Phosphorous acid, is the compound described by the formula H3PO3. This acid is diprotic, not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in the preparation of other phosphorus compounds. Organic derivatives of phosphorous acid, compounds with the formula RPO3H2, are called phosphonic acids.
H3PO3 is more clearly described with the structural formula HPO2. In the solid state, HP2 is tetrahedral with one shorter P=O bond of 148 pm and two longer P–O bonds of 154 pm. This species exists in equilibrium with an extremely minor tautomer P3. IUPAC recommends 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. Other important oxyacids of phosphorus are phosphoric acid and hypophosphorous acid. The reduced phosphorus acids are subject to similar tautomerism involving shifts of H between O and P.
Preparation
HPO2 is the product of the hydrolysis of its acid anhydride: . 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.
Reactions
Acid–base properties
Phosphorous acid has a pKa in the range 1.26–1.3. It is a diprotic acid, the hydrogenphosphite ion, HP2− is a weak acid: The conjugate base HP2− is called hydrogen phosphite, and the second conjugate base,, is the phosphite ion.. The hydrogen atom bonded directly to the phosphorus atom is not readily ionizable. Chemistry examinations often test students' appreciation of the fact that not all three hydrogen atoms are acidic under aqueous conditions, in contrast with H3PO4.
Disproportionation
On heating at 200 °C, phosphorous acid disproportionates to phosphoric acid and phosphine: This reaction is used for laboratory-scale preparations of PH3.
Both phosphorous acid and its deprotonated forms are good reducing agents, although not necessarily quick to react. They are oxidized to phosphoric acid or its salts. It reduces solutions of noble metal cations to the metals. When phosphorous acid is treated with a cold solution of mercuric chloride, a white precipitate of mercurous chloride forms: Mercurous chloride is reduced further by phosphorous acid to mercury on heating or on standing:
As a ligand
Upon treatment with metals of d6 configuration, phosphorous acid is known to coordinate as the otherwise rare P3 tautomer. Examples include Mo5 and 2+.
Uses
The most important use of phosphorous acid is the production of basic lead phosphite, which is a stabilizer in PVC and related chlorinated polymers. It Used in the production of basic lead phosphonate PVC stabilizer, aminomethylene phosphonic acid and hydroxyethane diphosphonic acid. Also Used as a strong reducing agent and in the production of raw materials of phosphorous acid, synthetic fibres and organophosphorus pesticides etc. It used in the production of high efficient water treatment agent amino trimethylene phosphonic acid.
Organic derivatives
The IUPAC name is phosphonic acid. This nomenclature is commonly reserved for substituted derivatives, that is, organic group bonded to phosphorus, not simply an ester. For example, PO2 is "methylphosphonic acid", which may of course form "methylphosphonate" esters.
