Phosphite

A phosphite in inorganic chemistry is a salt of phosphorous acid, H₃PO₃ and following the IUPAC naming recommendations the phosphite ion would be PO3−
3, a salt of P(OH)₃.

Historically phosphite has referred to salts containing HPO2−
3 and this is because aqueous H₃PO₃ is not triprotic P(OH)₃ but almost exclusively the diprotic HP(O)(OH)₂ (IUPAC recommended name of phosphonic acid). The IUPAC-recommended name for the HPO2−
3 ion is phosphonate and this naming convention is becoming more common. In the US the IUPAC naming conventions for inorganic compounds are taught at high school, but not as a 'required' part of the curriculum. A well known university level text book follows the IUPAC recommendations. In practise any reference to "phosphite" should be investigated carefully to determine which naming convention is being employed.