CURRENT-POTENTIAL CURVES FOR FACILITATED ION TRANSFER ACROSS OIL-WATER INTERFACES IN THE PRESENCE OF SUCCESSIVE COMPLEX-FORMATION

A theoretical analysis of the current-potential curves for facilitated ion transfer across an oil/water interface is presented for the case where two complex species having an ion-to-ligand ratio of 1:1 and 1:2 are allowed to exist in the oil phase. Cyclic voltammograms are calculated numerically by using a finite difference method assuming that the ion transfer is reversible and the complex formation equilibria are maintained throughout the oil phase. When the ligand concentration in the oil phase is much smaller than the ion concentration in the aqueous phase and the former limits the ion-transfer current, two well separated peaks may develop even under the assumption of equilibrium conditions for the complex formation. The shape of the voltammogram is determined primarily by the ratio of the two reduced stability constants: b1 = K1bc(L)O and b2 = K1K2(bc(L)O)2, where K1 and K2 are the stability constants for 1:1 and 1:2 complexes in the oil phase and bc(L)O the ligand concentration in the bulk of the oil phase. The degree of peak separation thus depends not only on K2/K1 but also on bc(L)O. A method for estimating the two stability constants from the voltammograms is proposed.