1. Phosphorous acid (or phosphonic acid) is the compound described by the formula H3PO3. It is diprotic (readily ionizes two protons), not triprotic as might be suggested by its formula. Phosphorous acid is an intermediate in the preparation of other phosphorus compounds. Organic derivatives of phosphorous acid, compounds with the formula RPO3H2, are called phosphonic acids. == Nomenclature and tautomerism == Solid HP(O)(OH)2 has tetrahedral geometry about the central phosphorus atom, with a P−H bond of 132 pm, one P=O double bond of 148 pm and two longer P−OH single bonds of 154 pm. In common with other phosphorus oxides with P−H bonds (e.g.hypophosphorous acid and dialkyl phosphites), it exists in equilibrium with an extremely minor tautomer P(OH)3. (In contrast, arsenous acid's major tautomer is the trihydroxy form.) IUPAC recommends that the trihydroxy form P(OH)3 be called phosphorous acid, and the dihydroxy form HP(O)(OH)2 phosphonic acid. Only the reduced phosphorus compounds are spelled... from wikipedia.org