Indium sesquisulphide, In₂S₃

This compound was prepared by Winkler as a brown solid by heating either indium or its sesqui-oxide with sulphur. By igniting a mixture of sodium carbonate, indium oxide and sulphur, Winkler obtained the sesquisulphide of indium in shining, yellow crystals. Thiel has prepared the sulphide in scarlet-red shining crystals by igniting indium sesqui-oxide in a current of hydrogen sulphide. The sulphide is not volatile at a red heat, but when heated in air it is decomposed with the ultimate formation of indium sesqui-oxide.

When hydrogen sulphide is passed into an aqueous solution of an indium salt, indium sesquisulphide is thrown down as a yellow precipitate, resembling cadmium sulphide in appearance. The precipitation is prevented by the presence of mineral acids except when the acidity of the solution is very slight, but acetic acid does not inhibit the precipitation. The yellow sulphide is soluble in concentrated mineral acids.

Indium sesquisulphide is obtained in colloidal solution by passing hydrogen sulphide into water in which indium hydroxide is suspended. Excess of hydrogen sulphide may be removed by boiling, but the addition of an electrolyte leads to the precipitation of the yellow sulphide.

Indium sesquisulphide forms either double or complex salts with alkali sulphides. Indium potassium sulphide, K₂S.In₂S₃ or K₂(In₂S₄), is obtained as insoluble, hyacinth-red, quadratic tables when indium sesqui-oxide (1 pt.), potassium carbonate (6 pts.), and sulphur (6 pts.) are fused together. By using sodium carbonate, the product obtained is soluble in water, but a white precipitate of indium sodium sulphide, Na₂(In₂S₄).2H₂O, slowly separates out. Potassium sulphide gives a white precipitate, probably of the composition K₂(In₂S₄).xH₂O, when added to the solution of an indium salt. In excess of potassium sulphide the precipitate dissolves. The addition of mineral acid to this solution leads to the precipitation of yellow indium sulphide, which dissolves in excess of the acid. Potassium hydrosulphide also produces a white precipitate with an indium salt, but it is insoluble in excess of the precipitant. An analogous white precipitate is produced by ammonium sulphide; it dissolves in excess of the hot, but not of the cold precipitant.