Trithionates

Trithionates are salts of the general type X₂S₃O₆, where X represents the equivalent weight of a metal; the alkali salts are the best known.

When heated alone the salts decompose with liberation of sulphur and sulphur dioxide:

K₂S₃O₆ = K₂SO₄ + SO₂ + S.

In aqueous solution alkali trithionates gradually undergo decomposition with formation of sulphate, hydrogen sulphite and tetrathionate. The course of the main reaction may be expressed ionically as follows:

(a) S₃O₆'' + H₂O = S₂O₃'' + SO₄'' + 2H^•,
(b) S₃O₆'' + S₂O₃'' + H^• = S₄O₆'' + SO₃H'.

A trace of pentathionate is also found in the solution after the lapse of several days, probably formed according to

5S₂O₃'' + 6H^• = 2S₅O₆'' + 3H₂O.

The decomposition is accelerated by the presence of acid, and sulphur is deposited:

(c) S₂O₃'' + H^• = HSO₃' + S.

In the presence of alkali decomposition is fairly rapid, sulphite and thiosulphate being the chief products:

2S₃O₆'' + 6OH' = S₂O₃'' + 4SO₃''.

Thiosulphate is the sole product on treatment with an alkali sulphide:

K₂S₃O₆ + Na₂S = K₂S₂O₃ + Na₂S₂O₃.

The decomposition of trithionates in aqueous solution is only slightly influenced by the addition of sulphites or thiosulphates, although the latter catalytically accelerate the decomposition of tetra- or penta- thionates under similar conditions.

In the presence of hydrogen sulphide the reaction is extremely slow, and is retarded by the presence of acid:

S₃O₆'' + H₂S = 2S₂O₃'' + H^•,
S₂O₃'' + 2H₂S + 2H^• = 3H₂O + 4S.

On adding potassium trithionate to a cold solution of copper acetate and keeping for several days in the dark, transparent yellow monoclinic prisms separate. These have the composition K₂S₂O₃.Cu₂S₂O₃.2H₂O, i.e. potassium cuprous thiosulphate. They are unstable in air or on boiling with water, cupric sulphide being formed. This reaction demonstrates the formation of thiosulphate during the hydrolysis of trithionate solutions.

The trithionates may be quantitatively precipitated by boiling for one hour with a mixture of copper sulphate and barium chloride solutions containing free hydrochloric acid:

S₃O₆'' + 8Cu^•• + 6H₂O = 3SO₄'' + 8Cu^• + 12H^•.

Tetrathionates and thiosulphates interfere, but not penta- or hexa- thionates.

The salts derived from metals other than the alkali metals are relatively little known; all are stated to be easily soluble in water with the exception of the mercurous, mercuric and silver salts, which are sparingly soluble. In describing the properties of the acid mention has already been made of the precipitation reactions of the salts with silver nitrate and other salts, and of the behaviour of the salts towards the alkali hydroxides and sulphides.