The main purpose of this paper is to develop the equations for the half-wave potential of facilitated proton transfer or protonated species transfer across liquid vertical bar liquid interfaces, including ion pairing. The main equation developed in this research allows simulating different chemical systems (hydrophilic and hydrophobic neutral base, multiple protonated species, ion-pair formation in the organic phase). Five representative chemical systems are studied in detail in order to demonstrate the usefulness of the equations developed. In addition, the effect of the hydrophilicity of neutral weak bases on the domains of predominance of each species is analysed. Finally, the half-wave potential as a function of the pH obtained using the developed equations, are compared to simulated results and to pH-potential diagram. (C) 2014 Elsevier B.V. All rights reserved.