How can I get the value of pe in Solution ?

[Wang]

I'm new to PHREEQC. Now I have a stupid question and wish someone could help me ^_^

I want to know what the pe in Solution means? Is it calculated by other parameters or measured ?

If calculated, how it works ?

I'm looking forward your help! Thanks ~

[dlparkhurst]

Think of pe as the ratio of oxidized to reduced species for an element. Thus, if you specify pe 12 in a SOLUTION definition and the total amount of a redox active element, such as Fe, then you will end up with predominantly Fe(3) (ferric or the more oxidized form of dissolved iron) in solution with a very small amount of Fe(2) (ferrous or the more reduced form of dissolved iron).

In SOLUTION, you specify the pe, either by the pe itself, or using the ratio of a redox pair to calculate a pe. It is possible to specify the concentrations of Fe(3) (total of ferric species) and Fe(2) (total of ferrous species), and the calculated ratio of Fe+3/Fe+2 (aqueous species) will determine the pe.

For SOLUTION definitions, it is possible to have redox disequilibrium. You can define the concentrations of redox states of various elements, and there is no requirement that the definitions meet the criteria of redox equilibrium. Each redox pair that you define concentrations for will probably give a different pe.

For reaction calculations (reaction of a solution with minerals, exchangers, kinetics, etc) the pe will be determined by the reaction. If the reaction tends to increase oxidized species relative to reduced species then the pe will go up; if reduced species increase relative to oxidized species then the pe will go down. pe will also vary with pH.

In reaction calculations, redox equilibrium is calculated. One way to think of it is that, after the reaction, you can calculate a pe from every redox pair in solution and they will all give the same pe.

Here is an exercise for you to study closely and experiment with:

SOLUTION
-units mmol/kgw
pH   7 charge
Fe(3)    1
Fe(2)    1e-6
END
USE solution 1
REACTION 1
H2(g) 1
0 0.5 mmol
END