Tungsten Trisulphide, WS₃

Tungsten Trisulphide, WS₃, may be obtained by treating a solution of an alkali thiotungstate (see below) with excess of acid. The thiotungstate may first be obtained by fusing wolframite (3 parts) with sodium sulphate (7 parts) and carbon (1.2 parts). The mass is extracted with water and the solution treated with excess of sulphuric or hydrochloric acid. The precipitate is filtered rapidly, washed with water containing hydrogen sulphide, and dried at 100° C. in a current of carbon dioxide. It is necessary to exclude air as much as possible or the product is found to contain oxygen.

Tungsten trisulphide is a brown powder which becomes black when dried. When heated in absence of air it gives up sulphur and forms the disulphide WS₂. It may be reduced to the metal by heating with lime in an electric furnace. When heated in the air it yields tungsten trioxide. It is slightly soluble in cold water, more readily in hot, forming a colloidal solution (see below). It is easily soluble in alkali hydroxides, carbonates, and sulphides, forming dark brown solutions which contain thiotungstates and colloidal tungsten trisulphide.

It was observed by Berzelius that when the freshly precipitated trisulphide is washed with water, it dissolves to a slight extent, forming a yellow solution. If the precipitate is boiled with pure water it dissolves in considerable quantity, yielding a brownish-yellow solution. Winssinger obtained a dark brown solution of tungsten trisulphide by adding to a solution of sodium thiosulphate a little more dilute hydrochloric acid than was necessary to saturate the alkali. The solution could be passed through filter paper. On boiling, or on the addition of electrolytes, the tri-sulphide is precipitated.

The existence of oxysulphides of tungsten, described by Berzelius and others, is doubtful.

A sulphochloride of tungsten, 3WS₃.WCl₆, has been obtained by heating the hexachloride with liquid hydrogen sulphide in a sealed tube for thirty-six hours at 60° to 65° C. It is a brown powder, insoluble in carbon disulphide, alcohol, or benzene; it is decomposed by water. If heated to redness in the air, it takes fire. It is readily oxidised by nitric acid or fused potash.