The Impossibility of Measuring Individual Ion Activity Coefficients Using Ion Selective Electrodes

Abstract

The pressing aim of this work is to prevent an error from proliferating. In arecent paper, a new procedure for calculating the junction potentials has beenpresented, whose purported goal is to allow individual ion activities and activitycoefficients to be identified using the appropriate ISEs. As convincing and usefulas this procedure may seem, it is, however, fallacious. The present analysisconsiders the results obtained when applying the procedure to typical friendlycases liable to precise thermodynamic–mathematical treatments. It is shown thatthe “individual ion activities” found are a mere artifact of the initial settings and,therefore, change accordingly. The implications raised from the values of suchindividual ion activities and, in particular, those concerned with the modifiedHydration Theory, are, in turn, devoid of any real experimental validation.