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Activity coefficient calculation vs databases
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Topic: Activity coefficient calculation vs databases (Read 894 times)
IliasBouchkira
Top Contributor
Posts: 41
Activity coefficient calculation vs databases
«
on:
September 06, 2023, 10:28:45 AM »
Dear Dr
I would like to know whether the activity coefficient model used depend on the selected database ?
I have seen the keywords -gamma and -davies, do they allow to select the model to use for activity coefficient calculation?
many thanks in advance.
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dlparkhurst
Global Moderator
Posts: 3802
Re: Activity coefficient calculation vs databases
«
Reply #1 on:
September 06, 2023, 05:44:20 PM »
The databases distributed with PHREEQC have several different activity-coefficient models.
Most complex is the Guggenheim or Pitzer model. It accounts for non-ideality with numerous parameters for ion pairs and ion triplets (although it uses very few aqueous species). pitzer.dat, ColdChem.dat, and frezchem.dat databases have the Pitzer parameters defined in a PITZER data block.
A simplified version of the specific-interaction approach is implemented in the sit.dat database. It has only one parameter for pairs of ions.
All of the other databases use some form of Debye-Huckel activity coefficients.
llnl.dat has its own Debye-Huckel formulation that has species-specific ion-size parameters, but a single bdot parameter for ions and a single bdot parameter for uncharged species.
All other databases use a combination of the -gamma identifier and -davies identifiers in the SOLUTION_SPECIES data block. By default the Davies expression is used, which has no adjustable parameters. Alternatively, a Debye-Huckel ion-size parameter and a bdot parameter can be defined for a species using the -gamma identifier.
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Activity coefficient calculation vs databases