A complete discussion of the rationale supporting the experimental determination of individual ionic activities

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

“…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.…”

“…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.…”

Activities of aqueous Na+ and Cl- ions from homoionic measurements with n

“…The term in the first brackets of the right-hand side is a constant and, as discussed elsewhere (Wilczek-Vera et al, 2006a) the second term is negligible in comparison with the last term. Thus, differentiating with respect to ln(m k ±,k ) gives…”

“…Following exactly the same algebra presented to obtain Eq. (15) in a previous publication (Wilczek-Vera et al, 2006a), mutatis mutandis, for the response of the cationic electrode we write for a 1:1 electrolyte:…”

“…As it is discussed below, the crux of the problem for measuring this property is the possible bias that could be introduced by a poor estimate of the junction potential between the sample solution and the solution used in the reference electrode. We will not repeat the arguments here, but the interested reader can find in recent literature (Malatesta, 2006a;Wilczek-Vera et al, 2006a) all details and pertinent references of previous discussions on the subject. Although independent measurements in other laboratories (Lee et al, 2002;Schneider et al, 2003Schneider et al, , 2004 have confirmed the repeatability of results obtained by the method proposed by Khoshkbarchi and Vera (1996) to measure individual ionic activities, it may be argued that the same biased assumptions were used in all cases.…”