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Re: Phreeqci 2.6 charge balance question



David,
 
Thank you for your response, I believe I'm getting the hang of things.  I'd like to ask one other thing.
 
As a test case I'm trying to simulate the effects of sulfate reduction due to sulfate reducing bacteria on acid mine drainage.  The equation sometimes put forth as occurring is
 
SO4-2  +  2CH2O  =   2HCO3-  +  HS-  +  H+
 
Where the CH2O represents some organic material.  Using the REACTION keyword I'm taking out sulfate, and adding bicarbonate, bisulfide, and protons.  Since it's a redox reaction that's why I was concerned about charge.  If the program ignores charges, then for example if I specify sulfur as participating in the reaction, how does it know if I'm talking about sulfate or sulfide?  Incidentally, when I do put them in as SO4-2, HCO3-, HS-, and H+, it does seem to give reasonable results.
 
Thanks,
Brian Park
 
----- Original Message -----
From: "David L Parkhurst" <dlpark@xxxxxxxx>
To: "Brian Park" <bpark@xxxxxxxxxx>
Sent: Monday, February 10, 2003 5:47 PM
Subject: Re: Phreeqci 2.6 charge balance question

>
> > Where I'm a little confused is, if I specify a solution and equilibrium
> phases and no charge balance specie (or pH), it speciates the solution,
> gives saturation indices, etc.  No problem.  But then I use the REACTION
> keyword and add/subtract some species (essentially putting in an
> irreversible equation with the charge properly balanced on each side), the
> program goes ahead with the calculations and seems OK.  The charge balance
> is still the same as it was with the original speciation, but this time in
> the printout it says "Charge Balance" next to the calculated pH.
>
> It says "Charge Balance" next to pH for equilibrium_phases or reaction,
> basically any "reaction" calculation, as defined in the manual.
>
> Most people define reactions incorrectly. You should not use any charges on
> the things that are added or removed from solution; the charges are
> ignored. You must define the changes in elemental totals in the reaction.
>
> > Does that mean it has adjusted pH to achieve a charge balance somehow?
>
> Yes.
>
> > In a nutshell, I thought that if I did not specify anything to be used to
> achieve a zero charge balance, that the program would maintain the original
> charge balance throughout.
>
> That is correct (unless you are using SURFACE, where charge accumulates on
> the surface and the solution is adjusted to account for the loss of
> charge). Now think a bit, if I add HCl to a solution, the pH can not stay
> the same. How do I calculate the pH of the solution? ... I adjust the pH to
> the point where the charge imbalance is the same as the initial solution.
>
> > I am a little unnerved by seeing "Charge Balance" in the printout next to
> the pH resulting from the reaction.
>
> You can set the equations up in different ways. I chose mass balance on
> hydrogen and oxygen, and charge balance as three fundamental equations.
> Other people use "total H", redox balance, or frequently drop an equation
> and assume a constant mass of water, or something else. At this point, I am
> convinced the equations are implemented correctly, you just have to puzzle
> on it a bit to learn how I think about these things.
>
> David
>
>
> David Parkhurst (
dlpark@xxxxxxxx)
> U.S. Geological Survey
> Box 25046, MS 413
> Denver Federal Center
> Denver, CO 80225
>
> Project web page:
http://wwwbrr.cr.usgs.gov/projects/GWC_coupled
>
>
>
>                                                                                                            
>                       "Brian Park"                                                                         
>                       <
bpark@xxxxxxxxxx        To:       <dlpark@xxxxxxxx>                                 
>                       >                        cc:                                                         
>                                                Subject:  Phreeqci 2.6 charge balance question              
>                       02/10/03 05:28 PM                                                                    
>                                                                                                            
>                                                                                                            
>
>
>
>
> Hello,
>
> I am a new user of PhreeqcI 2.6 and wanted to ask a question.  While I
> wasn't able to find an official technical support contact (if there is
> one), your e-mail address showed up in a number of places and I believe you
> were involved in developing the software.
>
> My question is fairly simple and relates to charge balance.  I think I
> understand how charge balance is handled in aqueous speciations, from
> looking at the manual.  If you put in analytical data, and do not specify
> anything to be used to be added/subtracted to make the charge balance zero,
> the program maintains the original charge balance (or should I say
> imbalance) through the calculations.  Where I'm a little confused is, if I
> specify a solution and equilibrium phases and no charge balance specie (or
> pH), it speciates the solution, gives saturation indices, etc.  No problem.
> But then I use the REACTION keyword and add/subtract some species
> (essentially putting in an irreversible equation with the charge properly
> balanced on each side), the program goes ahead with the calculations and
> seems OK.  The charge balance is still the same as it was with the original
> speciation, but this time in the printout it says "Charge Balance" next to
> the calculated pH.  Does that mean it has adjusted pH to achieve a charge
> balance somehow?  I thought the charge balance was constrained to the value
> calculated for the original solution and that the program just calculated
> pH as part of the overall speciation problem, with the added/subtracted
> species included.
>
> I don't know if that was very clear.  In a nutshell, I thought that if I
> did not specify anything to be used to achieve a zero charge balance, that
> the program would maintain the original charge balance throughout.  I am a
> little unnerved by seeing "Charge Balance" in the printout next to the pH
> resulting from the reaction.  I attached the input file if it helps any.  I
> would really appreciate any guidance on this, I will look at the manual
> some more tonight.
>
> Thanks,
> Brian T. Park, P.E.
> Senior Process Engineer
> MSE Technology Applications, Inc.
> 200 Technology Way
> P.O. Box 4078
> Butte, MT  59702
>
> (406) 494-7415
> (406) 494-7230  [fax](See attached file: test run.pqi)
>
>
>


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