Miscellaneous Oxo-Acids

Hypodiphosphoric acid, H4P2O4, 2/P-2/P, can be isolated from the hydrolysis products of phosphorus tetraiodide and may be regarded as the tautomer of the hydroxy derivative (5.247). This 

Crystalline sodium diphosphite, Na3HP205 I2H2O, was first isolated from the hydrolysis products of PI3. The diphosphite ion is converted to hypophosphate by boiling in NaOH or by oxidation with bromine in NaHC03 solution. Acid hydrolysis produces phosphorous acid. 

Isohypophosphoric acid (3/P-0-5/P acid) can be prepared by heating a mixture of phosphorous and phosphoric acids under appropriate conditions (5.249). Isohypophosphoric acid and its salts are readily hydrolysed (reverse of (5.244). The trisodium salt can be prepared by reacting sodium diphosphite and sodium orthophosphate (5.250), and it is stable in the solid or in neutral solution. 

Several condensed oxo-acids based on more than two P atoms are known. These include 4/P-3/P-4/P and 3/P-0-4/P-4/P and cyclic rings such as (-3/P-). This last ring is formed when a suspension of red phosphorous in KOH solution is oxidised with bromine. The cyclic structure (5.251a) has been established in an x-ray study of (CsP02)e. 

The potassium salt of (4/P-4/P-0-) ring acid (5.251b) is formed by the dehydration of two molecules of H4P2O6 (4/P-4/P acid) with acetic anhydride. Hydrolysis of the ring salts in NaOH at 25°C opens the ring to give salts of 4/P-4/P-0-4/P-4/P acid (5.251c). 

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