Tetrathionates

The Tetrathionates are generally soluble in water and can be caused to crystallise from solution by the addition of alcohol. When heated in solution the alkali salts decompose into trithionate and sulphur; further change may also give rise to pentathionate, sulphate and sulphite. Aqueous solutions are less stable in the presence of thiosulphate. The salts of the other metals, for example barium or copper, are much less, stable, the former readily forming thiosulphate and the latter sulphide.. The tetrathionates are converted by nascent sulphur into penta- thionates, whilst by potassium sulphite solution they are reduced to trithionate. The latter reaction explains the erroneous assumption that trithionate could be directly produced by the action of iodine on an aqueous mixture of potassium thiosulphate and sulphite, With equivalent quantities of tetrathionate and sulphite,, the reaction leads to the equilibrium

S₄O₆'' + SO₃'' ⇔ S₃O₆'' + S₂O₃'';

but very slight excess of sulphite serves to send the reaction completely to the right. With a bisulphite the following equilibrium is slowly set up:

S₄O₆'' + HSO₃' ⇔ S₃O₆'' + S₂O₃'' + H^•,

but this is soon disturbed by decomposition and reformation of the trithionate, thus:

S₃O₆'' + H₂O = SO₄'' + S₂O₃'' + 2H^•,
S₂O₃'' + 4HSO₃' + 2H^• = 2S₃O₆'' + 3H₂O.

Thus the H^•-ion concentration increases and sulphur separates:

S₂O₃'' + H^• ⇔ HSO₃' + S,
S₂O₃'' + 2HSO₃' = ₂SO₄'' + 2S + H₂O.

Addition of excess of mercuric chloride to a solution of a tetra- thionate causes precipitation of the compound Hg₃S₂Cl₂.