Re: Log K and more help please!

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

> Thank you for explaining the logK concept to me, however, I am still a
little confused. In theory, I should convert the K into the zero ionic
strength K before I enter it into the input file to be consistent with
the Ks listed in the Phreeqc database.   In reality, all my experiments
are carried out in 0.1 M sodium nitrate medium.  Does PHREEQC take the K
at zero ionic strength condition, convert it to the value that suits the
ionic strength at which my experiments are conducted and hence does the
output file list the species distribution at 0.1 M, i.e. the ionic
strength I used?


> If I enter the K at 0.1 M into the input file, does Phreeqc assume this
is the K at zero ionic strength and hence go through the normal
conversion process, i.e. convert this K that is already at 0.1 M to a
'wrong' value at 0.1 M before it predicts the species distribution?

Essentially, yes, it will convert to the "wrong" value. However, PHREEQC
does not actually correct the K. The K is assumed to be the true
thermodynamic K (zero ionic strength, or the K calculated from free
energies of formation). All of the corrections are made through the
activity coefficients of the aqueous species. At ionic strength of 0.1, the
K will be the K that is calculated for the given temperature, and the
activities for the IAP are calculated using activity coefficients for 0.1
ionic strength. If you want to use an approach that corrects the K, then
the activity coefficients are folded into an apparent constant, but you
would use molalities in the mass-action expression.

David


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