I think what you are seeing is the oxidation of N2 by O2 to produce nitric acid. Thermodynamics says that N2 and O2 do not coexist, even though they are quite happy together in our atmosphere. To do your calculation, you want to have redox disequilibrium, which is not as easy as we would like. You must define the redox states of nitrogen as separate elements. The following example splits N(0), the redox state of N2, and redefines it as a separate element. All of the species and phases of N(0) are redefined. When this input file is run, there is no redox connection between Ngas and NO3 for example. Any "N" in solution will obtain equilibrium among NO3, NO2, N2, and NH4, but "Ngas" will react independently, in this case, between aqueous Ngas and gaseous Ngas only. To allow denitrification, it would be necessary to write a KINETIC reaction that consumed N and produced Ngas. Hope this helps. David SOLUTION_MASTER_SPECIES Ngas Ngas2 0 Ngas2 14 SOLUTION_SPECIES Ngas2 = Ngas2 log_k 0 delta_h 0 kJ PHASES Ngas(g) Ngas2 = Ngas2 log_k -3.26 delta_h -1.358 SOLUTION 1 Water & Air temp 25 pH 7.0 EQUILIBRIUM_PHASES Ngas(g) -0.1 # N2(g) -0.1 O2(g) -0.68 CO2(g) -3.5 END David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Yves Dudal <Yves.Dudal@xxxxx To: dlpark@xxxxxxxx on.inra.fr> cc: In-Reply-To: <184.108.40.206.0.20020412110049.00a05010@xxxxxxxxxxxxxxx> Subject: 04/12/02 03:10 AM 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) 220.127.116.11.28 fax: +33 (0) 18.104.22.168.12 E-mail : Yves.Dudal@xxxxxxxxxxxxxxx Site internet: www.avignon.inra.fr ----------------------------------------------------------------------------------------
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