Voltammetry of Ion Transfer across a Polarized Room-Temperature Ionic Liquid Membrane Facilitated by Valinomycin: Theoretical Aspects and Application

Cyclic voltammetry is used to investigate the transfer of alkali-metal cations, protons, and ammonium ions facilitated by the complex formation with valinomycin at the interface between an aqueous electrolyte solution and a room-temperature ionic liquid (RTIL) membrane. The membrane is made of a thin (similar to 112 mu m) microporous filter impregnated with an RTIL that is composed of tridodecylmethylammonium cations and tetrakis[3,5-bis-(trifluoromethyl)phenyl]borate anions. An extension of the existing theory of voltammetry of ion transfer across polarized liquid membranes makes it possible to evaluate the standard ion-transfer potentials for the hydrophilic cations studied, as well as the stability constants (K-i) of their 1:1 complexes with valinomycin, as log K-i = 9.0 (H+), 11.1 (Li+), 12.8 (Na+), 17.2 (K+), 15.7 (Rb+), 15.1 (Cs+), and 14.7 (NH4+). These data point to the remarkably enhanced stability of the valinomycin complexes within RTIL, and to the enhanced selectivity of valinomycin for K+ over all other univalent ions studied, compared to the conventional K+ ion-selective liquid-membrane electrodes. Selective complex formation allows one to resolve voltammetric responses of K+ and Na+ in the presence of an excess of Mg2+ or Ca2+, which is demonstrated by determination of K+ and Na+ in the table and tap water samples.