Redox Equilibria > Oxidation and Reduction

Calculation of the Eh value and the pH using the redox couple FeII/FeIII



I have a question regarding the calculation of the Eh value (redox potential).

Is it possible to replace the OPV (operational valuence) used in earlier versions of Phreeqe with a master redox couple - representing all redox pairs to calculate the Eh value?
We do not want to use the Mix command from PhreeqC, but to transport / mix all master species in a separate mass transport model and then call PhreeqC with a new solution. In our example, we would like to use ΣFeII and ΣFeIII as the transport unit instead of ΣFe and OPV and use the command
Redox FeII / FeIII?

So basically I had two solutions which I mixed 1:1. On step was to manually calculate the mixed solutions elemnt concentration and then calculating the new pH value by using the redox couple FeII/FeIII. The other step was to just use the MIX command in Phreeq.
When I compare the two pH values they do not fit verry well.

Has somebody tried anything simmilar like this or can somebody give me a hint here?


It is sufficient to transport H, O, charge, and the totals of all other elements. In PhreeqcRM, there is also an option to transport H2O, H - 2H2O, and O - H2O, which can be numerically more favorable because H2O is large relative to non H2O H and O. Transporting these components accurately will allow Phreeqc to calculate pH and pe.

Now, it is not possible to define total H, O, or charge with a SOLUTION definition. It would be necessary to use SOLUTION_RAW or SOLUTION_MODIFY to define these quantities.

If you do not transport H, O, and charge, I think you will have trouble recreating pH and pe. The MIX calculation should be correct.


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