Phosphorous acid is a clear to yellowish crystalline solid with a
garlic like odour melting at 73℃, decomposes at 200℃. It is very
soluble in water and in alcohol. This compound contains one direct
P-H bond (which is not very acidic) and only two hydrogens bonded
to oxygen (which are acidic). The structure of this material is
more correctly written (HO)2HPO.
For this reason, this dibasic acid forms two series of salts, one
containing the dihydrogen phosphite ion, H2PO3 -, and the other
containing the hydrogen phosphite ion, HPO32-. It is prepared by
hydrolysis of phosphorus trichloride (or tetraphosphorus hexaoxide)
with alcohols or phenols. Phosphorous acid esters are called
phosphite with the formula (RO)3P.
Phosphorous acid and phosphite are used as reducing agents in
chemical industry because of easy oxidation property to phosphoric
acid. They are used as antioxidant, stabilizer and chelating agent
in plastic system. They are used as solvent in paint and as flame
retardant on fibres. They are used as a chemical intermediate in
the production of pharmaceutical ingredients, pesticides, optical
brighteners and in lubricant additives and adhesives.