Calculation of the exchanger composition changes pe?

[MichaelZ20]

Hello!
When I run the attached file for calculation the exchanger composition, from the unknown reason pe changes from the default value to -2.261  "Adjusted to redox equilibrium". Is this a bug?

[dlparkhurst]

Let's call it a feature. Your solution has no redox buffering, that is no redox element with two (or more) redox states with significant concentrations. The pe can range from -2 to over 10, without generating a significant amount (usually around 1e-14 mol/kgw) of two redox states. It picked -2 because H2(aq) is beginning to appear in solution at that pe, but it could also range up to a high pe where O2(aq) begins to appear at a concentration of 1e-14. The direction it goes is somewhat random.

So, if you added some DO to your SOLUTION, or any other redox couple, the pe would be poised, and would not change so much. One artificial way to add a redox species is with the following definition:

Code: [Select]

SOLUTION_SPECIES
H2O + 0.01e- = H2O-0.01
log_k -8

This ficticious species will have a concentration of about 1e-8, and provides sufficient redox buffering to keep the pe near 4.