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Phosphrous Acid s 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  phosphorus compounds.

Phosphoric acid is a colorless, odorless crystalline liquid. ... Most of soda's acidity also comes from phosphoric acid. Phosphoric acid is made from the mineral phosphorus, which is found naturally in the body. It works with calcium to form strong bones and teeth.