Conceptual Models > Design of conceptual models

Activity coefficient calculation vs databases

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IliasBouchkira:
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.

dlparkhurst:
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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