Lead Thiosulphate, PbS₂O₃

Lead Thiosulphate, PbS₂O₃, is formed as a white precipitate, which may be amorphous or crystalline, when sodium thiosulphate is added to a lead salt solution. When heated it turns black, decomposing into lead sulphide and sulphate, together with sulphur and sulphur dioxide. It burns in the air; and when mixed with an oxidising agent has been employed in the manufacture of matches free from phosphorus. When sodium thiosulphate is added to a boiling solution of a lead salt containing ammonium chloride the whole of the lead is precipitated as sulphide, together with free sulphur. Lead thiosulphate is very slightly soluble in water; according to Rammelsberg, 1 part dissolves in 3266 parts of water. Hot water decomposes this salt, producing sulphide, sulphate, and free sulphur, with small amounts of sulphuric acid, the main reaction being:

4PbS₂O₃ = PbS + 4S + 3PbSO₄.

It is more soluble in thiosulphate solutions, owing to the formation of complex salts, a number of which are known.

PbS₂O₃.Li₂S₂O₃ is a crystalline salt which readily absorbs and is decomposed by water with formation of lead monosulphide.

PbS₂O₃.2Na₂S₂O₃, obtained by adding lead acetate to concentrated sodium thiosulphate solution, is crystalline, and slightly soluble in water. PbS₂O₃.2K₂S₂O₃.2H₂O crystallises from a saturated solution of lead thiosulphate in potassium thiosulphate solution. The following salts also exist:

PbS₂O₃.2(NH₄)₂S₂O₃.3H₂O, PbS₂O₃.2Cs₂S₂O₃.3H₂O, PbS₂O₃.2Rb₂S₂O₃.2H₂O, PbS₂O₃.2CaS₂O₃.4H₂O, PbS₂O₃.Cs₂S₂O₃.2H₂O.