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Dear Dr. Parkhurst,

I am getting familiar with the PHREEQC model and so far, I am extremely pleased with its capabilities.
I would like to report a problem I am experiencing regarding the use of gas equilibrium phases. Here is a simple example of the problem:
SOLUTION 1 Water & Air
        temp    25
        pH      7.0     
EQUILIBRIUM_PHASES
        N2(g)           -0.1
        O2(g)           -0.68
        CO2(g)  -3.5
END
When I run this little program trying to equilibrate air with water, the results are surprising: pH of 1.065, a pe of 19.546, leading to the formation of a lot of nitrate (0.1 M).  By decomposing the program, I find that O2(g) is responsible for this result, but everything I do to keep the redox adjusted to the O(0)/O(-2) couple does not solve it.  This result might be due to a mistake that I do, using the software and I would be very happy if you could help me solve it.  Is there a way not to adjust the charge balance to the redox equilibrium when using the PHASES?

Thank you for your time,

Yves Dudal, Ph.D.

Chargé de Recherche
Institut National de la Recherche Agronomique
Unité Climat Sol et Environnement - Bâtiment Sol
Equipe "Couplages entre Transferts et Cycles Biogéochimiques"
Domaine Saint-Paul - Site Agroparc
84914 Avignon cedex 9
France

tél: +33 (0) 4.32.72.22.28
fax: +33 (0) 4.32.72.22.12
E-mail : Yves.Dudal@xxxxxxxxxxxxxxx
Site internet: www.avignon.inra.fr
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