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 > >
Please note that some U.S. Geological Survey (USGS) information accessed through this page may be preliminary in nature and presented prior to final review and approval by the Director of the USGS. This information is provided with the understanding that it is not guaranteed to be correct or complete and conclusions drawn from such information are the sole responsibility of the user.
Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.
The URL of this page is:
Last modified: $Date: 2005-09-13 21:04:21 -0600 (Tue, 13 Sep 2005) $
Visitor number 2039 since Jan 22, 1998.