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Re: Enquiry

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 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                                                                                         
                      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

Tel: 510-486-6472

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