Ferrous tetrathionate, FeS₄O₆

Ferrous tetrathionate, FeS₄O₆, results in unstable solution on adding ferrous thiosulphate solution in small quantities at a time to ferric chloride: -

2FeCl₃ + 2FeS₂O₃ = FeS₄O₆ + 3FeCl₂.

The product readily decomposes, evolving sulphur dioxide. Thus: -

FeS₄O₆ = FeSO₄ + SO₂ + 2S.

Ferrous dithionate, FeS₂O₆, is readily obtained by double decomposition of solutions of the barium salt and ferrous sulphate. The dithionate is also formed by dissolving ferric hydroxide in sulphurous acid, and allowing to stand for some hours. If the ferric hydroxide is pure and well washed, it dissolves exceedingly slowly; the ordinary, freshly precipitated hydroxide dissolves rapidly. On standing, the yellow solution becomes green, as the ferric iron becomes reduced to ferrous. The reaction apparently takes place in stages involving the intermediate formation of ferric sulphite - which salt, however, has not been isolated. Thus: -

2Fe(OH)₃ + 3SO₃ = Fe₂(SO₃)₃ + 3H₂O,
Fe₂(SO₃)₃ = FeS₂O₆ + FeSO₃.

On passing sulphur dioxide into cold solutions of ferric sulphate, ferrous dithionate is formed. With a 2 per cent, solution of ferric sulphate, 80 per cent, of the sulphate is converted into dithionate at 0° C., but this amount diminishes as the temperature rises, none being formed at 95° C. The equilibrium may be represented by the equation: -

Fe₂(SO₄)₃ + 3SO₂ + 2H₂O = 2FeS₂O₆ + 2H₂SO₄.

It is interesting to note that ruthenium sulphate behaves in a precisely similar manner. On concentration bluish green crystals are obtained, which are unstable in air. They contain 5 or 7 molecules of combined water, and belong to the triclinic system. At 18.5° С., 1 part of the heptahydrate dissolves in 0.59 part of water.

The following double salts have been prepared: -

FeS₂O₆.3(NH₄)₂S₂O₆.6H₂O (monoclinic), and
4FeS₂O₆.18(NH₄)₂S₂O₆.33H₂O.