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Geochemist's Workbench Support Forum

Replicating Eh-pH from Literature

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As an originator of the second of the diagrams (which was done before GWB, actually), I can point to some important strategies and concepts for generating similar diagrams.  First, know what solids and aqueous species you want to include in the model, and then make sure they are in the thermodynamic data base called by Act2.  It is quite possible that some of the solids and aqueous species are not contained in any or all of the default databases that come with GWB.  In our research lab we have been building our own as we need it for different metals, drawing aqueous complexes and solid phases from either others of the thermo databases, or we add them from the literature. Notably, there are widely-varying solubility constants and aqueous complex formation constants in the literature.  You could get very different-looking diagrams depending on your choices.  There are certainly differences behind the last two diagrams you show.  We have found it useful to include multiples of the same solid phase with different Ksp's, but name them differently so that you can test the different choices in a diagram by disallowing unwanted solid phases.  When ligands are weak acids or weak bases, like the carbonate system or phosphoric acid system, you will have to allow the form to vary with pH when you set up your Act2 script.  One additional reminder, and that is to make sure that the activities chosen for Pb and the carbonate (or other species) are as specified in the original diagrams you're trying to reproduce, or set to values for new diagrams of your choice.  For your reference, most of the thermodyamic data for the first diagram likely originates in the book chapter:  Schock, M. R.; Wagner, I.; Oliphant, R. The Corrosion and Solubility of Lead in Drinking Water; In Internal Corrosion of Water Distribution Systems; Second ed.; AWWA Research Foundation/DVGW Forschungsstelle: Denver, CO, 1996.

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Thank you for the reference. I was able to find a more complete representation with the minteq database. I'm interested in your suggestion of presenting multiples of the same solid phase. Could you share a script where you did so? Even if not lead, I can adapt it. I'm just real new to this software and found it more effective to start from functional scripts than from scratch.

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