Facilitated Ion Transfer Reactions Across Liquid|Liquid Interfaces Assisted by a Neutral Weak Acid: A Theoretical Approach

Abstract

We present a model that describes facilitated transfer of two cations across liquid|liquid interfaces in presence of a neutral weak acid and its conjugate base. The non-trivial calculation of the initial equilibrium state is included, since it determines which processes are observed during voltammetry. In certain experimental conditions, the ion transfer reactions involve an interfacial cation exchange. The mechanism that takes place can be identified by measuring the peak-to-peak potential difference and the dependence of transfer potential with pH. We also derive a set of equations that allow to locate each experimental condition within a pH-potential diagram that describes the system at a glance, according to the hydrophobicity of the ligand. Our results can explain potentiometric slopes obtained with ion-selective electrodes and charge transfer processes with apparent super-Nernstian responses.