Stannic Sulphide, SnS₂

Stannic Sulphide, SnS₂, has long been known and valued as a pigment. It is prepared for this purpose by heating together 12 parts of tin, 6 parts of mercury, with which the tin forms an amalgam, 7 parts of sulphur, and 6 parts of sal ammoniac. Stannous chloride, mercuric chloride, and the remaining sal ammoniac sublime, and golden-yellow scales of stannic sulphide remain. It may also be obtained by heating stannous sulphide with corrosive sublimate, and in other ways. The product crystallises in hexagonal tablets, which are translucent and of a golden colour. It has, therefore, been known since the eighteenth century as mosaic gold, aurum mosaicum or musivum. The density of this substance is about 4.5, and it is employed as a bronzing powder for articles of wood, gypsum, etc. When it is heated sufficiently it is partly decomposed into stannous sulphide and sulphur, whilst the rest sublimes. It dissolves in aqua regia and in caustic alkali, but not in hydrochloric or nitric acid. Stannic sulphide is met with in analysis as a dull yellow precipitate, formed when, hydrogen sulphide gas is passed into stannic chloride solution. According to Jorgensen, the precipitate obtained with hydrogen sulphide and a hydrochloric solution of meta-stannic acid contains variable amounts of metastannic acid and stannic sulphide, its composition depending on the concentration of the hydrogen sulphide and of the hydrochloric acid, as well as upon the temperature and the time. Although the precipitate contains little stannic sulphide at first it is gradually transformed into this substance in contact with hydrogen sulphide solution, though the process requires about two months for completion.

Precipitated stannic sulphide dissolves in concentrated hydrochloric acid, though less readily than stannous sulphide. It dissolves easily in caustic alkali solutions, forming a mixture of thiostannate and stannate, thus:

3SnS₂ + 6NaOH = 2Na₂SnS₃ + Na₂SnO₃ + 3H₂O;

and in alkali sulphide solutions forming thiostannate only. Stannous sulphide may also be converted into thiostannate, and so more easily dissolved, by heating it with alkali polysulphide solution or with alkali hydroxide and flowers of sulphur.

Thiostannic Acid and its Salts. - When a solution of precipitated stannic sulphide in alkali sulphide is reprecipitated with acid a yellowish brown solid separates, which on drying becomes almost black. This is thiostannic acid, H₂SnS₃, which on heating, in absence of air, loses hydrogen sulphide, yielding the yellow disulphide. Potassium thiostannate is obtained in colourless prisms, having the composition K₂SnS₃.3H₂O, from its solution formed by boiling concentrated potassium sulphide solution with tin and sulphur. The sodium salt, Na₂SnS₃.2H₂O, is formed similarly; whilst by fusing together sodium sulphide, stannous sulphide and sulphur, extracting the mass with water, and concentrating the solution, colourless crystals of the ortho-salt, Na₄SnS₄.12H₂O, are obtained. The ammonium salt (NH₄)₂SnS₃.3H₂O is obtained in unstable yellowish tablets by precipitating a solution of stannic sulphide in yellow ammonium sulphide by alcohol.