Selenotrithionic Acid, H₂SeS₂O₆

In the preparation of potassium selenosulphate as just described, a certain small amount of a potassium selenotrithionate, K2SeS2O6 (potassium salt of Selenotrithionic Acid, H2SeS2O6), is obtained, which generally crystallises first on evaporation of the solution at the ordinary temperature. The yield is much better if potassium hydrogen sulphite is also present with the selenium and normal potassium sulphite. The corresponding free acid is formed by the interaction of aqueous solutions of sulphurous and selenious acids, the former in excess: 3H2SO3 + H2SeO3 = H2SeS2O6 + H2SO4 + 2H2O. In the description of the properties of selenious acid it was mentioned that the acid is reduced by sulphurous acid in the presence of hydrochloric acid. In the absence of hydrochloric acid the separation of selenium may be far from complete or even entirely lacking, on account of the formation of selenotrithionic acid. Selenotrithionic acid is known only in aqueous solution and in the form of its salts. The solution is not very stable and on exposure to light gradually deposits selenium. Evaporation also causes decomposition. Hydrochloric acid in considerable quantity causes decomposition (compare above) with liberation of selenium: H2SeS2O6 = H2SO4 + SO2 + Se. The selenotrithionates are rather more stable than the acid, but they yield a precipitate of selenium when warmed with hydrochloric acid. As the barium salt is soluble in water, no precipitate is produced on the addition of barium chloride. From the general behaviour of the acid and its salts the constitution is presumably