The main purpose of this paper is to develop a model for the ion transfer of weak adds across liquid vertical bar liquid interfaces including ion pairing and non-ideal electrolyte solutions to calculate the half-wave potential. The main equation derived in this model allows the simulation of different chemical systems, comprising multiple acid base equilibria, hydrophilic and hydrophobic neutral weak acids, and different anions, including ion-pair formation in the organic phase. Hence, several representative chemical systems are analysed in detail in order to demonstrate the usefulness of the model. In addition, simple criteria to evaluate the presence of ion-pairs in the organic phase as well as the calculation of the apparent ion-pair formation constants are included. (C) 2016 Elsevier B.V. All rights reserved.