Product Name: Phosphorous acid Other Names: Phosphonic acid; hydrogen phosphonate; phosphorus(+3) trihydride cation trihydroxide; Orthophosphorus; Orthophosphorous acid; Ortho-Phosphorous acid Molecular Formula: H3PO3 UN No.:2834 HS No.: 2811199090 CAS No.: 13598-36-2 EINECS No.:237-066-7 Hazard Class: 8 Purity:98% 99% Appearance: white crystal Physical and Chemical Properties Phosphorous acid (or phosphonic acid (singular)) is the compound described by the formula H3PO3. This acid is diprotic (readily ionizes two protons), not triprotic as 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 formula RPO3H2, are called phosphonic acids.