Oxygen : Oxygen, Technical Preparation

Various methods have been applied for the production of oxygen on the large scale, but at the present day two types of process are almost exclusively in use, viz., the electrolysis of water (with simultaneous production of hydrogen), and the fractional distillation of liquid air.

One type of electrolytic apparatus consists of an iron tank containing a solution of caustic soda, in which are immersed a number of iron bell-jars, each provided with an insulated iron electrode dipping below the mouth of the bell. The electrodes are connected to the positive and negative poles of a dynamo, and the oxygen and hydrogen gases liberated pass inside the bell-jars and are led off through pipes. The electrolyte is kept at about 60° and large currents (up to 1000 amperes) may be used.

A number of chemical methods were formerly used for the production of oxygen, the most important being the Brin process.

Boussingault in 1852 showed that baryta, BaO (which is a substance similar to quicklime, CaO), heated in a porcelain tube to dull redness, can absorb oxygen from air passed over it, giving barium peroxide: (i) 2BaO + O₂ = 2BaO₂, whilst the nitrogen is not absorbed. If the barium peroxide is now heated to bright redness, it gives off oxygen, leaving baryta: (ii) 2BaO₂ = 2BaO + O₂. The reaction is reversible, and proceeds in one direction or the other according to the temperature: 2BaO₂ < = > 2BaO + O₂. By using purified air, and iron retorts heated by gas at. about 700°, the brothers Brin in 1879 found that the process could be worked at one temperature if, during the absorption, the air was under 2 atm. pressure, whilst the peroxide was decomposed on reducing the pressure to about 2 in. of mercury. This Brin process was the principal technical method until 1902; it has now given way to the liquid air process.

A few physical processes were proposed before the present industrial method was adopted. Graham found that oxygen passes through an unvulcanised rubber membrane two and a half times as fast as nitrogen, and by pumping air through a rubber bag by means of a mercury pump he obtained a gas containing 42 per cent, of oxygen, which rekindled a glowing chip. This process, depending on the selective permeability of a membrane, is called dialysis. Again, if air is shaken with water, oxygen is dissolved more readily than nitrogen, and the gas liberated on heating or reducing the pressure is richer in oxygen. By working under pressure, and repeating the process four or five times, Mallet obtained a gas containing over 75 per cent, of oxygen.

When air is slowly passed through the stem of a clay tobacco-pipe enclosed in a partially exhausted glass tube, the lighter nitrogen diffuses through the porous tube more rapidly than the oxygen, in the inverse ratio of the square roots of the densities (Graham):

The issuing gas is therefore richer in oxygen than air. This process was called atmolysis by Graham.

Since oxygen is slightly heavier than nitrogen, Mazza (1901) attempted to separate air into the two gases by passing it through a centrifugal sieve: the method failed, owing to the small difference of densities.

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Oxygen
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