Activity coefficients calculated incorrectly for non 1:1 salts

[rkingsbury]

For salts with a cation : anion ratio other than 1:1 (e.g. MgCl2, K2SO4), activity coefficients calculated by the Pitzer Model (the default method) are incorrect.

>>> import pyEQL
>>> s1 = pyEQL.Solution([['Mg+2','0.5mol/L'],['Cl-','1mol/L']])
>>> s1.get_activity_coefficient('Mg+2')
<Quantity(0.727018603664255, 'dimensionless')>

The literature value (CRC Handbook) is 0.485. Similarly large discrepanices occur at other concentrations and with other salts.

[rkingsbury]

The problem is that the Pitzer model code has to compute the concentration of the SALT, not just the individual cation or anion. It does this by averaging the cation and anion concentration:

# determine the average molality of the salt
# this is necessary for solutions inside e.g. an ion exchange
# membrane, where the cation and anion concentrations may be
# unequal
molality = (self.get_amount(Salt.cation,'mol/kg')+self.get_amount(Salt.anion,'mol/kg'))/2

But, the above doesn't account for the stoichiometry of the salt. In 0.4-dev I've fixed it like this:

molality = (self.get_amount(Salt.cation,'mol/kg')/Salt.nu_cation+self.get_amount(Salt.anion,'mol/kg')/Salt.nu_anion)/2

With the above fix, 2:1 and 1:2 salts now work properly. Examples of these are now included in the test suite for activity coefficient as well.

[rkingsbury]

Fixed by 5fa742b