Re: Solution Keyword and Aqueous Species

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

Hi Jean-Philippe,

>I need to use as PHREEQC input some output from another geochemical code.
The problem is that I can only use concentrations that are given in terms
of a few individual aqueous species, i.e., Ca++ instead of Ca total or
SO4-- instead of S. Is there a simple way to use that kind of data as an
input apart from picking some larger number for total Ca... and iterate
until Ca++..... concentrations in PHREEQC output matches my input?

You can use a trick and fix the activities of Ca++ and SO4--. Below is a
file the makes the activity of Ca+2 ion 1e-4 and the activity of SO4-2
1e-5, for example. Sounds like you need the molalities to be fixed, so this
approach would differ by the activity coefficients in the solution. If you
need to get closer, you would have to figure out the ionic strength and
change the target for the activities to be the target molality times the
estimated activity coefficient.

David

SOLUTION 1
      Ca   1  Fix_Ca   -4  # -4 is log of target activity for calcium
      S    1  Fix_SO4  -5
PHASES
      Fix_Ca
      Ca+2 = Ca+2
      logk  0

      Fix_SO4
      SO4-2 = SO4-2
      logk  0

END

David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225