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 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, Bruno
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