Aspen Physical Property System - Physical Property Models

and for n-m electrolytes:
2 Thermodynamic Property Models 143
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The parameters � (0) , � (1) , � (2) , � (3) and also C, �, and � can be found in
Pitzer's articles .
After the activity coefficients are calculated, they can be converted to the
mole fraction scale from the molality scale by the following relations:
For solutes:
For water as a solvent:
Where:
�m = Activity coefficient (molality scale)
�x = Activity coefficient (mole fraction scale)
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Application of the Pitzer Model to Aqueous
Electrolyte Systems with Molecular Solutes
In aqueous weak electrolyte systems with molecular solutes, the second and
third virial coefficients in the basic Pitzer equation for molecule-ion and
molecule-molecule interactions must be considered. The following extensions
of Pitzer's interaction parameters are made.
The second-order parameters Bij are extended to include molecule-molecule
and molecule-ion interaction parameters.
The third-order parameters �ijk are extended to molecule-molecule-molecule
interactions. The following expressions relate �ijk to Pitzer's original �ijk:
�iii = 6�iii
However, molecule-molecule interactions were not taken into account by
Pitzer and coworkers. So �iii is an artificially introduced quantity.