Facilitated ion transfer reactions across oil/water interfaces - Part II. Use of the convoluted current for the calculation of the association constants and for an amperometric determination of the stoichiometry of MLjz+ complexes

Cyclic voltammetric experiments for reversible ion transfers across the aqueous/organic interface facilitated by a neutral macrocyclic ligand are presented for complexation reactions of 1:1 to 1:4 ion-to-ligand stoichiometries. The convoluted current is taken into account to derive general theoretical equations relating the half-wave potential to the initial concentrations of both the metal Mz+ and the ligand L. Analytical relationships are obtained for both limiting cases of ligand and, respectively, metal excess and for any type of reaction mechanisms. Likewise, considerations on the convoluted current provide a condition on the transition point between the diffusion regimes where either the metal or the ligand limits the transfer, which constitutes an amperometric determination of the complex stoichiometry. It is shown that the half-wave potential depends on the various over-all association constants, on the partition coefficient of the ligand and on the initial concentrations of both Mz+ and L. This dependence is the same for the TIC, TOC and TID mechanisms, but differs in the case of the ACT mechanism. The theoretical predictions are corroborated by the results deduced from various calculated voltammograms and are verified experimentally for the transfer of Pb2+ assisted by the thioether ligand 1,4,7,10-tetrathiacyclododecane at the water/1,2-DCE interface. Thanks to the simulation, it is shown that the experimental current waves are due to 1:1 and 1:2 complex formation and that the first association constant in the organic phase is log K-1(0) approximate-to 5. (C) 1998 Elsevier Science S.A. All rights reserve