Electrochemistry of Ionophore-Based Ion-Selective Electrodes: Response Limits, Slope and Potential Stability
Authors: Mikhelson Konstantin N. , Peshkova Maria A. , Levin Mikhail B. , Koltashova Evgeniya S. , Ivanova Nataliya M. 
Published: February 2020

The contribution is devoted to the electrochemical issues related to the response span, slope, and potential stability of ionophore-based ion-selective electrodes (ISEs). The discussion is primarily focused on those research topics in this area which have been studied by the late R.P. Buck.
Theoretical and experimental efforts aimed at the improvement of the lower detection limit of the ISE response constitute the current mainstream in the ISE theory and practice. This is mostly due to high environmental relevance of the trace level analysis. The galvanostatic polarization of ISEs, as the most flexible approach to the low detection problem, was proposed first by R.P. Buck et al [1]. Since then, the method was largely improved and modified in different ways. We recently proposed tuned galvanostatic polarization [2, 3] which proved to be suitable also for ISEs with crystalline membranes [4]. The idea of the method will be discussed and illustrated with several applications.
It will be shown that the upper detection limit of ISEs (which is mostly of industrial relevance) can also be improved in a very similar way. Thus the so-called anion interference (or cation interference in the case of anion-sensitive ISEs): the effect studied originally by R.P. Buck in 1970s [5], can be overpassed successfully. Furthermore, simple theoretical modeling of the trans-membrane fluxes effect in the ISE potentiometric response allows for a unified description of both lower and upper limits of the ISEs response, together with the response slope. Theoretical conclusions on the origin of slightly sub-Nernstian slopes typically observed for real-world ISEs, and on the relation between the slope and the response span, will be supported with the direct experimental evidence.
When studying the so-called solid contact ISEs (those without internal solution), R.P. Buck suggested that a reversible and fast RedOx reaction at the interface between ionically-conducting membrane and electronically-conducting substrate is a prerequisite for the electrode potential stability over time [6]. This idea is widely recognized among the ISE researchers community. However, our recent studies showed that this RedOx reaction is the necessary, but not the sufficient condition, and low diffusional polarizability at the interface is also needed for the solid contact ISEs stability [7, 8].
Besides the experimental facts which can be treated positively in the frames of the existing concepts of the ISE response mechanism, some peculiar observations will be presented. These evidences obtained by impedance and chronopotentiometric studies appear challenging and require further thorough studies.

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