Calculate redox potential from equilibrium solid

[peterwadeuk]

Greetings people,

I have a straightforward set of chemical analyses that derive from an experiment in which columns were planted with sorghum and watered. The leachates were analysed.

I wish to obtain a rough estimate for the redox potential in the columns just before the leachates emerged.

The only element I have available to assist is the concentration of iron in the leachates.

Is there a way I can calculate what the redox potential was by speciating Fe(II) (soluble) and Fe(III) (would have precipitated as Fe(OH)3)?

I tried modifying pO2 to no avail.

I have achieved a result by manually modifying the ratios of Fe+2/Fe+3 until I achieve a zero saturation index for Fe(OH)3(a). This is laborious and I have sixteen solutions to analyse.

Can anyone please advise how I may obtain a pe for these solutions?

All the best,
Peter

[dlparkhurst]

The following will produce a pe in the "Redox couples" section of the output.

SOLUTION
pH      7.0
Fe(2)   1
Fe(3)   1   Fe(OH)3(a)
END