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Re: SI S

Hi Greg,

I think you want to adjust the pe, rather than the pH, when you find the
fixed SO4/H2S ratio.

See if the following does what you want.

TITLE        sulfur SI
             H2O + SO4-2 = H2S
             -log_k            0
             -file                         int_S.sel
             -reset                        false
             -ph                     true
             -totals                       S(-2) S(6)
             -activities             H2S SO4-2
             -saturation_indices           S
             units                         mol/kgw
             temp                    25
             S                       0.1
             pH                      1
            pe          1      fix_S      0
             units                         mol/kgw
             temp                    25
             S                       0.1
             pH                      2
            pe          1      fix_S      0
             units                         mol/kgw
             temp                    25
             S                       0.1
             pH                      3
            pe          1      fix_S      0
             units                         mol/kgw
             temp                    25
             S                       0.1
             pH                      4
            pe          1      fix_S      0
             units                         mol/kgw
             temp                    25
             S                       0.1
             pH                      5
            pe          1      fix_S      0


I tried just adjusting the pe to the phase boundary in the reaction step by
adding H2. Apparently, this affects the pH (SO4-2 -> S-2) more than it
affects the ratio (SO4/H2S), and it does not converge. I think the program
is working ok, it is simply not possible to attain a SO4/H2S ratio in this
way. At any point you can adjust both the pe and the pH with the following
and I think it should work.

            fix_S 0.0 H2 10
            fix_pH -5 NaOH 10  # with appropriate definition of fix_pH

But you have to choose the pH that you want, in this case pH 5.

I expect at some point all the polysulfides and other sulfur redox states
will cause some problems.


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

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