The isotherms of CO 2 in the neighbourhood of the critical point and round the coexistence line

Abstract

The isotherms of CO 2 between 0 and 150°C. and up to 3000 atm. have been previously published by two of the authors (Michels, A. and C. 1935). The method used for these measurements was not suitable, however, for determinations in the neighbourhood of the critical point and the coexistence line. A second method has therefore been developed by which both the critical data and the coexistence line can be determined. This method and the results obtained are described in the present paper. The Method and Apparatus The method was based on the one developed by Michels and Nederbragt (1934) for the determination of the condensation points of a binary mixture. While, however, for the measurements of condensation points, it was not necessary to know the quantity of gas in the apparatus, this knowledge is essential for the determination of isotherms. A new apparatus was therefore constructed in which this quantity could be determined. A diagrammatic sketch showing the principle employed is given in fig. 1. In a steel vessel A , a glass bell B is suspended which is connected through the steel valve H and the capillary J to a cylinder containing a supply of the gas to be examined. A steel capillary C connects A with a second steel vessel D , placed on one scale pan of a balance. Inside D a steel tube E , which is coupled to C , reaches to the bottom. The capillary F is connected to the top of D and leads to a cylinder of pure nitrogen and to an apparatus for measuring the gas pressure. The capillaries C and F are flexible, and are supported at G at such a distance from the scale pan that the variations in the forces acting on the latter during the swinging can be neglected. Before starting the measurements, the vessels A , the glass bell B and the tube C are completely, and the vessel D is partly filled with mercury. The valve H is then opened and CO 2 gas admitted to the glass bell, driving mercury out of A into D . The pressure in D is balanced by nitrogen introduced through F . When sufficient CO 2 has entered the glass bell, the valve H is shut. As the filling operation is carried out at a temperature and pressure at which the isotherms of CO 2 are known, the amount of gas in B can be calculated from a knowledge of the volume.