Vanadic Sulphates

Two of these are known, viz. V₂O₅.3SO₃ and V₂O₅.2SO₃. The former has been prepared in ruby-red, transparent octahedra by boiling vanadium pentoxide with excess of sulphuric acid; if the boiling is continued for some time, golden-yellow needles separate out. The crystals absorb water rapidly and simultaneous formation of vanadyl sulphate takes place, so that analysis is difficult. Miinzing doubts the existence of V₂O₅.3SO₃, because by the same process he obtained the substance V₂O₅.2SO₃, which is also extremely deliquescent, and exists either as a red, crystalline mass or as a yellow powder. If the sulphuric acid solution is strongly heated, a dark-brown modification is obtained which changes slowly into the red variety. It has recently been shown that when heated in sulphuric acid solution, tetravalent vanadium is partially oxidised and pentavalent vanadium partially reduced to an equilibrium mixture. A crystalline form of the compound V₂O₅.2SO₃, with rather different properties, has been obtained by the decomposition of alkali vanadyl sulphates. The action of concentrated sulphuric acid on vanadium pentoxide has also given a hydrate, V₂O₅.3SO₃.3H₂O, in either yellow or red, deliquescent crystals. This hydrate can be alternatively written V₂O₅.3H₂SO₄, but no corresponding salts are known, although by addition of the respective alkali sulphate to its solutions two complex salts have been prepared, which can be looked upon as double sulphates of V₂O₅.SO₃, namely,

(NH₄)₂O.V₂O₅.2SO₃.4H₂O and K₂O.V₂O₅.2SO₃.6H₂O.

A compound intermediate in composition between vanadic sulphate and vanadyl sulphate has been obtained by reducing a solution of a vanadic salt in sulphuric acid with sulphur dioxide, or by mixing solutions of the pentoxide and the dioxide in sulphuric acid.