Chapter 6 : Solution Thermodynamics and Principles of Phase Equilibria
Section 17 : The Activity Coefficient
As we have seen in the last section that the value of the function is relatively larger compared to thus comparing eqns. 6.167 and 6.168 with 6.175 and 6.176 respectively, one may conclude that the activity coefficients are far more sensitive to changes in temperature than to changes in pressure. For this reason for phase equilibria computations at low to moderate pressures, the activity coefficients are assumed invariant with respect to pressure.
Since the activity coefficients are partial molar properties, they are related by the Gibbs Duhem equation (at constant temperature and pressure) as follows:
(6.177)
The above equation may be used to validate or check the consistency of experimental data on isothermal activity coefficients for a binary system. The following equation may be derived from eqn. 6.177 for this purpose (by assuming negligible effect of pressure on the liquid phase properties):
is relatively larger compared to 
thus comparing eqns. 6.167 and 6.168 with 6.175 and 6.176 respectively, one may conclude that the activity coefficients are far more sensitive to changes in temperature than to changes in pressure. For this reason for phase equilibria computations at low to moderate pressures, the activity coefficients are assumed invariant with respect to pressure.
