Re: Use of PHREEQC

["David L Parkhurst" <dlpark@xxxxxxxx>]

> 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