Potassium polysulphides

Several polysulphides have been described. According to Thomas and Rule, the complete series K₂S_X exists, x being any whole number up to a maximum of 6. The values obtained for the molecular weights in alcoholic solution by the ebullioscopic method favour the simple formula K₂S_x., and not K₄S_x.

The trisulphide, K₂S₃, is formed by the action of excess of sulphur on a solution of potassium in liquid ammonia. The tetrasulphide, K₂S₄, the most stable member of the series, can be prepared by heating a mixture of potassium carbonate and sulphur at 800° C. in a current of carbon dioxide, or by the action of hydrogen sulphide on the fused pentasulphide. It yields an octahydrate, a trihydrate, and a semihydrate. Its heat of formation from the elements is 116.6 Cal.

The pentasulphide, K₂S₅, is formed at lower temperatures from the carbonate by the action of excess of sulphur, thiosulphate being also produced by heating the two substances in a closed vessel at 250° C., the mixture constituting one variety of the so-called " liver of sulphur." A better method for preparing the pentasulphide is to saturate a solution of potassium ethoxide with hydrogen sulphide and add the equivalent amount of sulphur. By this process it has not been found possible to prepare the disulphide, trisulphide, tetrasulphide, or hexasulphide, even when the calculated proportions of the materials are employed, the product being invariably the pentasulphide. Thus prepared, the substance forms an orange-red, very hygroscopic, crystalline solid. At 220° C. it melts with decomposition. When in contact with air, or in aqueous solution, it undergoes gradual decomposition, with liberation of sulphur.

Polysulphides of the formulae K₄S₅ and K₄S₈ have also been prepared by the interaction of the solid primary sulphide with hydrogen sulphide. Aqueous solutions of the monosulphide are unsuited for preparing individual polysulphides.