Hypophosphorous Acid H3PO2

a strong monobasic acid. The formula representing the structure of hypophosphorous acid may be written in the form H[H₂PO₂], which indicates that only one hydrogen atom can be replaced by a metal in a molecule. Anhydrous hypophosphorous acid occurs as colorless crystals, with a density of 1.49 g/cm³, a melting point of 26.5°C, and a dissociation constant of K = 8.9 × 10^–2 at 25°C. Hypophosphorous acid is readily soluble in water; the concentration of commercial hypophosphorous acid is 30–50 percent. Hypophosphorous acid decomposes upon heating to form phosphine, red phosphorus, orthophosphoric acid, and hydrogen; the heating of aqueous solutions essentially leads to the formation of phosphorous acid, orthophosphoric acid, and hydrogen.

Hypophosphorous acid is obtained by the interaction of concentrated solutions of its salts—hypophosphites, for example, Ca(H₂PO₂)₂—with sulfuric acid. Under laboratory conditions, hypophosphorous acid can be prepared by the oxidation of phosphine with an aqueous iodine suspension. Solutions of pure H₃PO₂ are prepared from NaH₂PO₂ with the aid of ion-exchange resins. Hypophosphites are used as reducing agents in the application of thin metallic coatings.