Note 10 bis — Chapter XIX · PHOSPHORUS AND THE OTHER ELEMENTS OF THE FIFTH GROUP

If there be a deficiency of oxygen, phosphorous anhydride P2O3 is formed. It was obtained by Thorpe and Tutton (1890) and is easily volatilised, melts at 22°·5, boils without change (in an atmosphere of N2 or CO2) at 173°, and is therefore easily separated from P2O3, which volatilises with difficulty. The vapour density shows that the molecular weight is double, i.e. P4O6 (like As2O3). Although colourless, phosphorous anhydride (its density in a state of fusion at 24° = 1·936) turns yellow and reddens in sun-light (possibly red phosphorus separates out ?), and decomposes at 400° forming hypophosphorous anhydride P2O4 (Note 11) and phosphorus. It passes into P2O5 in air and oxygen, and when slightly heated in oxygen becomes luminous, and ultimately takes fire. Cold water slowly transforms P2O3 into phosphoric acid, but hot water gives an explosion and leads to the formation of PH3, (P4O6 + 6H2O = PH3 + 3PH3O4). Alkalis act in the same manner. It takes fire in chlorine and forms POCl3 and PO2Cl, and combines with sulphur at 160°, forming P2S2O3 (the molecular formula is double this) a substance which volatilises in vacuo and is decomposed by water into H2S and phosphoric acid, i.e. it may be regarded as P2O5, in which O2 has been replaced by two atoms of sulphur. Judging from the above, the mixture of P2O3 and P2O5 formed in the combustion of phosphorus in air is transformed into P2O5 in an excess of oxygen.