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Abstract
The electrochemical properties of ion-exchanger-based solvent polymeric ion-selective electrodes (ISEs)—bulk and interfacial resistance, capacitance, and polarization under a galvanostatic current step—are studied, with a nitrate ISE based on tetradecylammonium nitrate (TDANO3) as a model system. The study is performed by chronopotentiometric and impedance measurements, and focuses on the dependence of the aforementioned properties on the concentration of NO3− anions in solution. The impacts from the bulk and the interfacial charge transfer to the overall membrane resistance are revealed. It is shown that the bulk resistance of the membranes decreases over an increase of NO3− concentration within the range of a Nernstian potentiometric response of the ISE. This fact, also reported earlier for K+- and Ca2+-selective ISEs, is not in line with current views of the mechanism of the ISE response, or of the role of ion exchange in particular. The origin of this effect is unclear. Estimates are made for the concentration of ionized species (NO3− and TDA+) and, respectively, for the TDANO3 association constant, as well as for the species diffusion coefficients in the membrane.
Highlights
Ion-selective electrodes (ISEs) with solvent-polymeric membranes are routinely used as sensors of ion activity in a broad range of applications, primarily in clinical chemistry, environmental monitoring, and industry
The classical theory of the ISE response focuses primarily on ion-exchange processes at the interface between the membrane and aqueous solution. It takes into consideration a co-extraction of the aqueous electrolyte from the sample or calibration solution, which governs the upper limit of the ISE response [1,2,3,4]
These ISEs have been thoroughly characterized with regard to their potentiometric response and selectivity, and widely used in practical applications [1,23,24]
Summary
Ion-selective electrodes (ISEs) with solvent-polymeric membranes are routinely used as sensors of ion activity in a broad range of applications, primarily in clinical chemistry, environmental monitoring, and industry. The classical theory of the ISE response focuses primarily on ion-exchange processes at the interface between the membrane and aqueous solution. It takes into consideration a co-extraction of the aqueous electrolyte from the sample or calibration solution, which governs the upper limit of the ISE response [1,2,3,4]. It was shown that the lower limit of the ISE response is due to trans-membrane fluxes of electrolyte co-extracted from the internal solution [5,6,7], or from the internal reference system in the case of solid-contact ISEs [8]. Among numerous approaches aimed at the improvement of the lower detection limit, galvanostatic polarization appears the most flexible [9,10,11,12,13].
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