Aspen Physical Property System - Physical Property Models

�ijk is written for three different ions �ijk = �kij = �jki , and �ikk = 0. �ijk = 0
if the sign of zi =sign of zj =sign of zk. The factor of 1/6 is different from 1/2
in the last term in brackets in Pitzer's original expression. Pitzer distinguishes
between cations and anions. In Pitzer's original model this parameter appears
twice, as �cc'a and �c'ca. In this modified model, it appears six times, as �cc'a;
�c'ca; �acc'; �ac'c; �cac'; and �c'ac. Fürst and Renon's expression, equation 4,
calculates the expressions for activity coefficients and osmotic coefficients.
Pitzer (1975) modified his model by adding the electrostatic unsymmetrical
mixing effects, producing this modified Pitzer equation for the excess Gibbs
energy:
Calculation of Activity Coefficients
The natural logarithm of the activity coefficient for ions is calculated from
equation 4a to give:
140 2 Thermodynamic Property Models
(5)
(4a)
Where � is neglected and �ij and � ' ij are the electrostatic unsymmetric mixing
effects:
The X parameters are calculated differently on the option code.
For option code = –1, there is no unsymmetric mixing correction term: