Phosphorous acid 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.

H₃PO₃ is more clearly described with the structural formula HPO(OH)₂. This species exists in equilibrium with a minor tautomer P(OH)₃. IUPAC recommendations, 2005, are that the latter be called phosphorous acid, whereas the dihydroxy form is called phosphonic acid. Only the reduced phosphorus compounds are spelled with an "ous" ending.

The P(OH)₃ tautomer has been observed as a ligand bonded to molybdenum. Other important oxyacids of phosphorus are phosphoric acid (H₃PO₄) and hypophosphorous acid (H₃PO₂). The reduced phosphorus acids are subject to similar tautomerism involving shifts of H between O and P.

In the solid state, HP(O)(OH)₂ is tetrahedral with one shorter P=O bond of 148 pm and two longer P–O(H) bonds of 154 pm. The central phosphorus atom is assigned an oxidation state of +3.

HPO(OH)₂ is the product of the hydrolysis of its acid anhydride:

(An analogous relationship connects H₃PO₄ and P₄O₁₀).

On an industrial scale, the acid is prepared by hydrolysis of phosphorus trichloride with water or steam:

Potassium phosphite is also a convenient precursor to phosphorous acid:

In practice aqueous potassium phosphite is treated with excess hydrochloric acid. By concentrating the solution and precipitations with alcohols, the pure acid can be separated from the salt.

...
Wikipedia