Re: PHREEQC and pH

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

I think you've checked out most of the options, and none of them are great
for doing what you want. (1) I'd do it like example 8, where you define one
solution, then a series of USE solution 1; EQUILIBRIUM_PHASES with Fix_H+.
You need a few lines of input for each pH.

(2) You can define a series of solutions with different pH, optionally
adjusting to charge balance with one of the elements. I think this is what
you are doing now.

(3) You can use REACTION to titrate an acid or base in varying amounts. The
problem here is that you do not get nice even pH values without some trial
and error on the reaction amounts.

(4) With more effort, you can add the REACTION in tiny steps, and then
write a USER_PUNCH data block to save values that are sufficiently close to
the desired pH (the PUT command in Basic). You probably want to turn off
all the printing during this stage. As a last step, do a trivial
calculation, turn on printing, and write another USER_PUNCH or USER_PRINT
data block to print out the results (GET and PRINT or PUNCH). This works in
concept, but it would take some effort to write the Basic and it still
might be tough to get increments that produce pH values close to even pH
values.

MINTEQA2 allows you to sweep through a set of fixed pH values I believe. I
know Geochemist's Workbench allows you to fix activities, but I'm not sure
how complicated it is to sweep through a series. We chose not to do this
because it is possible to inadvertently imply moles of reactant to maintain
a pH or to step out of the stability field of water. If you only want a
handful of pH values, you can put together (1) or (2) with cutting and
pasting and a little editing. If you want hundreds, I'd probably write some
sort of script to generate the input files.

Long answer to tell you what you already suspected,

David




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


                                                                                                                          
                      "Richard                                                                                            
                      Johnston"                To:      <dlpark@xxxxxxxx>                                                 
                      <johnstonrh@xxxx         cc:                                                                        
In-Reply-To: <000201c15e68$16a052e0$505b0298@xxxxxxxxxxx>
                      spring.com>              Subject: PHREEQC and pH                                                    
                                                                                                                          
                      10/26/01 03:49                                                                                      
                      PM                                                                                                  
                                                                                                                          
                                                                                                                          



Hi Dr. Parkhurst -

I'm just starting to learn PHREEQC and am making diagrams to show
parameters
(e.g. speciation) changing with pH. Right now I'm defining a new solution
for each pH value, but this takes up a lot of lines. Is there a way to tell
PHREEQC to sweep through a range of pH values? I know you can compactly
vary
temperature using REACTION_TEMPERATURE, and can tell it what size step to
take. I also know you can fix pH by defining a phase like FIX_H+ and then
invoking EQUILIBRIUM_PHASES for a given solution, but if I want say 20
different pH values I have to define 20 different SOLUTION blocks. Can I
use
REACTION or some other trick to 'titrate' with Fix_H+? Perhaps somehow
using
a BASIC loop?

I can imagine wanting to do the same thing with other constituents (e.g.
adding incremental amounts of Cu+2 to a system), and it seems like it
should
be possible to do it compactly, I just haven't been able to figure out how
to do it yet.

Thanks in advance, and have a good weekend.


Rick Johnston


-------------------------------------------------------
Richard Johnston
Doctoral Candidate
Department of Environmental Sciences and Engineering
University of North Carolina - Chapel Hill
CB #7400
Chapel Hill, NC 27599-7400
Tel: 919-967-5468
Email: johnstonrh@xxxxxxxxxxxxxx
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