> 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. It wouldn't hurt to use the Pitzer approach for a third opinion. I believe GWB allows calculation with the Pitzer equations. PHRQPITZ is also a Pitzer model, although it is rather old and hard to use. It can be found at the web address given below. I think the Pitzer calculations should be consistent with little or no CaCl+ pairing in an ion association model, but it is worth a check. > 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 looked in a version of the EQ3/6 database, which has the same numbers as used in llnl.dat, the PHREEQC database that was translated from the GWB database. The EQ3/6 database gives the following reference for the aqueous species CaCl+ and CaCl2. It also indicates that further extrapolation was done using the techniques of SUPCRT, which I assume is for temperature dependence. 94sve Sverjensky, D.A., 1994, Thermodynamic properties of aqueous inorganic metal complexes: Geochim. Cosmochim. Acta (in prep.), . 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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