Acid Strength's Influence on logK Value

[OmranS]

Hi all !

My question is in regard to the log value of the dissolution constant that is presented in the output of a simple REACTION_ input. I first outlined my concentrations of Ba, S, as I was interested in the concentrations of barite, precipitated in HCl. Yet I noticed that even if I were to range the pH’s through the change of the acid's strengths, the barite's logK value does not change and consistently remains -9.84. So my question is, when I calculate the pK values for Ba and SO4 in various concentrations of HCl, the pK values do not change as function of acid strength. Why is that the case?

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SOLUTION 1 Pure water
units ppm
pH 7.0
temp 25.0
pe 8.451
redox O(0)/O(-2) # as redox couple
Ba 0.00028566
S 0.00028566
O(0) 1.0 O2(g) -0.7

REACTION 1
HCl 12.0
135 mmol
END

[dlparkhurst]

PHREEQC uses intrinsic log K values that depend only on the free energies of formation for the minerals and species in a reaction; the values apply to an idealized state at zero ionic strength. 

All of the concentration effects are included in the activity coefficients, for example:

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log10K(Barite) = [Ba+2][SO4-2] = gamma(Ba+2)*(Ba+2)*gamma(SO4-2)*(SO4-2)

whre brackets [] indicate activity, gamma indicates the activity coefficient, and parentheses indicate molality (mol/kgw).

These equations apply to all PHREEQC databases, although the theory and implementation of the activity coefficients vary by database. Pitzer.dat, coldchem.dat, and frezchem.dat use a Guggenheim virial expansion and lots of parameters to calculate activity coefficients; Sit.dat uses a simpler virial expansion; all of the rest use an expanded Debye-Huckel approach, but still with some differences in parameterization among the ion-association-model databases phreeqc.dat, llnl.dat, minteq.V4.dat, and others.