Vanadous Sulphate, V₂(SO₄)₃

Vanadous Sulphate or vanadium sesquisulphate, V₂(SO₄)₃, is readily obtained in solution by reducing a solution of vanadium pentoxide in sulphuric acid either by means of magnesium or electrolytically. The solid has been obtained by slowly heating to 180° C. in an atmosphere of carbon dioxide a solution of green acid vanadous sulphate, V₂(SO₄)₃.H₂SO₄.12H₂O (see next page), in water which has been slightly acidified with sulphuric acid. Ammonium vanadous sulphate, (NH₄)V(SO₄)₂, can also be used. Vanadous sulphate forms a micro-crystalline, yellow powder, which is insoluble in water; it decomposes on being heated in air with formation of vanadium pentoxide. By the action of concentrated sulphuric acid on vanadic acetate several green or yellowish- green hydrates of vanadous sulphate have recently been prepared: V₂(SO₄)₃.3H₂O; V₂(SO₄)₃.xH₂O; V₂(SO₄)₃.9H₂O; V₂(SO₄)₃.10-11H₂O; of these the tri- and tetra-hydrates appear to correspond in general behaviour to the green tri- and tetra-hydrates of chromium sulphate.

Electrolytic reduction of solutions of vanadyl sulphate, VOSO₄, has given two green, crystalline, acid vanadous sulphates, according to the amount of sulphuric acid also added:

HV(SO₄)₂.4H₂O or V₂(SO₄)₃.H₂SO₄.8H₂O.
HV(SO₄)₂.6H₂O or V₂(SO₄)₃.H₂SO₄.12H₂O.

By electrolysing a solution of ammonium metavanadate in sulphuric acid instead of vanadyl sulphate, the ammonium salts of these acid sulphates are obtained, viz. NH₄V(SO₄)₂.4H₂O and NH₄V(SO₄)₂.6H₂O. The latter is referred to again below. These acids (and their salts) differ from one another in their general properties, for instance in the acidity of their solutions, in the electrical conductivity of their solutions, and in the ease with which they undergo oxidation. From a study of their analogy with the corresponding compounds of chromium, the constitution [V(SO₄)(H₂O)₄]SO₄H has been ascribed to the tetrahydrate acid, and the constitution [V(SO₄)₂(H₂O)₂]H.4H₂O to the hexahydrate acid. By the action of sulphuric acid in varying quantity on solutions of vanadic acetate in glacial acetic acid, compounds having the compositions HV(SO₄)₂.5H₂O and HV(SO₄)₂.8H₂O have been prepared; dehydration of these gives rise to HV(SO₄)₂.2H₂O and HV(SO₄)₂. The ammonium salt of the pentahydrate acid, NH₄V(SO₄)₂.5H₂O or [V(H₂O)₄](SO₄)(SO₄NH₄)-H₂O, has also been isolated.

When the reduction of the vanadyl sulphate is allowed to proceed in the presence of the sulphates of the alkali metals or of thallium, two series of double salts are obtained, the composition of which again varies with the amount of sulphuric acid present and with the other conditions.

Series I. has the general formula RV(SO₄)₂.12H₂O or R₂SO₄. V₂(SO₄)₃.24H₂O, which corresponds to the composition of the alums formed by the metals of Group III. and other groups. The vanadium alums are very similar in their behaviour to ferric, chromium and aluminium alums. They crystallise in regular, pentagonal hemihedra, varying in colour, with increase in atomic weight of the alkali metal, from the violet of the ammonium alum to the ruby-red of the caesium alum. The following table includes those alums which have hitherto been prepared:

Vanadium Alums

9° C.

Formula	Solubility in 2 litres of Water at 25° C. (gram- mols.).	Temperature at which Crystals Melt in their Water of Crystallisation.	Colour	Density
V2(SO4)3.(NH4)2SO4.24H2O	1.210	20° C.	Violet	1.687
V2(SO4)3.Na2SO4.24H2O	. . .	45° C	. . .	. . .
V2(SO4)3.K2SO4.24H2O	3.480		Greenish-violet	1.782
V2(SO4)3.Rb2SO4.24H2O	0.177	64° C.	Bluish-violet	1.915
V2(SO4)3.Cs2SO4.24H2O	0.020	82° C.	Ruby-red	2.033
V2(SO4)3.Tl2SO4.24H2O	0.573	48° C.	Reddish-violet	2.342

Bultemann observed that vanadium ammonium alum separates out in blue crystals from a solution containing sulphuric acid, but from solutions containing a weak acid, or from neutral solutions, red crystals are obtained. (The chromium alums can also be prepared in differently coloured modifications.) The analytical data, melting-point, electrical conductivity, rate of efflorescence, and general behaviour of both kinds of crystals are identical, so that it is difficult to ascribe different constitutions to them. Meyer and Markowitz have shown that both forms separate out when the molecular proportion of sulphuric acid in the solution is less than that theoretically required, and attribute the red colour to the presence of traces of vanadous oxide, V₂O₃, or its hydroxide, V(OH)₃. Vanadium rubidium and vanadium caesium alums behave in the same way. A vanadium guanidine alum has also been prepared.

The general constitutional formula for the vanadium alums is [V(H₄O₂)₆](SO₄)(SO₄R), which differs from the formula for other double vanadium sulphates in that the sulphato groups are not attached to the nuclear vanadium atom.

Series II. - This appears to comprise the salts of the acid HV(SO₄)₂.6H₂O mentioned above, since they can be obtained by boiling this acid with the respective alkali sulphates. Only two members of the class have been examined:

(NH₄)₂SO₄.V₂(SO₄)₃.12H₂O or (NH₄)V(SO₄)₂.6H₂O.

(Rb)₂SO₄.V₂(SO₄)₃.12H₂O or RbV(SO₄)₂.6H₂O. By heating the alkali vanadyl sulphates, compounds have been obtained which can be looked upon either as the dehydrated forms of members of this series, or as salts of the acid sulphate, HV(SO₄)₂, mentioned above:

(NH₄)V(SO₄)₂ or (NH₄)₂SO₄.V₂(SO₄)₃.
NaV(SO₄)₂ or Na₂SO₄2.₂(SO₄)₃.
KV(SO₄)₂ or K₂SO₄.V₂(SO₄)₃.

These have the general formula [V(SO₄)₂]R.

It is of some interest to note that iron and aluminium also give rise to double sulphates in which the number of molecules of water is less than in the alums.