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quick question re: PHREEQ



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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