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Data required for calculating activities


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I am trying to use Spece8 to calculate activities using the Bdot model. I have in situ data of chemical composition. I'm altering the Debye-Huckel coefficients for different temperatures. I am only going to use geochemists workbench to calculate the log(a) values for these species. I notice that logK values don't appear in the Bdot equations. So my question is, would I need to go through and alter all the logK values for the secondary species and redox reactions in thermo.dat? Are these logK values important in the Bdot model?


Many thanks,



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Hi Natasha,


The default thermodynamic dataset included with GWB, thermo.dat, spans temperatures from 0 to 300 C. The log Ks for all the reactions span this range, as do the Debye-Huckel parameters. Do you have a reason to alter the coefficients? Are you looking to work outside of this temperature range?


If not, then you can just add components for each of your analyses to the Basis pane, perform any swaps if necessary, then set the constraints (bulk/ total concentrations, or concentrations for free species) and the temperature. See Section 7 "Using SpecE8" in the GWB Essentials Guide for more info on how this is done. A

on our YouTube channel shows how this is done.


As for using the B-dot equation to calculate activity coefficients, I don't think you'll need to alter the log Ks unless you're working outside the temperature and pressure range of the thermodynamic dataset. Craig Bethke's Geochemical and Biogeochemical Reaction Modeling text discusses what goes into the Debye-Huckel equation (this applies as well to the B-dot equation), saying "Equation 8.2 is notable in that it predicts a species' activity coefficient using only two numbers (zi, the species' charge, and ai, the ion size parameter) to account for the species' properties and a single value I (the true ionic strength) to represent the solution."


Hope this helps,


Brian Farrell

Aqueous Solutions LLC


P.S. I moved your topic from the archive of old posts to the front page.

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