Zinc Thiosulphate, ZnS₂O₃

Zinc Thiosulphate, ZnS₂O₃. - When sulphur dioxide is passed through an aqueous suspension of zinc sulphide, insoluble crystals of zinc sulphite separate,

ZnS + H₂SO₃ = ZnSO₃+H₂S,

that finally dissolve as the acid sulphite. Some thiosulphate is simultaneously produced: reactive sulphur produced by reaction between the hydrogen sulphide and sulphur dioxide acts on the acid sulphite according to the equation

Zn(HSO₃)₂+S = ZnS₂O₃+H₂O+SO₂.

If the zinc sulphide suspension is added slowly to aqueous sulphurous acid, so that the latter is always in excess, the resulting solution contains principally thiosulphate and but little acid sulphite -

2ZnS+3SO₂ = 2ZnS₂O₃+S.

Zinc thiosulphate has also been obtained in solution by dissolving the metal in aqueous sulphurous acid. Solutions of the salt are colourless and transparent and decompose readily -

2ZnS₂O₃ = ZnS+ZnS₃O₆.

The double salts, 3Na₂S₂O₃.2ZnS₂O₃.10H₂O, K₂S₂O₃.ZnS₂O₃.H₂O, and (NH₄)₂S₂O₃.ZnS₂O₃.H₂O, have been prepared.