Sodium phosphites

Disodium hydrogen phosphite, Na₂HPO₃,5H₂O, is obtained by concentration of a solution of phosphorous acid neutralized with sodium carbonate, and over sulphuric acid changes to the anhydrous salt. The pentahydrate is very deliquescent. The anhydrous salt melts at 53° C., and above this temperature is oxidized to sodium phosphate, with evolution of phosphine. The heat of formation from the elements is 285.1 Cal., and from the acid and sodium hydroxide in solution 28.45 Cal. The heat of hydration of the anhydride to the pentahydrate is 13.7 Cal.