Hexathionic Acid, H₂S₆O₆

The potassium salt of Hexathionic Acid, H₂S₆O₆, has been prepared by adding a solution containing potassium nitrite and potassium thiosulphate (1KNO₂ to 3K₂S₂O₃) to well-cooled hydrochloric acid, the mixture being vigorously shaken until the colour changes through brown and green to yellow. Nitrous fumes are removed by passing air through the solution, which then has an odour of sulphur dioxide. On keeping in a freezing mixture potassium chloride separates and the solution becomes almost colourless. After filtration and concentration under reduced pressure, potassium hexathionate contaminated with potassium chloride separates; the latter is removed by water and the hexathionate washed with alcohol and ether. Potassium tetrathionate is contained in the mother-liquor, its formation probably being due to the intermediate formation of nitrosylthiosulphuric acid, but the mechanism of the reaction is obscure.

Potassium hexathionate is stable when dry, but in aqueous solution undergoes decomposition with formation of sulphur or a higher polythionate. The addition of acid stabilises the solution, and the salt is best crystallised from solutions containing considerable amounts of mineral acid; from such solutions it may be salted out. It resembles the pentathionates in its reaction with alkalis and® with sodium carbonate, sulphur being deposited; also in giving no precipitate with copper sulphate. It gives a yellow precipitate with mercurous nitrate, a yellowish-white precipitate with mercuric chloride, and a brown coloration followed by a black precipitate with ammoniacal silver nitrate. It is decomposed by aqueous ammonia, the primary product being tetrathionate; the reaction is more rapid than in the case of pentathionates. Sulphites produce trithionates. Hexammine-cobaltic and -chromic chlorides and nitrates give crystalline precipitates.

Hexathionic acid itself is as yet unknown.