Re: quick question re: PHREEQ

["David L Parkhurst" <dlpark@xxxxxxxx>]

> 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