Ferrous thiosulphate, FeS₂O₃

Ferrous thiosulphate, FeS₂O₃.5H₂O, may be prepared by passing hydrogen sulphide through a cold solution of ferrous sulphate and sodium thiosulphate. The precipitate consisting of sulphur and ferrous sulphide is washed and suspended in water through which a current of sulphur dioxide diluted with hydrogen is passed. The reactions are: -

2FeS + 3SO₂ = 2FeS₂O₃ + S,
FeS + 3SO₂ = FeS₄O₆,
FeS + 3SO₂ = FeS₃O₆ + S.

A further quantity of the sulphide is then added to remove excess sulphur dioxide and to reduce the polythionates: -

FeS₄O₆ + FeS = 2FeS₂O₃ + S,
FeS₃O₆ + FeS = 2FeS₂O₃.

The salt may also be obtained by triturating a concentrated solution of ferrous sulphate with barium thiosulphate, but it is less pure, as it contains some tetrathionate as well. It results when sulphur is digested with ferrous sulphate solution, and when iron is dissolved in aqueous sulphurous acid. This latter reaction is somewhat complicated, ferrous sulphite being first produced, thus: -

Fe + H₂SO₃ = FeSO₃ + 2H.

The nascent hydrogen then attacks either the sulphurous acid (or ferrous sulphite), reducing it to thiosulphuric acid (or ferrous thiosulphate). Thus: -

Fe + 2H₂SO₃ + 2H = FeS₂O₃ + 3H₂O.

On concentration ferrous sulphite, being much less soluble, crystallises out first, leaving the thiosulphate in solution.

The salt yields green crystals, readily soluble in water. They belong to the triclinic system, their crystallographic elements being as follow: -

a: b: с = 0.37673: 1: 0.57844. α = 104° 57'. β = 98° 8'5'. γ = 89° 40'.

The salt readily oxidises in air, yielding a basic ferric compound.

The double salt, Fe₂S₂O₃.3Na₂S₂O₃.8H₂O, is obtained by precipitating a mixed solution of ferrous iodide and sodium thiosulphate with alcohol. It forms bright green crystals, readily soluble in water.