Stannous Sulphide, SnS

Stannous Sulphide, SnS, is formed in the dry way when tin-foil is heated with sulphur; the metal, indeed, takes fire spontaneously when brought into sulphur vapour. This sulphide may be sublimed in a current of hydrogen, and forms a shining crystalline mass, resembling graphite. Glistening, metallic scales are also formed when the sulphide is heated in the electric furnace. Stannous sulphide melts at 881° C., and boils at 1090° C., producing a green vapour; and when heated to 265° C. under pressure it decomposes into tin and stannic sulphide, thus:

2SnS = Sn + SnS₂.

Stannous sulphide is precipitated in a brown, hydrated form when a slightly acidified solution of stannous chloride is saturated with hydrogen sulphide gas; on drying, the precipitate turns black. The solubility of this precipitate in acids and alkalis is instructive. Thus it readily dissolves even in somewhat diluted hydrochloric acid, and consequently is imperfectly precipitated if much acid is present, the reaction

SnCl₂ + H₂S ⇔ SnS + 2HCl

being a reversible one which proceeds to an equilibrium.

This solubility may be referred to the base producing properties of tin; and consequently, as might be inferred, stannous sulphide is indifferently soluble in alkali solutions, though its solubility varies with its physical state. The freshly precipitated sulphide dissolves slowly on boiling with dilute caustic soda, solution, producing a mixture of stannite and thiostannite. Since the former salt is HSnOONa, the latter is probably constituted similarly, so that the reaction is:

2SnS + 2NaOH = HSnOONa + HSnSSNa.

The sulphide is reprecipitated from this solution by acid. Strong alkali may, however, decompose stannous sulphide into stannic sulphide and tin, the former of which dissolves, forming thiostannate, whilst the latter reacts with the alkali, producing stannate with evolution of hydrogen, thus:

6SnS + 6NaOH = Na₂SnO₃ + 2Na₂SnS₃ + 3H₂O + 3Sn
3Sn + 6NaOH + 3H₂O = 3Na₂SnO₃ + 6H₂.