Re: Manual calculations versus phreeqc

["David L Parkhurst" <dlpark@xxxxxxxx>]

Can you tell me if this difference is significant and if so where I might
be
going wrong?

I would not consider the difference significant. Both numbers indicate
calcite is reacting fast enough to maintain equilibrium.

I'm not sure how you are  doing the hand calculation, but PHREEQC prints
the molalities, activities, and log activity coefficient for each aqueous
species. You should be able to compare directly to the hand calculation.

David


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


                                                                                                                       
                      "Garrett Kilroy"                                                                                 
                      <kilroyg@xxxxxxx         To:      <dlpark@xxxxxxxx>                                              
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                                               Subject: Manual calculations versus phreeqc                             
                      07/21/01 10:50                                                                                   
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                      to kilroyg                                                                                       
                                                                                                                       
                                                                                                                       



Dear Sir/Madam,
I would be very grateful if you could assist me in this query.

I am using Phreeqc to calculate calcite saturation indices for my PhD.
However, my manual calculations are coming out different to the Phreeqc
results.
As a test I used data from a hydrogeology textbook (Fetter(2001) applied
hydrogeology, p360-1) which calculates calcite saturation manually. The
textbook gives a saturation index of 0.05 while Phreeqc gives a saturation
index of -0.02.
Can you tell me if this difference is significant and if so where I might
be
going wrong?
I would welcome any suggestions.
Best regards,

Garrett Kilroy
PhD Candidate

Department of Geology/
Centre for the Environment
Trinity College
Dublin 2
Ireland
Tel: +353 1 608 2403
Fax: +353 1 671 8047