Preparation
HPO(OH)2 is the product of the hydrolysis of its acid anhydride:
-
- P4O6 + 6 H2O → 4 HPO(OH)2
(An analogous relationship connects H3PO4 and P4O10).
On an industrial scale, the acid is prepared by hydrolysis of phosphorus trichloride with water or steam:
- PCl3 + 3 H2O → HPO(OH)2 + 3 HCl
Potassium phosphite is also a convenient precursor to phosphorous acid:
- K2HPO3 + 2 HCl → 2 KCl + H3PO3
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.
Read more about this topic: Phosphorous Acid
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