The bdot form of the activity coefficient equation allows activity coefficients to increase at higher ionic strength and it does produce activity coefficients greater than 1. This is in accordance with mean-activity coefficient data that also shows mean activity coefficients greater than 1 at high ionic strength for many electrolytes. Uncharged species often have activity coefficient expressions that produce activity coefficients greater than 1 at all ionic strengths. It sounds like PHREEQC is doing the correct calculation. However, if ions have activity coefficients greater than 1, you are in a range of ionic strength where the ion-association model may not be reliable, probably greater than 1.0 molal. You should consider a Pitzer model (EQ3/6, Geochemists Workbench, TEQUIL all have Pitzer options). Note that no Pitzer model allows redox reactions or alumino silicate reactions. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 zuoping zheng <ZZuoping@xxxxxx To: dlpark@xxxxxxxx v> cc: In-Reply-To: <3C72EE9C.8E6D9797@xxxxxxx> Subject: Enquiry 02/19/02 05:32 PM Please respond to ZZuoping Dear Dr.David Parkhurst: How are you. After finishing my study in Oslo, I moved to the States last month as a post-doctor currently at Lawrence Berkeley National Lab. When I use PHREEQC to solve some problems, I got some trouble in explaining the results, for example, activity higher molality for the same species. The reason for this is due to the limitation of PHREEQC in handling high ion-strength solution. Do we have some way to solve this problem? or if you have other programs which can be used to solve this problem. Please let me know. Thanks. Best Regards, Zuoping Zheng LBNL Tel: 510-486-6472
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