[OLD] Ion parameter

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From: Yoshida Yasushi

Subject: ion parameter

would like to know the function of ion parameter in GWB database and how to set its values for complicated species. For example, if we added new basis species, redox couples or aqueous species to GWB database, we need set its own value for ion parameter. In the point of view for consistency, ion parameter should be determined by same rule as original database. If the value of ion parameter is not important in GWB calculation, we can set its value as blank or dummy. So, I would like to know how ion parameter used in GWB calculation and how to determine its value in original database.

 

From: Craig Bethke

Subject: Re: ion parameter

The ion size parameter is used to calculate activity coefficients for aqueous species. You can read about it and the calculation of activity coefficients on pages 109-114 of the "Geochemical Reaction Modeling" text. When adding data to the GWB database, you should chose a value for the ion size parameter consistent with the derivation of the equilibrium constant for the species' reaction. In other words, since an activity coefficient must be assumed in order to specify from experimental data the log K for the reaction to form a species, you should use an ion size parameter consistent with that assumption.