Potassium hyposulphite, K₂S₂O₄

The Potassium hyposulphite, K₂S₂O₄, is obtained in the form of trihydrate by a method analogous to that employed for the sodium salt. A solution of potassium sulphite containing free sulphurous acid is treated with zinc-dust, and filtered after precipitation of the zinc by addition of slaked lime. On mixing the solution with alcohol, the trihydrate is deposited in very unstable yellow crystals. When boiled with acetone or methyl alcohol and dried in vacuum, they yield the anhydrous salt in the form of very stable white crystals, also produced by the interaction of sulphur dioxide and potassium hydride.