Chapter 7 : Vapour-Liquid Equilibria
Section 2 : Single Component System Phase Equilibria
 

On comparing eqns. 7.14 and 7.15, it follows that:  while A is the constant of integration. These are generally regarded as constants for a given species. A plot of experimental values of lnPsat vs. 1/T generally yields a line that is nearly straight between the triple and critical points. However, the validity of eqn. 7.15 is questionable at relatively high pressures, and certainly in the critical region. Thus the accuracy of the Clausius-Clapeyron equation reduces at higher pressures. A modified form of eqn. 7.15, called called the Antoine Equation, has proved to be more accurate, including at higher pressures, has the following form:
(7.16)

A, B, and C are readily available for a large number of species. Appendix-VI provides values of Antoine constants for select substances. More complex forms of equations relation temperature and vapour pressure of pure substances have been reported in the literature, which provide even greater accuracy. An example of such an equation is the Wagner equation, which is given by:
, where   (7.17)

The constants for the Wagner equation for specific substances are available in several reference texts (see R.C. Reid, J.M. Prausnitz and B.E. Poling, Properties of Gases and Liquids, 4th ed., McGraw-Hill, 1987).