Partial equilibrium

["David L Parkhurst" <dlpark@xxxxxxxx>]

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: <5.0.0.25.0.20020412110049.00a05010@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) 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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