Electrochemical Properties of Nitrate-Selective Electrodes: The Dependence of Resistance on the Solution Concentration

摘要

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.