Re: Phreeqci 2.6 charge balance question

["Brian Park" <bpark@xxxxxxxxxx>]

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: https://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)
> 
> 
>