Zinc Tetrathionate, ZnH₂(S₂O₆)₂

The acid salt, ZnH₂(S₂O₆)₂, is obtained by half neutralising Wackenroder's solution (aqueous sulphurous acid decomposed by passing hydrogen sulphide) with zinc carbonate. It is very soluble in cold water or alcohol, and crystallises in small, slightly hygroscopic needles from its alcoholic solution. It decomposes above 100° C., and sulphur separates when its aqueous solution is treated with potassium hydroxide.