Ferrous sulphite, FeSO₃

Ferrous sulphite, FeSO₃.3H₂O, is obtained by dissolving iron in aqueous sulphurous acid in the absence of air. Upon concentration the salt is deposited as colourless crystals. The reaction is somewhat complicated, for no hydrogen gas is liberated during the solution of the iron, the nascent hydrogen reducing some of the sulphurous acid (or ferrous sulphite) to thiosulphuric acid (or ferrous thiosulphate). Thus: -

2Fe + 3H₂SO₃ = FeSO₃ + FeS₂O₃ + 3H₂O.

The thiosulphate, being very soluble in water, remains in solution.

Ferrous sulphite also results when solutions of ferrous salts and sodium sulphite interact, and when ferrous hydroxide is dissolved in aqueous sulphurous acid. In these circumstances a red solution is usually obtained, probably because of interaction with dissolved oxygen. The colour quickly disappears, however, particularly on warming. On concentration the salt crystallises out.

On passing a current of sulphur dioxide into an aqueous suspension of freshly precipitated ferrous sulphide, the latter passes into solution and ferrous sulphite is gradually deposited: -

FeS + SO₂ + H₂O = FeSO₃ + H₂S.

On prolonging the passage of sulphur dioxide the salt dissolves and the acid sulphite, Fe(HSO₃)₂ is produced in solution. The normal salt is re-deposited on boiling, but the solution now contains ferrous thiosulphate in consequence of the reaction between a portion of the dissolved acid salt and sulphur produced by hydrogen sulphide and sulphur dioxide inter-reacting. Thus: -

Fe(HSO₃)₂ + S = FeS₂O₃ + H₂O + SO₂.

Ferrous sulphite solution readily oxidises in air, yielding a red solution. From its colourless solutions alkalies precipitate ferrous hydroxide.