Ferric sulphite, Fe₂(SO₃)₃

Ferric sulphite, Fe₂(SO₃)₃, has not been isolated. It is obtained in unstable solution on dissolving ferric hydroxide in sulphurous acid. If the ferric hydroxide is pure and well washed, it dissolves exceedingly slowly, yielding a yellow solution. As ordinarily precipitated from solution, however, the hydroxide dissolves rapidly. The solution is unstable, the yellow colour changing to green as the ferric salt undergoes reduction to ferrous dithionate in the course of a few hours: -

Fe₂(SO₃)₃ = FeS₂O₆ + FeSO₃.

Basic ferric sulphites, such as Fe₂(SO₃)₃.7Fe(HO)₃andFe₂(SO₃)₃.6Fe(HO)₃, have been prepared.