Aspen Physical Property System - Physical Property Models

Taking the derivative of equation 7 with respect to I, gives:
So that:
142 2 Thermodynamic Property Models
(8)
(9)
(10)
This equation is used in equation 6. In equations 7 and 9, is the usual Debye-
Hückel constant for the osmotic coefficient, determined from:
Where:
NA = Avogadro's constant
dw = Water density
�B = Dielectric constant of solvent B
(11)
b is an adjustable parameter, which has been optimized in this model to equal
1.2.
B and B' need expressions so that equations 5 and 6 can completely be solved
for the activity coefficients. The parameter B is determined differently for
different electrolyte pairings. For 1-n electrolytes (1-1, 1-2, 2-1, and so on)
the following expression gives the parameter B:
with �1=2.0.
(12)
For n-m electrolytes, n and m>1 (2-2, 2-3, 3-4, and so on), B is determined
by the following expression:
with �2 = 12.0 and �3 = 1.4.
(13)
By taking appropriate derivatives, expressions for B' can be derived for 1–n
electrolytes:
(14)