Use of PHREEQC

["Ainsworth, Calvin C" <calvin.ainsworth@xxxxxxx>]

Title: Use of PHREEQC

Dr. Parkhurst:

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

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 ClO_4^-, the FeClO_4²⁺ 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?

<<Phrqc1_104.pqo>>

I very much appreciate your help in this matter.

Sincerely,
Cal Ainsworth, Ph.D.
Environmental dynamics and Simulations, MSIN K3-61
Pacific Northwest National Laboratory
P.O. Box 999
Richland, WA 99352
Phone: (509) 375-2670
Fax: (509) 375-6954
E-mail: calvin.ainsworth@xxxxxxx

Attachment: Phrqc1_104.pqo
Description: Phrqc1_104.pqo