Sodium monosulphide, Na₂S

The Sodium monosulphide is formed by the interaction of sulphur and a great excess of sodium, the superfluous metal being removed by distillation; by the action of hydrogen sulphide on sodium hydroxide; and by the action of sulphur on excess of sodium dissolved in liquid ammonia, and evaporation of the solvent.

A quantitative yield of sodium monosulphide is obtained by heating sodium hydrogen sulphide very gradually, the evolved hydrogen sulphide being continually removed:

2NaSH=Na₂S+H₂S.

The product is a solid of faint buff colour. It dissolves readily in water and in alcohol.

Sodium monosulphide is manufactured by reducing sodium sulphate with charcoal, and can also be obtained in solution from the alkali waste of the soda manufacturer.

According to Rengade, the anhydrous sulphide forms microscopic, white needles. It melts at the temperature of softening of glass, and dissolves in water with a hissing sound. It is readily oxidized, and is combustible.

Four hydrates have been described by Parravano and Fornaini, containing respectively 9, 6, 5½, and 5 molecules of water. The pentahydrate was also prepared by Bottger by adding alcohol to a solution of sodium hydroxide saturated with hydrogen sulphide. Sabatier mentions a 4½-hydrate, obtained by drying the 9-hydrate over sulphuric acid.

The solution of the sulphide in water has an alkaline reaction due to hydrolytic dissociation. Atmospheric oxygen converts the dissolved sulphide into thiosulphate, and electrolytic oxidation yields the sulphate. The solution dissolves sulphur, forming poly sulphides.

The melting-point of sodium monosulphide is 920° C. For the anhydrous salt the density is given by Filhol as 2.471, a modern determination by Rengade and Costeanu being 1.856. The heat of formation from its elements is given by Sabatier as 88.2 Cal., and by Rengade and Costeanu as 89.7 Cal. The investigators last mentioned found the heat of solution to be 15.5 Cal. The heat of hydration of the anhydrous salt to the 9-hydrate is 31.72 Cal. For the heat of formation in aqueous solution from the elements, Thomsen gives 101.99 Cal.

In aqueous solution sodium monosulphide reacts with iodine to form sodium iodide, the liberated sulphur dissolving in excess of the sulphide solution. A double sulphide of the formula Na₂S,Cu₂S has been prepared. It melts at 700° C.

Other investigations of solutions of sodium sulphide include the concentration of the hydroxyl-ions and the depression of the freezing-point, the solubility of ammonia, and the density.