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(H4P2O6), a tetrabasic acid of medium strength. Anhydrous hypophosphoric acid occurs as colorless crystals, which melt at 70°C with decomposition; the crystal hydrates H4P2O6·H2O and H4P2O6·2H2O have a melting point of 62°–62.5°C and 79.5°–81.5°C, respectively. The dissociation constants of hypophosphoric acid at 25°C are K1 = 6 × 10–3, K2 = 1.5 × 10–3, K3 = 5.4 × 10–8, and K4 = 9.3 × 10–11. When heated, hypophosphoric acid is converted into phosphoric acid (H3PO4) and phosphorous acid (H3PO3). At temperatures above 180°C it decomposes with the release of phosphine (PH3). The anhydride of hypophosphoric acid is not known. Salts of hypophosphoric acid are called hypophosphates.
Hypophosphoric acid is formed by the slow oxidation of H3PO3 in air or by the oxidation of solid phosphorus in a limited quantity of air (the phosphorus is partially immersed in water). The process is faster under the effect of a calcium hypochlorite mixture on red phosphorus.