PhreeqcUsers Discussion Forum
Redox Equilibria => Oxidation and Reduction => Topic started by: BrunoA on June 22, 2018, 01:22:35 PM

Hi,
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?
Greets

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.

Hello Mr. Parkhurst,
first of all thank you for the answer.
So your answer would mean, if we understand correctly, that the calculation of a correct Eh (or pe) value when mixing two solutions is not possible by using the normal SOLUTION definition and using a masterredox pair (redox Fe2 / Fe3). We still use the old OPV definition and hoped to use 100% only Phreeq by mixing the solution externally from Phreeq and then passing just the mixed solution to Phreeq (and not using the mix command). Is it possible (preferably using the solution definition) to come up with a simple way to realize this, e.g. via the redox command (redox Fe2 / Fe3) representing the entire redox system.
Greetings
Bruno

I am not sure what you are , transporting, but my feeling is that you cannot do it with SOLUTION alone. Mixing two solutions would normally result in redox reactions between Fe and other redox elements, so the ratio of Fe(2)/Fe(3) would not remain constant.
I think the following would work. If you transport all redox states of all elements (including O(0) and H(0)) so that you define only redox states and no total concentrations of any redox element in SOLUTION, then the choice of pe will not affect the SOLUTION calculation. Following your SOLUTION definition, running an empty reaction calculation (REACTION with no reactants for example), will generate redox equilibrium and the correct distribution of redox states.
SOLUTION 1
...
END
USE solution 1
REACTION 1
SAVE solution 1
END