RE: turning off charge balance

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

I was afraid that example 8 wasn't what you needed, but there is no way to
easily turn off the charge balance, without getting into the program. I
guess I still don't quite understand what you need. If you are running a
"batch-reaction" calculation and you turn off charge balance, the pH will
be fixed at the initial pH and will not depend on anything. This is because
the unknown activity(H+) is associated with the charge balance equation in
PHREEQC. It seems like you want the pH to vary with whatever equilibrium
constants are being adjusted.

I'm not sure you really have a problem; the pH of the reacted solution is
independent of the initial charge imbalance. The calculations carry along
whatever charge imbalance was calculated for the initial solution, so that
each reacted solution has exactly the same charge imbalance. The variations
in pH that you are calculating are due to the changes in your independent
variables. However, if you are adding REACTIONS or KINETICS in your
reactor, it is essential that the reactants be charge balanced; it is not
possible to add a charged ion, like H+, you must add for example HCl.

David


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


                                                                                                                                  
                      "Jon Schwantes"                                                                                             
                      <plutonium@xxxxx         To:      "David L Parkhurst" <dlpark@xxxxxxxx>                                     
                      et>                      cc:                                                                                
In-Reply-To: <MABBJCOBOKMBDNHOOGCMGEBACBAA.plutonium@xxxxxxx>
                                               Subject: RE: turning off charge balance                                            
                      04/27/01 10:52                                                                                              
                      AM                                                                                                          
                                                                                                                                  
                                                                                                                                  



Dr. Parkhurst,

Thanks for your timely response.  I am aware of that example.  However,
when
using my inverse model to regress on "best" equilibrium constants, sets of
measured data are used as dependent variables (including pH) and are
assumed
to contain a bit of error.  In a hypothetical case where datasets used for
basing the regression are not exactly defined to zero charge balance, it
would be advantagous for the user to manually turn off charge balance
during
regression.  In such cases where datasets are closely defined to within a
few percent of charge balance, if charge balance was turned off regression
could occur on dependent variables including pH.  Currently, the user must
define pH exactly as in example 8 and are not able to regress on pH in such
cases.

Thanks

Jon Schwantes
Environmental & Water Resources Engineering
Civil Engineering Department, MS-3136
Texas A&M University
College Station, Texas 77843-3136
h: 979-695-8264, w: 979-845-9772
e: plutonium@xxxxxxxx

"That that you give to others, does not die with you"


-----Original Message-----
From: David L Parkhurst [mailto:dlpark@xxxxxxxx]
Sent: Friday, April 27, 2001 8:35 AM
To: plutonium@xxxxxxx
In-Reply-To: <MABBJCOBOKMBDNHOOGCMGEBACBAA.plutonium@xxxxxxx>
Subject: Re: turning off charge balance



> I am currently working on an inverse model that uses PHREEQC to back
calculate equilibrium and kinetic constants.  In order for my model to
work, an input file describing several reactor solutions (in the case of EQ
constants) is described and saved and then equilibrated with several
unknown solids.  In the first equilibration, we charge balance on a
fictitious species.  However, during equilibration with the solids, pH is
changed to satisfy charge balance.  Is there a way we can "turn this off"?
The general idea for my model is that it will call PHREEQC to perform
equilibrium several times in order to regress on the real values of the
constants for specified unknowns.  To do this, we regress on several
dependent variables, including pH, so it is imperative that pH is not
affected by charge imbalance.

There is a way to fix the pH, which is demonstrated in example 8 of the
manual.

David


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