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