Potassium sulphide, K₂S

If a solution of caustic potash is saturated with sulphuretted hydrogen, so much of this is absorbed that the salt KHS, potassium hydrosulphide or the acid salt of sulphuretted hydrogen, is formed. By evaporation it can be obtained as a very deliquescent salt, containing ½H₂O of crystallisation. If as much caustic potash is added as had been originally taken, and the solution evaporated, potassium sulphide, K₂S, which is also very soluble, can be obtained with 5H₂O of crystallisation. Anhydrous potassium sulphide is obtained by the reduction of potassium sulphate with pure charcoal: K₂SO₄ + 4C = K₂S + 4CO. It is, however, very difficult to obtain a pure product, because the fused potassium sulphide attacks vessels of all kinds and becomes contaminated with the constituents of these.

The aqueous solution of potassium sulphide has a strongly alkaline reaction, and for the most part does not contain the ions K^• and S''. On the contrary, the latter react with the solvent water and form HS' and OH', so that the solution contains hydroxidion in large quantity. Here, as in all such cases, we are dealing with a chemical equilibrium in which all possible ions are present - sulphidion, S'', therefore, as well; the amount of the latter is, however, very small.

The solutions of potassium sulphide rapidly oxidise in the air, the sulphide being converted into the potassium salts of the oxygen acids of sulphur. In the first instance, thiosulphanion is formed from the ion HS' by the absorption of oxygen: 2HS' + 2O₂ = S₂O₃' + H₂O.

In analytical chemistry, potassium sulphide is employed in order to obtain those sparingly soluble metallic sulphides which are decomposed by acids: e.g. K₂S + FeCl₂ = FeS + 2KCl. For this purpose, it is of no consequence that only a small amount of sulphidion is present in the solution, for when this is removed in the precipitate, a fresh quantity is immediately formed from HS' and OH'. For the above reaction, however, potassium sulphide is less used than the similarly acting ammonium sulphide, since the latter, or its products of transformation, can be much more readily removed from the analysis than can potassium sulphide.

If a solution of potassium sulphide is warmed with sulphur, large quantities of the latter are taken up, and the potassium salts of the ions S₄'' and S₅'' are formed, according to the amount of sulphur dissolved. They are all characterised by the fact that they give yellow-red solutions. If the liquids be poured into hydrochloric acid, the corresponding acids H₂S₄ and H₂S₅ separate out as oily liquids, which very readily decompose into sulphur and sulphuretted hydrogen, and are not known in the pure state. If, on the other hand, the hydrochloric acid is added to the sulphide solutions, sulphur and sulphuretted hydrogen are immediately obtained. The former is very finely divided, and constitutes milk of sulphur.

The peculiar difference of these two reactions is due to the fact that, in the first case, the acids formed, H₂S₂ to H₂S₅, are always in contact with the acid liquid; in the second case, with the liquid containing potassium sulphide. The latter, however, acts catalytically on the poly sulphides of hydrogen, and accelerates their decomposition into sulphuretted hydrogen and sulphur.