Metal Polysulphides

Sulphur shows a remarkable power of combining with metals, especially the alkali and alkaline earth metals, in proportions greater than the usually recognised equivalent; thus, the existence has been established of a complete series of poly-sulphides of general formula R₂S_x, where x ranges from 2 to 5 in the sodium series and from 2 to 6 in the potassium series. Iron pyrites, FeS₂, may be regarded as belonging to the class of polysulphides.

The simplest method, in practice, for the production of the alkali polysulphides is supplied by the interaction of sulphur and the alkali sulphide in hot aqueous or alcohol solution. "Liver of sulphur," obtained by fusing sulphur with potassium carbonate, is, when freshly prepared, mainly a mixture of potassium polysulphides with potassium thiosulphate. Solutions of the hydroxides of the alkali or alkaline earth metals also dissolve sulphur, yielding solutions of the polysulphides and thiosulphates of the corresponding metals. When a suspension of sulphur in aqueous ammonia is treated with hydrogen sulphide in the absence of air, a red solution is obtained, which on cooling yields yellow crystals of ammonium pentasulphide, (NH₄)₂S₅. Bloxam claimed to have separated tetra-, penta-, hepta- and nona- sulphides in this way, whilst Thomas and Riding, using alcoholic ammonia, obtained only what they considered to be di-, penta- and hepta-sulphides. Mills and Robinson, however, were unable to obtain evidence of the formation of any polysulphide other than the pentasulphide.

The solutions of these polysulphides are yellowish-brown, and on treatment with acid in the usual manner they yield hydrogen sulphide and a precipitate of sulphur. In many cases polysulphides of the alkali and alkaline earth metals have been isolated in the solid state; the types M₂S₄ and M₂S₅ appear to be the most stable as a general rule.

Fused mixtures of alkali sulphide and sulphur attack glass, but not glazed porcelain, above 400° C.; this action is greatest when the composition corresponds to the disulphide.

Alkali polysulphides react in solution with sodium sulphite, forming thiosulphate and sulphide; by determining the amount of thiosulphate produced, the polysulphide sulphur in the original solution may be estimated.