Acid-base Properties
Phosphorous acid is a diprotic acid, since the hydrogen bonded directly to the central phosphorus atom is not readily ionizable. Chemistry examinations often test students' appreciation of the fact that all three hydrogen atoms are not acidic under aqueous conditions, in contrast with H3PO4, phosphoric acid. The hydrogenphosphite ion, HP(O)2(OH)− is a moderately strong acid.
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- HP(O)(OH)2 → HP(O)2(OH)− + H+ pKa = 1.3
- HP(O)2(OH)− → HPO32− + H+ pKa = 6.7
The HP(O)2(OH)− species is called the hydrogenphosphite, and the HPO32− the phosphite ion. (Note that the IUPAC recommendations are hydrogenphosphonate and phosphonate respectively)
The IUPAC (mostly organic) 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, (CH3)PO(OH)2 is "methylphosphonic acid", which may of course form "methylphosphonate" esters.
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.
Read more about this topic: Phosphorous Acid
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