Sodium pyrosulphite, Na₂S₂O₅

When excess of sulphur dioxide is passed into sodium-carbonate solution at low temperature, anhydrous sodium pyrosulphite separates; at ordinary temperature the hydrate Na₂S₂O₅,½H₂O is formed. This substance may be regarded as an anhydride of sodium hydrogen sulphite: 2NaHSO₃ - H₂O=Na₂S₂O₅. The heat of formation of the anhydrous salt from its elements is given by de Forcrand as 348.4 Cal., and the heat of solution at 10° C. as 5.2 Cal. The electric conductivity has been investigated by Walden. The salt is employed to retard oxidation of photographic developers.