On the measurement of individual ion activities

On the measurement of the real values of individual ionic activities: A chemical engineering perspective

“…In both cases, even with the correction term set to zero, one gets a reasonable measure of the real activity coefficients of the species. As discussed elsewhere, the reason for this negligible effect on ionic activities of the value assumed for the junction potential is that deviations from its true value cancel out, to a large extent, due to the calibration of the ISE response (Rodil and Vera, 2003;Wilczek-Vera and Vera, 2005;WilczekVera et al, 2006aWilczekVera et al, , 2006b. Understandably, without computers it was not easy in the past to assert the insensitivity of the ionic activity values to the value of the junction potential.…”

“…The three unknowns are the activity coefficient of the cation, the activity coefficient of the anion, and the numerical value of E J. As discussed previously (Wilczek-Vera and Vera, 2005;Wilczek-Vera et al, 2006a) and emphasized below, one interesting case is that of homoionic measurements of KCl solutions, for which Eq (3) is too sensitive to small changes in the values of the activity coefficients of the ions while the values of the ionic activities are quite insensitive to the value of the junction potential E J.…”

“…Similarly, Harned (1926) in the introduction of one of his manuscripts, wrote: ''Since the difficulty in estimating liquid junction potentials is the only cause of our not being able to estimate exactly the individual activity coefficient of the ions, an analysis of the problem of liquid junction potentials will be made.'' As we have commented in a previous work (Wilczek-Vera and Vera, 2005) after an interesting and passionate discussion in the literature, this paradigm was incorporated into the regular teaching of physical chemistry once Taylor's algebra was reproduced in the authoritative textbook by Harned and Owen (1958). By the same time, Pitzer and Brewer (1961), in their revised second edition of the textbook Thermodynamics changed the original text of the Lewis and Randall (1923) version and introduced a dubious concept regarding the measurement of activities.…”

“…Surprisingly, even after this shattering vision, they concluded: ''Nevertheless, singleion properties are potentially measurable.'' In a previous publication (Wilczek-Vera and Vera, 2005) we listed some of the most representative texts that, one way or another, kept repeating the idea that ionic activities could not be measured. The trend of repeating this paradigm, without mentioning that there are claims of having measured ionic activities, can be found also in recent texts (Laidler et al, 2003).…”

“…Our purpose here is to present new facts showing that the actual values of the individual ionic activities are accessible with proper experimental measurements and data treatment. In particular, we consider the case of homoionic measurements of KCl solutions for which previous experience (Wilczek-Vera and Vera, 2005;Wilczek-Vera et al, 2006a) showed that the general method for reducing the experimental data failed and, thus, special considerations are needed.…”