Thallous sulphide, Tl₂S

Thallous sulphide, Tl₂S, is obtained as a black precipitate by passing hydrogen sulphide into, or adding ammonium sulphide to, a solution of a thallous salt. In the presence of a trace of sulphuric acid, hydrogen sulphide precipitates the sulphide from cold solutions as microscopic tetrahedra. The amorphous sulphide becomes crystalline when heated for some hours to 150°-200° with excess of colourless ammonium sulphide.

Thallous sulphide forms bluish-black crystals of density 8.0. It melts at 448°, and is very brittle. Heated in hydrogen, it is reduced to thallium. It is readily decomposed by dilute mineral acids, but not by acetic acid, and the moist sulphide rapidly oxidises in the air. It is insoluble in ammonium sulphide, alkali hydroxides, carbonates, and cyanides.

Thallous sulphide has been obtained in colloidal solution. Thallous sulphide forms definite compounds of the formulae 3Tl₂S.As₂S₅ (or Tl₃AsS₄) and 2Tl₂S.SnS₂ ( or Tl₄SnS₄) with arsenic and stannic sulphides. They may be employed for the gravimetric estimation of thallium. With arsenious, antimonious, antimonic, and stannous sulphides, thallous sulphide forms series of solid solutions. They are precipitated from mixed thallous and arsenic, antimony and tin solutions by alkali sulphides. Thallous sulphide also forms a double sulphide of the formula Tl₂S.4CuS with cupric sulphide, and two series of solid solutions probably of the compound Tl₂S.2CuS in Tl₂S.4CuS and Tl₂S in Tl₂S.2CuS.