The effect of foreign gases on the lower critical oxidation limit of phosphorus vapour

Abstract

The oxidation of phosphorus has for long been the object of experimental study. It is only within recent years, however, that it has been found possible to apply the laws of chemical kinetics to the problem in order to elucidate its mechanism. This advance was made possible by the study of oxidation of the vapour under carefully regulated conditions. In particular, the experiments of Chariton and Walta, Semenoff, and of Kowalski, have established the fact that the luminous oxidation of phosphorus vapour at a given pressure takes place only between two limiting pressures of the reacting oxygen—the lower and upper critical oxidation limits. At pressures less than the lower limit the oxidation is immeasurably slow, whereas at pressures above the upper limit Lord Rayleigh has shown that the oxidation is slow and is unaccompanied by luminescence. It was Semenoff who first definitely realised that the mechanism of the luminous oxidation was to be explained on the conception of chain reactions. He therefore applied the theory in a quantita­tive manner in order to find how the lower critical oxidation limit varied with the pressure of the phosphorus vapour, the diameter of the reaction tube, the presence of an inert gas and the temperature of the reaction mixture. The result of his investigation gave the equation p P p 0 (1 + p x / p 0 + p P ) d 2 = constant, where p 0 is the lower critical oxidation pressure, p P and p x the pressures of the phosphorus vapour and of the inert gas respectively, d is the diameter of a cylindrical reaction vessel. The “constant” in the equation is independent (within the experimental error) of temperature in the range 0-100° C. Somewhat later the same equation was deduced by Dalton and Hinshelwood by a different method but in a more rigorous way, for the oxidation of phosphine at the lower critical limit. The method, on account of its generality, is equally suitable for application to the oxidation of phosphorus. In order to derive this equation Semenoff assumed that the reaction chains terminated on the walls of the containing vessel and that the diffusion of the propagators of the chains to the walls was hindered by the presence of an inert gas X. This assumption resulted in the appearance of d 2 . For simplicity it was further assumed that the diameters of all molecular species present were equal and that the mean free path in the gas mixture was inversely proportional to the total pressure. The experimental results agreed excellently with the theoretical equation except that they tended to show that the critical oxidation pressure varied inversely as the square root of the pressure of phosphorus vapour. Argon was used as the inert gas.