Vanadyl Sulphates

By treating hypovanadic oxide, VO₂, with sulphuric acid at about 200° C., or by reducing a sulphuric acid solution of vanadium pentoxide, V₂O₅, with sulphur dioxide, two distinct compounds are obtainable, according to the quantity of sulphuric acid present:

1. Neutral vanadyl sulphate, VOSO₄.
Acid vanadyl sulphate, 2VOSO₄.H₂SO₄.xH₂O.

Vanadyl Sulphate, VOSO₄, exists in two modifications, insoluble and soluble. The insoluble variety is best obtained by heating any of the acid vanadyl sulphates to 260° C. It is a greyish-green, micro-crystalline powder, which is insoluble both in hot water and in hot dilute acids. It is readily decomposed by alkalis to form brown, hydrated hypovanadic oxide. On being heated at 130° C. in a sealed tube with a little water, it undergoes conversion into the soluble form, which can also be obtained by evaporation of a solution of the hydrate, 2VOSO₄.7H₂O, to dryness, or by evaporation of a solution of vanadium pentoxide in sulphuric acid in the presence of alcohol. This modification is a blue, amorphous, gummy mass, completely soluble in water, giving a blue solution which becomes green in air and slowly deposits the green oxide, VO₂. On being heated in the absence of air it again forms hypovanadic oxide; in the presence of air the pentoxide, V₂O₅, is produced.

Several blue, crystalline hydrates of vanadyl sulphate have been prepared from hypovanadic oxide, VO₂: VOSO₄.2H₂O; VOSO₄. 2½H₂O; VOSO₄.3H₂O; VOSO₄.3½H₂O; VOSO₄.5H₂O; VOSO₄. 6½H₂O; VOSO₄.7H₂O. The dihydrate, VOSO₄.2H₂O, has been prepared electrolytically.5 Most of these give a greyish-green monohydrate, VOSO₄.H₂O, on being heated to 150° C., and all of them undergo dehydration to form insoluble vanadyl sulphate at 260° C. It is doubtful if these are all individual chemical compounds; they may be mixtures of hydrates. They are, however, characterised by their crystalline forms, and in that they can, in several cases, be prepared by different methods.

Acid Vanadyl Sulphates, of general formula 2VOSO₄.H₂SO₄.xH₂O, are prepared by reducing solutions of vanadium pentoxide in sulphuric acid of such concentration that there are more than three molecules of sulphuric acid per molecule of vanadium pentoxide.6 Evaporation on the water-bath yields the pentahydrate, 2VOSO₄.H₂SO₄.5H₂O, which, on being heated to 200° C., undergoes dehydration and forms the compound 2VOSO₄.SO₃ or 2VO₂.3SO₃. By treatment at intermediate temperatures the following hydrates have also been prepared: 2VOSO₄. H₂SO₄.3H₂O; 2VOSO₄.H₂SO₄.2H₂O; 4VOSO₄.2H₂SO₄.H₂O. Gain7 has shown that hypovanadic oxide combines with sulphur trioxide to produce various other complexes all of which are blue, crystalline, fairly stable substances. They have the general formula 4VO₂.xSO₃.yH₂O, in which x varies between 4 and 10.

Two series of double salts of vanadyl sulphate and alkali sulphates are known. The members of the first series have the general formula 2 VOSO₄.H₂SO₄.xH₂O, and are obtained by reducing solutions of the alkali metavanadates in sulphuric acid with sulphur dioxide and then adding excess of the particular alkali sulphate desired. They form blue, crystalline compounds which undergo dehydration at 175° C. The following have been prepared: -

2VOSO₄.(NH₄)₂SO₄.H₂O;
2VOSO₄.(NH₄)₂SO₄;
4VOSO₄.2Na₂SO₄.5H₂O;
2VOSO₄.Na₂SO₄;
2VOSO₄.K₂SO₄;

Fischer, by the same method, prepared the double sulphates with calcium, magnesium and aluminium sulphates.

The members of the second series possess the general formula VOSO₄.R^•₂SO₄.xH₂O, and thus contain a smaller molecular proportion of vanadyl sulphate than the members of the preceding series, from which they are prepared by the prolonged action of alcohol on neutral solutions. They also yield dark blue, crystalline compounds, which undergo dehydration at 175° C. The following are known: -

2VOSO₄.2(NH₄)₂SO₄.7H₂O;
VOSO₄.(NH₄)₂SO₄;
VOSO₄.Na₂SO₄.4H₂O;
VOSO₄.Na₂SO₄;
VOSO₄.K₂SO₄.3H₂O;
VOSO₄.K₂SO₄.

The general compositions of these two series of double salts correspond to the compositions of the double sulphates of divalent metals, e.g. 2ZnSO₄.K₂SO₄ and ZnSO₄.K₂SO₄.6H₂O.