Lead Sulphide, PbS

Lead Sulphide, PbS, occurs in nature, and is the principal ore of lead. This ore has been known since the time of the ancients as galena, but that it contained sulphur was not recognized till modern times, although Boyle was aware that lead could be obtained from it by heating it with scrap-iron. Another name for the ore is lead glance. It occurs in dark grey masses, having a metallic lustre, crystallised in regular cubes or octahedra, and shows a well-defined cubic cleavage. Galena has a hardness of 2.5 and a density of 7.51 to 7.76.

Artificial lead sulphide may be prepared in the dry way, or by precipitating a lead salt solution with hydrogen sulphide or other soluble sulphide.

When iead is heated in sulphur vapour the metal burns and forms crystals or fused globules of the sulphide. The sulphide is also formed when lead oxide is heated with excess of sulphur. The formation of lead sulphide by the action of atmospheric hydrogen sulphide on lead pigments is well known; it accounts for the darkening of oil-paintings by age, and of structures coated with paint into the composition of which white lead enters. On this account no part of a chemical laboratory should be covered with lead paint. Further, it is frequently observed that powders containing lead compounds darken after exposure for a short time to the air of a laboratory in which hydrogen sulphide is being employed.

When lead sulphide is precipitated by hydrogen sulphide from a solution of lead acetate or nitrate the precipitate is amorphous, but when it is formed in a dilute solution containing free nitric acid, it is obtained crystallised in microscopic cubes.

Whilst crystallised lead sulphide is dark grey the precipitated sulphide is brownish black. The two forms differ in density; the density of the crystallised compound is 7.48 and of the amorphous 7.13. The melting-point of lead sulphide lies in the region of 1000° C.; that of galena has been estimated to be 1120° C.; the sulphide, however, sublimes below its melting-point, and a sublimate has been observed in a vacuum as low as 600° C., consisting of cubes which may be 1.5 mm. in diameter. On account of this property crystals of sublimed galena are often obtained in lead works. The molecular heat of formation of precipitated lead sulphide is 20,400 or 20,300 calories; its specific heat, which varies somewhat with temperature, has a mean value of about 0.068.

Lead sulphide is reduced to metal when heated in a current of hydrogen; chlorine converts it into chloride with the simultaneous formation of sulphur dichloride, SCl₂, but in presence of water-vapour forms the peroxide. Dilute nitric acid dissolves lead sulphide with the formation of nitrate in solution and separation of sulphur. At the same time some of this sulphur is oxidised to sulphuric acid with the consequent formation of lead sulphate; and this is specially the case when the concentrated acid is employed, so that the sulphide appears to be directly oxidised to sulphate. Hot concentrated hydrochloric acid dissolves lead sulphide with evolution of hydrogen sulphide; and this sulphide is by no means insoluble in the dilute acid, so that traces of lead may remain in solution after the passage of hydrogen sulphide if too much hydrochloric acid was present, and in the course of qualitative analysis will subsequently be precipitated as hydroxide by ammonia. Several other sulphides are less soluble in water than lead sulphide, e.g. cupric and silver sulphides. Consequently double decomposition occurs when freshly precipitated lead sulphide is shaken with solutions of cupric or silver salts. The solubility in water of precipitated lead sulphide at 18° C. is 3.60×10^-6 gram-molecules per litre; that of the crystallised sulphide is about a third this value. When lead sulphide is fused with caustic soda in the air, the sulphide is oxidised to sulphate, which dissolves in the soda. This is an example of the influence of a solvent upon oxidation, since the reaction depends upon the solubility of lead sulphate in caustic soda; it is analogous to the oxidation of manganese and chromic compounds to manganate and chromate under the same conditions. Similarly, precipitated lead sulphide is oxidised and dissolved when it is boiled with sodium peroxide and water.