Slow combustion of phosphorus

The slow combustion of phosphorus in moist air is an apparent exception to the law of mass action, for the rate of oxidation diminishes with rise of pressure, becoming practically nil at 500 mm. of oxygen. This is really a case of false equilibrium akin to passivity in metals. The surface of yellow phosphorus is exceedingly sensitive to the action of traces of impurity. The moisture appears to be the primary cause of the irregularity referred to, although the modus operandi is not understood. If, on the other hand, freshly distilled phosphorus is exposed to the action of air or oxygen dried by passage through sulphuric acid, and therefore having a moisture content of the order of 1 mg. per 400 litres, the oxidation proceeds at all pressures and is mono- molecular above 500 mm. It is accompanied by a very feeble glow and the formation of some unknown oxide. Below 500 mm. the reaction is accelerated, the glowing is more intense and phosphorus pentoxide is formed.

On the other hand, if the oxygen is very dry the oxidation proceeds exceedingly slowly, and probably if the system were entirely free from water no action would take place at all. Ozone and hydrogen peroxide, which are produced during the oxidation of moist phosphorus, do not occur in the dry reactions. The Slow combustion of phosphorus, or slow oxidation of phosphorus in various oils has also been studied.