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. David SOLUTION_MASTER_SPECIES Ngas Ngas2 0 Ngas2 14 SOLUTION_SPECIES Ngas2 = Ngas2 LogK 0.0 PHASES Ngas Ngas2 = Ngas2 log_k -3.260 delta_h -1.358 kcal SOLUTION units mg/l U 1 EQUILIBRIUM_PHASES 1 CO2(g) -3.5 O2(g) -0.69897 Ngas -0.1079 REACTION 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 PM 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 then 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 GAS_PHASE 1 Air -fixed_pressure -pressure 1.0 -temperature 25.0 CO2(g) 0.000316 O2(g) 0.2 N2(g) 0.78 as well as EQUILIBRIUM_PHASES 1 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 too. The first one comes out with the carbonate distribution as shown in attached 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 assumed acid or base ? Any advices if necessary? Thanks a lot for your precious time and help. Yours sincerely, Young Wang Department of Geology SUNY-Binghamton (See attached file: question.doc)
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