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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, D14°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
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