On the Facilitation Effect of Neutral Macrocyclic Ligands on the Ion Transfer across the Interface between Aqueous and Organic Solutions. I. Theoretical Equation of Ion-Transfer-Polarographic Current-Potential Curves and Its Experimental Verification.

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A theoretical equation of the reversible polarographic current-potential curves for the ion transfer across the aqueous/organic interface facilitated by the neutral macrocyclic ligand present in the o-phase is derived without any limitation on the magnitude of distribution constant of the ligand. In two limiting cases, which have been employed in common experimental practice, i.e., (A) the bulk concentration of cation, c*M, in the aqueous phase >> that of ligand, c*L, in the organic phase and (B) the reverse condition, c*M<<c*L, the equation of current-potential curves becomes the same in form as that of reversible D. C. polarographic waves. It is shown that the limiting current is controlled by diffusion of ligand in the organic phase for (A) and of cation in the aqueous phase for (B) and, on the other hand, the half-wave potential depends on c*M for (A) and on c*L for (B). Furthermore, an analysis method to determine the complex formation constants in the organic phase (and in the aqueous phase for favorable cases) from the concentration dependence of the half-wave potential is presented. The theoretical predictions are verified experimentally using dibenzo-18-crown-6 and 18-crown-6 as macrocyclic ligands and sodium, cesium, barium, and oxonium ions as transferred cations.
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