, and then continuing the reduction electrolytically in an atmosphere of carbon dioxide until the colour of the solution changes to violet. Concentration in vacuo deposits transparent, reddish-violet crystals, which become bluish-violet even with traces of oxygen. The crystals are monoclinic, and isomorphous with crystals of ferrous sulphate, FeSO₄.7H₂O. They undergo oxidation rapidly on exposure to air and are readily soluble in water; dilute aqueous solutions in the absence of oxygen evolve hydrogen, while concentrated solutions also yield hydrogen sulphide. When brought into contact with other compounds the sulphate exerts the same reducing action as hypovanadous chloride. Its solutions have the property common to solutions of ferrous sulphate and chromous sulphate of absorbing nitric oxide.

Hypovanadous sulphate gives rise to double sulphates with the sulphates of ammonium, potassium, and rubidium. Hypovanadous ammonium sulphate, VSO₄. (NH₄)₂SO₄.6H₂O, is formed when ammonium vanadate is electrolytically reduced in the presence of sulphuric acid. Hypovanadous potassium sulphate, VSO₄.K₂SO₄.6H₂O, is made by electrolytically reducing vanadium pentoxide in sulphuric acid solution, then adding potassium sulphate and continuing the reduction. In the same way hypovanadous rubidium sulphate has been prepared, but it has not been found possible to obtain it free from vanadium rubidium alum. These double sulphates form violet, monoclinic crystals, which are not stable in air; they are not so rapidly oxidised as the simple hypovanadous salt, however; they are less soluble in water than the simple salt, and give yellow or brownish-yellow solutions which may contain complex ions. Hypovanadous sulphate also has the property of forming mixed crystals with magnesium, ferrous and chromous sulphates; these have the formulae: (V.Mg)SO₄.7H₂O; (V.Fe)SO₄.7H₂O; (V.Cr)SO₄.7H₂O.