Reactions
Phosphorous acid on heating at 200 °C converts to phosphoric acid and phosphine:
- 4 H3PO3 → 3 H3PO4 + PH3
In practice this reaction yields a number of undefined phosphorus suboxides as well.
Phosphorous acid is a moderately strong dibasic acid. It reacts with alkalis forming acid phosphites and normal phosphites. Thus, reaction with sodium hydroxide gives sodium dihydrogen phosphite and disodium hydrogen phosphite, but not trisodium phosphite, Na3PO3 as the third (P-bound) hydrogen is not acidic.
- H3PO3 + NaOH → NaH2PO3 + H2O
- H3PO3 + 2 NaOH → Na2HPO3 + 2H2O
Phosphorous acid is a powerful reducing agent. When treated with a cold solution of mercuric chloride, a white precipitate of mercurous chloride forms:
- H3PO3 + 2 HgCl2 + H2O → Hg2Cl2 + H3PO4 + 2 HCl
Mercurous chloride is reduced further by phosphorous acid to mercury on heating or on standing:
- H3PO3 + Hg2Cl2 + H2O → 2 Hg + H3PO4 + 2 HCl
Read more about this topic: Phosphorous Acid
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