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Peter S

Davies and Debye Huckel with same water activity model

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Is there a database that can calculate activity coefficients with the Debye-Huckel model and the Davies equation while maintaining the same model for the activity of water?

I am trying to determine how Fe concentrations, buffered by the reaction:

Pyrite + 4Fe2+ + 4H2O = 8H+ + Magnetite + Pyrrhotite

change as a function of activity coefficient model.

So far I have used thermo.dat for the Debye-Huckel model, and PHREEQC for the Davies equation ( I set ion size = 0 for all charged species). I fix pH at a constant value. The differences in calculated activity of Fe2+ are small, but it would be great if there were no difference at all.

Thank you

 

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Hi,

I don't believe you can do that. If your fluid is dilute, the water activity might be 1 in either case, though. And if you're setting pH, that leaves a unique Fe++ activity at a given temperature (just make sure the stability of the minerals is the same in each database). You could use Rxn to figure that activity, and if you specify ionic strength you can solve for the Fe++ concentration.

Hope this helps,

Brian Farrell

Aqueous Solutions LLC

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