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Log K

> If I can only find one set of constants for a metal-ligand reaction, and
this set of constants is determined at 30 degrees celsius at an ionic
strength of 0.5 M.  When I construct an input file for PHREEQC to do
speciation modeling, if I define the reactions and put in this set of
constants, would PHREEQC modify the constants to adapt to zero ionic
strength and 25 degrees celsius conditions?  Or do I need to convert the
constants to suit the zero ionic strength, 25 degrees environment first
before I construct the input file?

Temperature dependence of log K is defined either by (1) the delta_H and
the Van't Hoff equation, or (2) by an analytical expression. It does not
sound like you have the information except at 30 degrees, so you probably
can't fit the coefficients for an analytical expression. The only
alternative would be to use or guess the enthalpy of reaction. The log_k
value for phreeqc is the log K at 25C, so you should correct the K from 30
to 25 degrees to be consistent. The K is also the zero ionic strength K, so
if you have the information, it should logically be corrected back to zero
ionic strength. In actuality, there are probably more uncertainties in the
K than you would find in these corrections. I'd probably just use the 30C
log K. You may want to do some sensitivity runs to determine the effects of
reasonable uncertainties.


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

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