Abstract
The dependence of the bulk resistance of membranes of ionophore-based ion-selective electrodes (ISEs) on the composition of mixed electrolyte solutions, within the range of the Nernstian potentiometric response, is studied by chronopotentiometric and impedance measurements. In parallel to the resistance, water uptake by the membranes is also studied gravimetrically. The similarity of the respective curves is registered and explained in terms of heterogeneity of the membranes due to the presence of dispersed aqueous phase (water droplets). It is concluded that the electrochemical equilibrium is established between aqueous solution and the continuous organic phase, while the resistance refers to the membrane as whole, and water droplets hamper the charge transfer across the membranes. In this way, it is explained why the membrane bulk resistance is not constant within the range of the Nernstian potentiometric response of ISEs.
Highlights
Ion-selective electrodes (ISEs) with solvent-polymeric membranes and containing ionophores and ion exchangers are routinely used as potentiometric sensors in a large variety of applications [1,2,3]
This study confirms the data on the non-constancy of the bulk resistance of ISE
It is shown that the bulk resistance of ISE membranes, as well as water uptake, are determined by the ionic strength of the aqueous solution, rather than by the concentration of the respective primary ion
Summary
Ion-selective electrodes (ISEs) with solvent-polymeric membranes (most commonly made of plasticized polyvinylchloride, PVC) and containing ionophores and ion exchangers (ionic additives) are routinely used as potentiometric (zero-current) sensors in a large variety of applications [1,2,3]. Analysis with ISEs in non-zero current modes of measurements, e.g., voltammetry [4,5,6,7] and coulometry [8,9,10,11,12,13], offers new advantages. Non-zero current measurements with ISEs are promising for analytical applications, zero-current (potentiometric) measurements with ISEs remain the most common mode of their practical use. Practical use of ISEs is largely limited to measurements within the Nernstian response range. According to the established views on how ISEs work, within the linear Nernstian response range, only the boundary potential at the membrane/sample interface varies
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