Determination of Critical Constants
Long before Andrews performed his experiments with carbon dioxide, Caignard de La Tour (1822) observed that when ether was heated in a sealed tube, the surface of separation between liquid and its vapor disappeared at a particular temperature; but on cooling the surface reappeared at about the same temperature. The significance of these observations was not realized until Andrews correlated this with his work on carbon dioxide. He observed that the temperature at which the surface of separation between liquid and its vapor disappears is the same at which he observed the Critical phenomenon.
These observations have been utilized for the determination of the critical constants. Critical pressure and temperature may be determined fairly easily by using an apparatus as shown in figure; similar to Caignard de La Tour’s apparatus.
It consists of a thick-walled glass U-tube one limb of which is longer than the other. The shorter end containing liquid in contact with its saturated vapor is sealed and is surrounded by a heating jacket. The other limb of the U-tube, which serves as manometer, has air at its upper end A, the rest of the tube being filled with mercury.
The end B is gradually heated by passing hot air or vapor through the jacket and the temperature at which the meniscus separating liquid and vapor just disappears and at which it reappears on cooling is noted. The average of the two is the critical temperature. The volume of the air in the limb A is also noted at this temperature and the critical pressure is calculated from this.
When the liquid and its vapor are gradually heated the density of the liquid decreases, but the density of the vapor increases because of compression until at the critical temperature and pressure the densities of both liquid and vapor become equal, and the liquid and vapor become indistinguishable from each other. This is the reason why, at the critical point the surface of separation vanishes.