Potassium pyrosulphite, K₂S>₂O₅

On treatment of a hot saturated solution of potassium carbonate with sulphur dioxide, the Potassium pyrosulphite, K₂S₂O₅, separates in monoclinic crystals, which are more stable than those of the primary sulphite. Its heat of formation from the elements is 369.4 Cal. It is employed in the preparation of photographic developers under the name " potassium metabisulphite."