> 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
Please note that some U.S. Geological Survey (USGS) information accessed through this page may be preliminary in nature and presented prior to final review and approval by the Director of the USGS. This information is provided with the understanding that it is not guaranteed to be correct or complete and conclusions drawn from such information are the sole responsibility of the user.
Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.
The URL of this page is:
Last modified: $Date: 2005-09-13 21:04:21 -0600 (Tue, 13 Sep 2005) $
Visitor number 1997 since Jan 22, 1998.