Activity Coefficient Models

[Salem]

Hi,

How can I know what activity coefficient model is used in speciation calculations?

[dlparkhurst]

The database that you use determines the activity coefficient model. There are basically three different kinds: (1) extended Debye-Huckel, (2) Pitzer, and (3) SIT (Specific Interaction Theory).

(1) Most databases distributed with PHREEQC use some form of extended Debye-Huckel, where there are at most two parameters: an ion-size parameter and an ionic strength parameter for each species. The details for each species are found in the SOLUTION_SPECIES definitions. llnl.dat and core10.dat have special definitions for some of the parameters defined in the LLNL_AQUEOUS_MODEL_PARAMETERS data block. You can look in the manual for these data blocks, and the version 2 manual (distributed with the code) has more details on activity coefficients.

(2) pitzer.dat, coldchem.dat, and frezchem.dat use the Pitzer approach, which uses many specific-ion-interaction parameters defined in the PITZER data block to calculate activity coefficients. In detail, this approach makes no claims for individual ion activity coefficients, only for mean-activity coefficients. The McInnis assumption that activity coefficient Cl- is equal to activity coefficient K+ can be used to estimate individual ion activity coefficients (the option -mcinnis in the PITZER data block).

(3) SIT, in sit.dat, is a stripped down Pitzer approach that has at most one specific-interaction parameter for each pair of ions.

[Salem]

Thanks!