Selectivity of lithium electrodes: correlation with ion-lonophore complex stability constants and with interfacial exchange current densities

Lithium-selective electrodes with solvent polymeric membranes based on two different dicyclohexylamide neutral ionophores are studied systematically. The selectivity of lithium response is studied by means of the ordinary potentiometric experiments. Stability constants of lithium, sodium, and potassium ions with the neutral ionophores are measured by means of the segmented sandwich membrane method. Charge transfer through the membrane bulk and across the membrane/solution interface is studied by means of electrochemical impedance spectroscopy. Well-resolved Faradaic impedance semicircles are obtained, allowing calculation of exchange current densities for lithium, sodium, and potassium. It is clearly demonstrated that the potentiometric selectivity coefficients correlate well with thermodynamic equilibrium parameters. The correlation with exchange current densities also exists, although it is low, and seems rather qualitative than quantitative. The results are treated in favor of equilibrium at the membrane/solution interface. It is also concluded (tentatively) that the kinetic description is equivalent to the equilibrium one, giving evidence that ion-ionophore complexes form directly at the interface.