Disodium hydrogen phosphite
Disodium hydrogen phosphite is the chemical compound Na2HPO3 which is commonly encountered as the pentahydrate. It is a salt of phosphorous acid, HP(O)(OH)2 and contains the anion HPO3. Its common name suggests that it contains an acidic hydrogen atom, as in sodium hydrogen carbonate. However, this is misleading as the hydrogen atom is bonded to phosphorus rather than oxygen. It is toxic along with all phosphites. Phosphites contain phosphorus in its +3 oxidation state and are therefore reducing agents.