David: Thanks a lot for your reply. I think the reason I see the difference in two approaches I was taking lies in your explanation that in the Reaction calculation Phreeqc does not differentiate between Fe(II) and Fe(0). But in the electrochemical process I am actually introducing ferrous iron into the solution directly instead of as a salt such as FeSO4. The reactions are: At the anode (oxidation reaction): Fe * Fe+2 (aq) + 2e- (1) At the cathode (reduction reaction): 2H2O + 2e- * H2 * + 2OH- (aq) (2) <<...OLE_Obj...>> How can I model this in Phreeqc? Regards, Biswajit > -----Original Message----- > From: David L Parkhurst [SMTP:dlpark@xxxxxxxx] > Sent: Tuesday, January 14, 2003 5:47 PM > To: Mukhopadhyay, Biswajit > Subject: Re: Using Phreeqc for modeling redox reactions > > > > My question is, for the experiment described above, is using the key > word > EQUILIBRIUM_PHASES with O2(g) Partial Pressure (10^-0.68) the correct > approach. I enclose the input file and the database [modified phreeqc.dat > as phreeqcc.dat to include all Cr(VI) and Cr(III) species] for your > viewing. > > If you include the O2(g), you are assuming that sufficient oxygen enters > the beaker to maintain atmospheric partial pressure of oxygen at all > times. > My guess is that the gas-water transfer of oxygen is relatively slow and > you might be better off not using the O2(g). If the transfer were really > fast, you would expect to see red iron hydroxide forming as well. > > Note that in the reaction calculation, PHREEQC does not consider any > charge > that you might enter ("Fe+2"). It ignores the charge and is actually > adding > elemental Fe to the solution, which is ok, that is the reaction you want. > The elemental iron must oxidize to Fe+2 (or Fe+3) and in the process > something must be reduced. If you include the oxygen, then oxygen will be > reduced, whereas leaving oxygen out will force the reduction the most > thermodynamically preferred electron acceptor available, Cr(IV) I should > think. > > David > > 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 > >
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