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On a recent purported determination of individual ion activity coefficients



In a recent article “On the activity of ions and the junction potential: Revised values for all data,” G. Wilczek-Vera, E. Rodil, and J. H. Vera proposed the amended version of a controversial method to derive ion activity coefficients from electromotive force data. Here we show that their method is incorrect. Using precise data available for HCl, we show that their approach does not identify an unequivocal single set of ion activity coefficients, but as many sets as those that are introduced as the input in the evaluation of the liquid junction potentials. That is, an infinite number of self-consistent sets. Indeed, the “ion activity coefficients” found echo the starting assumptions, with no relationship to the actual, but unknown real values. © 2005 American Institute of Chemical Engineers AIChE J, 2006