Theory of the Transient Potential Response of Ion-Selective Electrodes Based on the Polarizable Oil/ Water Interface

Abstract

The theory of the transient potential response of potentiometric ion-selective electrodes (J. Electroanal. Chem. Interfacial Electrochem., 378, 215 (1994)) is further elucidated. General solutions of the transient potential vs. time curves for a stepwise change in the activity (concentration) of a potential-determining ion in an external solution were numerically obtained for different cases concerning their ion-transfer kinetics. Also, the response time, defined as the time which is taken by the potential of an ion-selective electrode to change from its initial value to within a given limit of the final value, was numerically obtained, and is discussed in terms of the ion concentrations, the kinetic parameters of the ion-transfer reaction, and the electrical double layer capacity of the ion-selective electrode interface. Furthermore, the theory is extended so as to include the transient potential response of neutral-carrier-based ion-selective electrodes. The theory predicts the effect of the concentration of a neutral-carrier, which depends on the reaction mechanism of the carrierassisted ion transfer across the interface. In a previous papers, a theory of the transient potential response of ion-selective electrodes for a stepwise change in the activity (or concentration) of potential-determining ions in an external solution was presented. This theory is based on the phase-boundary potential model of the mechanism of the potential response.2-5 Recent electrochemical studies concerning ion-transfer processes across a liquid/liquid interface have enabled a more exact model of the phase-boundary potential mechanism to be presentedl,6-8, and, hence, to study the problem in more detail. Thus, a solution of the theoretical equation was derived for a specified case when the transfer coefficient of the ion transfer kinetics is equal to 0.5 and the transient potential vs. time (E(t)-t) curves were discussed in terms of the kinetic parameter (rate constant) of the ion-transfer reaction at the interface and the differential capacity of the interface as well as the potential-determining ion concentrations. In this paper, a discussion of the general solutions of the theoretical equation is given as well as that for cases different in their ion-transfer kinetics. Furthermore, the response time, which is defined in this study as the time taken by the potential to attain, for example, 95% of the entire change from its initial value to the final equilibrium value discussed (as well as its numerical solution) as a function of such parameters as the ratio of initial to final concentration of the potential-determining ion, the kinetic parameters of the ion-transfer reaction, and the electric double-layer capacity of the ion-selective electrode interface. Also, the theory is extended so as to include the transient potential response of neutral-carrier-based ion-selective electrodes. TheoreticalThe experimental setup of a potentiometric MZ ionselective electrode based on the polarizable interface between two immiscible electrolyte...