Arsenic Subsulphide, As₃S

Arsenic Subsulphide (Arsenic Tritasulphide), As₃S, was first obtained in 1900 by Scott, who treated an aqueous solution of an alkali arsenate with phosphorus trichloride and, after filtering, saturated the mixture with sulphur dioxide. The method of procedure was as follows: 150 c.c. of phosphorus trichloride were added to a solution containing 200 g. of sodium arsenate crystals dissolved in 3.5 litres of water; when cold, the mixture was filtered and sulphur dioxide was passed in. The product was kept for two days, when the supernatant liquor was poured off and the precipitate washed first with water and then with aqueous ammonia. It was then digested for an hour or so with a more concentrated solution of ammonia, hydrogen sulphide being passed into the liquid. Finally the precipitate was washed, first with water and then with alcohol and dried in a vacuum. The subsulphide may be obtained in a similar manner from arsenites, but owing to the precipitation of much arsenious oxide from the strongly acid solution the yield of subsulphide is very much smaller.

By the reducing action of sodium hydrosulphite, Na₂S₂O₄, on arsenious oxide or alkali arsenites, in moderately acid solutions, precipitates are obtained, insoluble in colourless ammonium sulphide and containing about 12.7 per cent, of sulphur. These appear to consist of arsenic subsulphide (As₃S = 12.45 per cent. S) with a small amount of arsenious sulphide. Compounds containing quinquevalent arsenic under similar treatment yield products which approximate in composition to As₂S₃.As₃S; in strongly acid solution the precipitates are mainly As₂S₃. In neutral solution only arsenates yield products containing the subsulphide, those from arsenites containing only 6 to 7 per cent, of sulphur; in alkaline solution precipitation occurs only at the boiling point and no subsulphide is produced. In the formation of the subsulphide, intermediate complex compounds of the types Na₃As(S₂O₄)₃, sodium arsenohydrosulphite, and Na₃As(S₂O₃)₃, sodium arsenothiosulphate, appear to be produced. In no case is the arsenic completely precipitated from the solution.

Arsenic subsulphide is a dark brown powder. It is insoluble in carbon disulphide, in aqueous ammonia and in colourless ammonium sulphide. It dissolves, however, in yellow ammonium sulphide, which is at the same time decolorised, and the addition of an excess of hydrochloric acid to the solution precipitates arsenious sulphide. It is decomposed by caustic alkali. When heated, arsenic subsulphide is resolved into realgar and arsenic.