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Oxygen : Liquefaction Of Gases
Most of the attempts relied on the application of pressure to the gases. Some gases may be liquefied by the application of pressure without very strong cooling; the pressures in atm. required to liquefy the gases at 0° are:
Sulphur dioxide: 1.53
Carbon dioxide: 34.55
The application of pressures up to 2000 atm. was tried without result by Natterer (1844) in the case of the gases nitrogen, oxygen, and hydrogen.
In 1869 Andrews discovered that a gas cannot be liquefied by any pressure, however high, unless it is previously cooled below what is called the critical temperature of the gas. Just below this temperature the gas is liquefied by the application of what is known as the critical pressure. The volume occupied by 1 gm. of a substance at the critical temperature and under the critical pressure is called the critical volume. The critical temperature of oxygen is -118.75° and the critical pressure 50.2 atm.
The critical temperatures of the so-called permanent gases lie below the lowest temperatures attained by older experimenters. As soon as it was clear that strong cooling was necessary and that high pressures alone could never succeed in the case of these gases, the problem was solved, independently, by Pictet and Cailletet in 1877.
Pictet used the apparatus shown in
Oxygen generated in the retort P by heating potassium chlorate, was compressed by its formation in a copper tube, cooled in liquid carbon dioxide L boiling under reduced pressure, and fitted with a pressure gauge Q and release valve N. The carbon dioxide was re-liquefied by a pump G in a second copper tube, EF, surrounded by liquid sulphur dioxide boiling under reduced pressure, and circulated by a second pump. Pictet reduced the temperature to -140°, and the pressure rose to several hundred atmospheres. On opening the release-valve N, a jet of liquid oxygen issued from it, at once boiling away.
Cailletet compressed the gas by a powerful pump forcing water into a strong steel vessel, B,
in which the gas was contained in a tube, T, sealed below by mercury. Water forced into B drove the mercury into T, and strongly compressed the gas. The pressure was then suddenly released by opening a valve which allowed the water to escape, and the gas expanded suddenly (adiabatic expansion). The cooling produced, by the gas doing work against pressure in expanding, reached the point of liquefaction of the oxygen. A fog of liquid droplets was seen momentarily in the tube, at once vanishing as heat was communicated from the walls of the latter.
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