Sodium dihydrogen phosphite, NaH₂PO₃

Sodium dihydrogen phosphite, NaH₂PO₃,2½H₂O, crystallizes when a solution of equimolecular proportions of phosphorous acid and sodium hydroxide is cooled to -23° C. The heat of formation of the anhydrous salt from the elements is 333.8 Cal., and from the acid and sodium hydroxide in solution 14.83 Cal. The heat of hydration of the hydrate is 6.05 Cal.

When anhydrous sodium dihydrogen phosphite is heated at 160° C. it is converted into disodium dihydrogen pyrophosphate, Na₂H₂P₂O₅. It forms microscopic crystals, reconverted by heating in solution into the parent substance. The heat of formation from the elements is 599 Cal.