Phosphorous acid(orphosphonic acid) is thecompounddescribed by theformulaH3PO3. Thisacidisdiprotic(readily ionizes two protons), nottriproticas might be suggested by this formula. Phosphorous acid is an intermediate in the preparation of other phosphorus compounds. Organic derivatives of phosphorous acid, compounds with the formulaRPO3H2, are calledphosphonic acids.