Vanadium Pentasulphide, V₂S₅

Vanadium pentasulphide is prepared by heating vanadium trisulphide with slight excess over the calculated quantity of sulphur at 400° C. for several hours; excess of sulphur is finally removed with carbon disulphide. This reaction is reversible, for on being heated in the absence of air the pentasulphide loses sulphur and re-forms the trisulphide. Heated in air it forms vanadium pentoxide. It is a black powder, of density 3.0. It does not differ from the lower sulphides in its behaviour towards acids; colourless ammonium sulphide also dissolves it to give the purple solution given by the other sulphides, but yellow ammonium sulphide produces a brownish-red solution. It differs markedly from the other sulphides in being readily dissolved by caustic soda, especially on warming, and in this respect acts in a manner comparable to vanadium pentoxide. Several thiovanadates analogous to the vanadates are known.

The order of stability of the sulphides of vanadium is not the same as that of the oxides; for whilst in the oxygen series the penta-compound is stable at a red heat, in the sulphur series the penta-compound is converted into the trisulphide at this temperature. Again, the trioxide is permanent in hydrogen at intense redness, whilst the trisulphide is reduced to the monosulphide under similar conditions.

A sulphide in which the vanadium is tetravalent, corresponding to the oxide VO₂, has not hitherto been prepared.