Phosphorous Acid

phosphorous acid (H₃PO₃), also called orthophosphorous acid, one of several oxygen acids of phosphorus, used as reducing agent in chemical analysis. It is a colourless or yellowish crystalline substance (melting point about 73° C, or 163° F) with a garliclike taste. An unstable compound that readily absorbs moisture, it is converted to phosphoric acid (H₃PO₄) in the presence of oxygen or when heated above 180 °C (360 °F). Phosphorous acid (H₃PO₃) forms salts called phosphites, also used as reducing agents. It is prepared by dissolving tetraphosphorus hexoxide (P₄O₆) or phosphorus trichloride (PCl₃) in water.