Dear David, I'm a postdoc with Patricia Dove at Virginia Tech. I'm studying growth of calcite at different temperatures in the AFM. I worked out my recipes for supersaturated solutions with Geochemist's Workbench, but plugged in solubility constants from Plummer and Busenberg. Niel Plummer suggested running the recipes in PHREEQ to compare results, and I've found that the main difference is that GWB predicts equilibrium concentrations of CaCl+ that are on the order of the concentration of Ca++, while PHREEQ does not include CaCl+ as a species at all. This makes a large difference in supersaturation log(aCa*aCO3/Ksp). Niel thinks the CaCl+ association is very weak. He suggested looking at some other versions of PHREEQ that include Pitzer parameters, but I am unable to find these on the web. I don't think I should need Pitzer equations, though, because my solutions have ionic strength of ~0.1. Do you know where I might get good data for the CaCl+ complex? I have no idea where the data come from in GWB, but it seems to think CaCl+ has even higher activities than Ca++. I'd very much appreciate any ideas or advice you can offer. Regards, --Laura -------------------------------------- Dr. Laura E. Wasylenki Postdoctoral Research Associate Department of Geological Sciences 4044 Derring Hall Virginia Tech Blacksburg, VA 24061 (540)231-2403 (540)231-3386 fax
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