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Re: diolution of Air composition

The experiment you describe fixes the partial pressures of the gases and
EQUILIBRIUM_PHASES is the right keyword data block to use.

The reason you are having numerical problems is related to the introduction
of N2 and O2. PHREEQC calculates redox equilibrium in reaction calculations
and thermodynamically, N2 and O2 should react to form NO3. Fixing partial
pressures of N2 and O2 generates huge amounts of NO3, and the program fails
to converge. If you assume the N2 is essentially inert, the following input
file will make the calculation without allowing N2 to react to NO3, by
defining a new "element" Ngas, which has only one aqueous species, Ngas2
and the gas component Ngas.


    Ngas          Ngas2            0     Ngas2           14
      Ngas2 = Ngas2
      LogK  0.0
        Ngas2 = Ngas2
        log_k           -3.260
        delta_h -1.358  kcal

      units mg/l
      U     1
             CO2(g)            -3.5
             O2(g)       -0.69897
             Ngas        -0.1079
      NaOH  1
      0 .0000001 .000001 .00001 .0001 .001 .01 .1 1 mol

David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled

                      "Yong Wang"                                                                               
                      <ywang2@xxxxxxxx         To:      "David L Parkhurst" <dlpark@xxxxxxxx>                   
                      on.edu>                  cc:                                                              
In-Reply-To: <FOEMLCJFIJMKLGPCIDHCCEAKCAAA.ywang2@xxxxxxxxxxxxxx>
                                               Subject: diolution of Air composition                            
                      08/02/02 12:08                                                                            

Mr. Parkhurst:

Hi, how are you !

I am very appreciated of your help on my question last time . I don't know
if you had time to think over the question on caron dioxide. I remembered
that you said you were out of town for two weeks. I sent a email to you
and i am not sure if you got it.

So I here sum it up in this way:

I am trying to simulate the disolution of the gas composition of air in
solution which is bubbled with a humidified air in lab experimental
condition. The concentration of carbonate is unknown.

I tried to set the initial data in

                         -pressure                     1.0
                   -temperature    25.0
                         CO2(g)          0.000316
                         O2(g)           0.2
                         N2(g)           0.78

as well as

        Fix_H+   -8.0   NaOH    10.0
             CO2(g)            -3.5
             O2(g)       -0.69897
             N2(g)       -0.1079

The pH are set as fixed from 2-10.

There are 1 mg/l U in the soultion and adsortion by iron oxide are allowed

The first one comes out with the carbonate distribution as shown in
file while the second one doesn't converge. I tried the KNOBS data but
didn't work. Butwhen I took away the O2(g) and N2(g) from the data set, it
gave a fairly good result.

Here is my questions:

1. which one data set should i use to represent the experimental condition

2. I used the Fixed_H+ for constant pH. In order to keep the acid-base and
calculation balance, I need to assumed some acid or base present as Manual
recommended. Don't  they influence the chemical reaction, such as to
increase or decrease the dissolution of CO2 ? how to avoid using the
acid or base ? Any advices if necessary?

Thanks a lot for your precious time and help.

Yours sincerely,

Young Wang
Department of Geology

(See attached file: question.doc)

Attachment: question.doc
Description: MS-Word document

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