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any of the salts or esters of phosphorous acid (H3PO3). The salts may be monosubstituted, for example, NaH2PO3·2.5H2O.or disubstituted, as in Na2HPO3·5H2O. Most phosphites are difficultly soluble in water; this is not true, however, of phosphites of alkali metals. When calcined, phosphites decompose into the corresponding phosphates and phosphorus derivatives with lower oxidation states, the process including the formation of phosphine (PH3). Phosphites are oxidized in aqueous solutions by halogens and mercury salts, for example, HgCl2, to phosphates. Phosphites are formed when phosphorous acid is neutralized by hydroxides. They are used as reducing agents in inorganic syntheses. Lead phosphite serves as a light stabilizer in the production of polyvinyl chloride.
The esters of phosphorous acid may be mono-[ROP(O)HOH], di-[(RO)2P(O)H], or tri-substituted [(RO)3P], They are obtained from the reactions of phosphorus trichloride with alcohols and alcoholates:
PCl3 + 3ROH → (RO)2P(O)H
PCl3 + 3RONa → (RO)3P
Phosphites are used as stabilizers of polymer plastics and oils and as intermediates in the synthesis of organophosphorus compounds.