What is "phosphorous acid"

n. (context chemistry English) a yellowish hygroscopic solid, H₃PO₃; a mild reducing agent

n. a clear or yellow-colored monobasic acid (H3PO2) [syn: hypophosphorous acid, orthophosphorous acid]

Phosphorous acid is the compound described by the formula HPO. 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.