Phosphorus Dioxide, [PO₂]_n

Phosphorus Dioxide, [PO₂]_n, or Tetroxide, P₂O₄.—The formula P₂O₄ is sometimes assigned to this oxide by analogy with N₂O₄, but by analogy with the other oxides of phosphorus the formula would be P₄O₈.

By heating in a sealed evacuated tube the products of the slow combustion of phosphorus, a crystalline sublimate is obtained, which was shown by Thorpe and Tutton to be a distinct new oxide, probably formed according to the equation

P₄O₆ = 3PO₂ + P

The oxide may also be formed directly by slow oxidation of phosphorus in oxygen at a pressure of 600 mm. and containing water vapour (0.1 mm.).

It sublimes at about 180° C. in colourless crystals which are of rhombic appearance. The density, D_14°^22.6°, is 2.537. The vapour density, determined at a temperature above 1400° C., corresponded to a molecular weight of 458.6, and therefore to the formula P₈O₁₆. This oxide is stable in oxygen at the ordinary temperature, but is oxidised to pentoxide at 350° to 400° C.

As the tetroxide has been shown to have the formula of a " mixed anhydride," it should yield phosphorous and phosphoric acids on treatment with water. Actually, the solution, after neutralisation with sodium hydroxide, showed the reactions of phosphites and meta- phosphates. The hydration may therefore be represented by an equation

2PO₂ + 2H₂O = H₃PO₃ + HPO₃