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phosphorous acid[′fäs·fə·rəs ′as·əd]
H3PO3, a weak dibasic acid. Anhydrous phosphorous acid occurs as colorless hygroscopic crystals with a density of 1.65 g/cm3 and a melting point of 70.1°C. The dissociation constants at 18°C are K1 = 5.1 × 10–2 and K2 = 1.8 × 10–7. The solubility at 20°C is approximately 80 percent. When heated to 250°C, anhydrous phosphorous acid decomposes into phosphoric acid and phosphine; its aqueous solutions decompose at this temperature into phosphoric acid and hydrogen. Phosphorous acid is readily oxidized by such agents as halogens and the oxides of nitrogen. It is produced by dissolving P4O6 (P2O3) in cold water, by hydrolyzing PC13, and by reacting phosphites with sulfuric acid. Phosphorous acid and its salts are used as reducing agents.