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