Nitrogen Pentasulphide, N₂S₅

Nitrogen Pentasulphide, N₂S₅, is formed by the decomposition of the yellow sulphide, N₄S₄, and its derivatives. It is formed, for example, when the yellow sulphide is exploded by friction, when the compounds of N₄S₄ with the halogens or with nitrous or nitric acid are boiled with water, and also when N₄S₄ is heated carefully with lead oxide.

The pentasulphide may be prepared by heating together yellow nitrogen sulphide and carbon disulphide under pressure:

N₄S₄ + 2CS₂ = N₂S₅ + S + 2CNS.

A deep red solution is obtained and a brown precipitate. The solution consists of nitrogen pentasulphide and sulphur in carbon disulphide, the pentasulphide being extracted by means of ether. If the product is pure, containing no sulphur, it crystallises from the ether solution in tablets of metallic appearance somewhat resembling iodine. The brown precipitate has the composition C₃N₃S₃.

Nitrogen pentasulphide may also be obtained by the action of carbon tetrachloride on N₄S₄ at 125° C. In this case a black complex by-product is obtained.

The action of zinc dust on a suspension of thiotrithiazyl chloride, N₃S₄Cl, also gives the pentasulphide.

Nitrogen pentasulphide is a deep red liquid, which solidifies to a crystalline mass strongly resembling iodine. It melts at 10° to 11° C. Its specific gravity at 18° C. is 1.901. It is insoluble in water but soluble in most organic solvents. It is unstable in solution, especially on exposure to light, the products of its decomposition being N₄S₄ and sulphur. When boiled with water or with aqueous solutions of the caustic alkalis, ammonia and sulphur are formed. An alcoholic solution of an alkali hydroxide added to an alcoholic solution of nitrogen pentasulphide produces a transient but intense violet-red coloration. So characteristic is this reaction that it may be used for the detection of very small quantities of the pentasulphide. Alcoholic solutions of the alkali sulphides yield ammonia and a polysulphide. With hydrogen sulphide, ammonium polysulphide and sulphur are formed, thus:

N₂S₅ + 4H₂S = (NH₄)₂S₅ + 4S.

The pentasulphide is violently oxidised by concentrated nitric acid, with formation of sulphuric acid, whilst dilute hydrochloric or sulphuric acid yields the ammonium salt and sulphur.

Cryoscopic determinations of the molecular weight, using benzene as solvent, give results agreeing with the formula N₂S₅.