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Re: selectivity coefficients

It is not necessary to bias the log K by 20 in PHREEQC. This was a trick
for the old PHREEQE program, which include X- in the mole balance
equations. The free X- ion was forced to be small by this adjustment to the
log K's of the exchange reactions. In PHREEQC, the X- ion is not added into
the mole balance equations at all. The cation exchange reactions are half
reactions referenced to sodium (log K 0.0) in the default data bases.


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled

                      "Bruno Haerens"                                                                            
                      <bruno.haerens@xxxxxxxx        To:       <dlpark@xxxxxxxx>                                 
                      uven.ac.be>                    cc:                                                         
                                                     Subject:  selectivity coefficients                          
                      11/10/2003 06:10 AM                                                                        

Dear David,

Here I am again with a question:
In Appelo's paper in WRR (1994), he mentions in the comments of table 1,
that to all log KIX, 20 is added in PHREEQM. I guess this is als still the
case in PHREEQC-2, but I didn't find that in the manual.

Am I correct here?

Best regards,

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