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GWB > Pitzer parameters alpha1/alpha2

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Hello Tom,

I have a question regarding the treatment of the Pitzer parameters alpha1 and alpha2 in GWB.

In Craig`s book "Geochemical and Biogeochemical Reaction Modeling" (2nd edn) I find the following

equation in Table 8.1 on page 125 (under step 4):

BMX = beta0 + beta1 * g * (alpha1 * sqrt(I)) + beta2 * g * (12 * sqrt(I))

Is it correct, that the "12" represents the value of alpha2 and is it fixed to that value for

every possible cation-anion pair? This would imply that the alpha2 values given in the parameter

file are not really used?

Thanks a lot for your help,

Christiane

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Dear Christiane:

The book uses a constant value because the parameter value is 12 for all cation-anion pairs (you can see this by inspecting the two Pitzer databases GWB uses, thermo_hmw.dat and thermo_phrqpitz.dat). The GWB code, however, picks up alpha_MX^(2) directly from the thermo dataset- so you can change these parameters to anything you want, and it will be reflected in the 2nd virial coefficient calculations.

I hope that helps,

Tom

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