Disolution of an amount of Fe

[luicho]

Hello,

I'm trying to understand how phreeqc do this calculation:

      Database   sit.dat
      Solution 1
      REACTION 1
           Fe(s)     1
           1 micromoles in 1 steps
      END   

I'm trying to find what semi-reactions use phreeqc to calculate the final pE?

Does Phreeqc use the Nernst Equation to calculate the final pE?
pE= log_k/ne - 1/ne * log({red}/{ox}) 

[dlparkhurst]

Yes, but not quite that way.

PHREEQC solves a set of mole balance and mass-action equations. For the system you are considering, PHREEQC would have the following equations:

Mole balance on H, O, Fe, and charge; and a set of mass action equations for OH-, Fe+3, FeOH+2, Fe(OH)2+, and so on.

The mass-action equations are effectively substituted into the mole balance equations, so you end up with 4 equations, and the 4 unknowns are pH, pe, a(Fe+2), and mass of water.

Now the mass-action equation for Fe+3 is your Nernst equation rearranged such that log K = .... . So the calculated pe is such that the Nernst equation will be satisfied. If you have multiple redox elements, the pe calculated from any of the appropriate Nernst equations will be the same.