Negative Deviations from the Debye-Hückel Limiting Law for High-Charge Polyvalent Electrolytes: Are They Real?

In the past few decades, electromotive force (emf, E) measurements using improved electrochemical cells have afforded the derivation of mean ionic activity coefficients (γ_±'s) of very dilute solutions of binary electrolytes, within the 10^-6-10^-3 molar range (Malatesta et al., J. Solution Chem. 1994, 23, 11 ). Such measurements are especially important for highly charged electrolytes whose behavior at very low concentration is not yet fully understood, and whose γ_± values, derived from E data, have been claimed by Malatesta and co-workers to exhibit "negative deviations" from the Debye-Hückel (DH) limiting law. Here I examine electrolytes studied by the Malatesta group, which belong to the 3-1, 1-3, 2-3, and 3-3 valence families, and analyze their E and γ_± data using the Smaller-ion Shell (SiS) theoretical treatment ("DH-SiS") of strong electrolyte solutions (Fraenkel, J. Chem. Theory Comput. 2015, 11, 178 and 193 ). The DH-SiS physical model incorporates all three ion-size parameters of a binary ionic solution as the "true" ion-ion collision distances, and leads to an improved DH-like electrostatic theory of ionic activity. Correcting Malatesta's data by better extrapolation of E to zero concentration results in a more accurate "standard potential", E°; this affords improved γ_± values that (1) fit well with the DH-SiS equations and (2) agree with the DH limiting law.