Physical Preparation of Oxygen

The atmosphere, as a comparatively simple gaseous mixture, naturally suggests itself as a source of oxygen. There are various physical methods available for the separation of two such gases as nitrogen and oxygen, chief among which are the following:

When liquid air is allowed to evaporate, the escaping vapour is exceedingly rich in nitrogen since this gas has a lower boiling-point (namely, 195-67° C.) than oxygen (b.-pt., 182.9° C.).

Oxygen is approximately twice as soluble in water as nitrogen, hence it follows that water, upon exposure to air, will absorb twice as much oxygen in proportion to nitrogen as corresponds to the partial pressures of these gases. If now the dissolved gases are expelled from the water by boiling or by the aid of a vacuum pump the resulting " air " will contain roughly one part of oxygen to two parts of nitrogen by volume. By repeating these processes several times, fairly pure oxygen can be isolated. Mallet, in 1869, took out a patent for the commercial preparation of oxygen based on the foregoing principle. He found that after eight absorptions with water under pressure, a gas containing 97.3 per cent, of oxygen could be obtained. His results for successive absorptions were as follows:

No. of absorptions	0	1	2	3	4	5	6	7	8
Percentage of oxygen	21	33.3	47.5	62.5	75.0	85.0	91.0	95.0	97.3
Percentage of nitrogen, etc	79	66.7	52.5	37.5	25.0	15.0	9.0	5.0	2.7

At the present time this method does not appear to have any commercial importance. The relative solubilities of oxygen and nitrogen in various other solvents have been determined, but the results do not encourage the idea that oxygen can be obtained any more readily than by the employment of water.

Thin layers of caoutchouc allow oxygen to diffuse through them about 2½ times as rapidly as nitrogen, and a rough separation of the gases can be effected in this manner.

When coconut charcoal is cooled to - 185° C., and exposed to pure, dry air, it absorbs oxygen more readily than nitrogen, and the gas recovered at 15° C. contains some 56 per cent, of oxygen. If allowed to escape slowly, the absorbed gas can be fractionated, the later fractions containing as much as 84 per cent, of oxygen.

It is also possible to separate oxygen and nitrogen by taking advantage of their differences in density as, for example, by direct diffusion through some inert, porous material, when the gases pass through at rates consonant with Graham's Law; or by centrifugal force.