> 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
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