Non-equilibrium responses for polyion detection

Development of various ionophores in the last four decades has allowed detection of more than 60 analyte ions with liquid membrane ISEs (3). Recent progress in the theory of liquid 

These theoretical advances have led to the development of potentiometric methods for quantifying fundamental membrane processes. The stoichiometry of the ionophore-ion complexation in the membrane phase can be determined by studying the effects of ionic sites on potentiometric selectivity. Such a study also reveals whether an ionophore serves as a neutral or a charged ionophore (64). Formation constants of the complexes with the corresponding stoichiometry can be determined from the unbiased selectivity coefficients or more directly by the sandwich method. Quantitative information about the complexation processes in the membranes, which eventually limits practical performances of the electrodes, will be useful for future design of selective ionophores. 

Non-equilibrium processes at the sample/membrane interface and across the bulk membrane bias the selectivity and detection limits of the electrodes. Elimination of these nonequilibrium effects by operating the electrodes under complete equilibrium conditions will be of both practical and fundamental significance. While non-equilibrium responses are useful for potentiometric polyion-selective electrodes, it is not obvious whether potentiometry based on mixed ion-transfer potentials is a better transduction mechanism than amperome-try/voltammetry based on selective polyion transfer (65, 66). Ion-transfer electrochemistry at polarized liquid/liquid interfaces is introduced in Chapter 17 of this handbook. 

Fawcett, Liquids, Solutions, and Interfaces From Classical Macroscopic Descriptions to Modern Microscopic Details, Oxford University Press New York, 2004. 

Umezawa, P. BUhlmann, K. Umezawa, K. Tohda, S. Amemiya, Pure Appl. Chem. 72, 1851 (2000). 

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