Wackenroders Solution

Whenever sulphur dioxide, water and "nascent" sulphur meet, for example in the action of water on sulphur chloride, of mineral acids on a thiosulphate, or of hydrogen sulphide on aqueous sulphur dioxide solution, formation of polythionic acids is likely to occur. Dalton in 1812 demonstrated that the last-named reagents gave rise to an acid liquid, a result which was confirmed later by Thomson; in 1846 Wackenroder proved the presence of pentathionic acid in the liquid, since which date the aqueous reaction product has been known as " Wackenroder's Solution."

The reaction was subsequently investigated by other chemists, particularly Debus, who found that the most satisfactory procedure was to treat an almost saturated cold solution of sulphur dioxide repeatedly with hydrogen sulphide on successive days until the sulphur dioxide was consumed. The resulting liquid contained sulphur in colloidal suspension, free sulphur, sulphuric acid, a little trithionic acid, tetrathionic acid and pentathionic acid, and an acid or acids still richer in sulphur-possibly a hexathionic acid.

Examination, of the progress of the reaction indicated that the first product is probably tetrathionic acid:

H₂S + 3SO₂ = H₂S₄O₆,

which subsequently undergoes partial decomposition by hydrogen sulphide with formation of water and sulphur; the sulphur, being in a "nascent" condition, converts some of the tetrathionate to pentathionate, whilst some of the tetrathionate also undergoes reduction to trithionate.

The composition of Wackenroder's solution varies with the conditions of preparation. When prepared in the manner described, the milky solution can be clarified by careful evaporation at the ordinary temperature in a vacuum, when the suspended sulphur separates and a solution containing mainly pentathionic acid is obtained.

The observed occurrence of pentathionic acid in natural waters is doubtless to be attributed to a reaction of this kind between sulphur dioxide and hydrogen sulphide.