Phosphorous acid (or phosphonic acid) is the compound described by the formula . It is diprotic (readily ionizes two protons), not triprotic as might be suggested by its formula. Phosphorous acid is an intermediate in the preparation of other phosphorus compounds. Organic derivatives of phosphorous acid, compounds with the formula , are called phosphonic acids.
Nomenclature and tautomerism
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Solid has tetrahedral geometry about the central phosphorus atom, with a bond of 132 pm, one double bond of 148 pm and two longer single bonds of 154 pm. In common with other phosphorus oxides with bonds (e.g.hypophosphorous acid and dialkyl phosphites), it exists in equilibrium with an extremely minor tautomer . (In contrast, arsenous acid's major tautomer is the trihydroxy form.) IUPAC recommends that the trihydroxy form be called phosphorous acid, and the dihydroxy form phosphonic acid. Only the reduced phosphorus compounds are spelled with an "-ous" ending.
: K = 10<sup>10.3</sup> (25°C, aqueous)
Preparation
On an industrial scale, the acid is prepared by hydrolysis of phosphorus trichloride with water or steam:
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could be produced by the hydrolysis of phosphorus trioxide:
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Potassium phosphite is also a convenient precursor to phosphorous acid:
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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.
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Reactions
Acid–base properties
Phosphorous acid has a pK<sub>a</sub> in the range 1.26–1.3.
: pK<sub>a</sub> = 1.3
It is a diprotic acid, the hydrogenphosphite ion, is a weak acid:
: pK<sub>a</sub> = 6.7
The conjugate base is called hydrogen phosphite, and the second conjugate base, , is the phosphite ion. (Note that the IUPAC recommendations are hydrogen phosphonate and phosphonate respectively).
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 .
Redox properties
On heating at 200 °C, phosphorous acid disproportionates to phosphoric acid and phosphine:
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This reaction is used for laboratory-scale preparations of .
Phosphorous acid slowly oxidizes in air to phosphoric acid.
Heating a mixture of potassium tetrachloroplatinate and phosphorous acid gives the luminescent salt potassium diplatinum(II) tetrakispyrophosphite:
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Uses
The most important use of phosphorous acid (phosphonic acid) is the production of basic lead phosphite, which is a stabilizer in PVC and related chlorinated polymers.