> I recently downloaded PHREEQC and am trying to come to terms with it (this is a chore in and of itself for me). At present I am attempting to utilize PHREEQC to investigate the reduction of Fe3+ and its various complexes by dihydroanathraquinone-2,6 disulfonate (AH2DS), which is the reduced form of anthraquinone-2,6-disulfonate (AQDS). We are following the oxidation of AH2DS to AQDS using a stopped-flow apparatus with UV/Vis detection. This is being done over a range of pH and Fe3+ concentrations. > The half-cell reduction of AQDS to AH2DS follows as: AQDS + 2e- + 2H+ = AH2DS E0 = 0.253 V From this equation AQDS and AH2DS should have the same charge, but you have written the input file with A-2 and Aq (uncharged). >I was hoping to utilize PHREEQC first model the equilibrium solution composition of a solution initially containing Fe3+, NaClO4, and AH2DS - a very simple system. After multiple attempts of building an input file that identifies ClO4-, the FeClO42+ complex, AH2DS, and AQDS. I still can't get it to converge. The output appears not to recognize the AH2DS [in the attached output file this species is designated as A(2)]. Two questions: 1) is this a problem that PHREEQC can handle, and 2) if it should be able to do this problem, what am I missing? I think PHREEQC should be able to handle the problem. First, there should be 3 entries in the SOLUTION_MASTER_SPECIES for anthraquinone, but I'm not sure exactly which three given the confusion above. Maybe something like the following: SOLUTION_MASTER_SPECIES A A-2 A(0) A-2 A(-2) AH2-2 The redox states in parentheses are arbitrary, but is seems like the reduced form should have a smaller number. I think the basic problem is that you should not define Aq as a separate "element" for the reduced form. Then, I expected to see the equation you listed above somewhere in SOLUTION_SPECIES A-2 + 2e- + 2H+ = AH2-2 -log_K -8.55 # log_k from E0 = .253 If you can clear up the confusion about the charge on the species, it should be possible to make a consistent thermo data definition. 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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