Rotating Ion‐Selective Membrane Electrodes for Trace‐Level Measurements

Abstract

Ion-selective solvent polymeric membrane electrodes (ISEs) are shown to exhibit improved response characteristics in a rotating electrode configuration. With a rather concentrated internal electrolyte solution of primary (analyte) ions, which induces their leaching from the membrane into the sample, the detection limit of the ISEs rotating at 3000 rpm is found to be better by about one order of magnitude than that of the analogous conventional ones in a stirred solution. On the other hand, with an internal solution inducing a strong inward flux of primary ions (i.e., from the sample to the ISE membrane), the apparent super-Nernstian response due to depletion of primary ions from the analyte solution near the membrane surface is shifted by about one order of magnitude to lower concentrations. Membranes of about 0.8 mm diameter positioned eccentrically with respect to the rotation axis give a further shift of the apparent super-Nernstian response step by about 0.3 logarithmic units. Simultaneously, the use of rotating electrodes substantially lowers the response time at submicromolar activities. An eccentrically placed Pb 2 -selective membrane on a rotating disk electrode with optimized internal solution shows a lower detection limit of a Pb 6.3 Â 10 À11 M.

Computer simulation of ion-selective membrane electrodes and related systems by finite-difference procedures

“…Actually, ISE responses were reported to be very sensitive to the flow-or stirring-rate in the sample solution, especially at lower ion activities [13,35]. For rotating ISEs, a similar dependence on the rotation frequency was observed [23,36]. Although boundary-layer influences are widely manifest in real experiments on ISEs, they have so far not been considered in the respective digital simulations.…”

Potentiometric sensors for trace-level analysis

“…Besides these experimentally most challenging points, the LOD may also be improved by reducing the thickness of the aqueous Nernst diffusion layer. This has been achieved by sample stirring [42], or using a rotating electrode configuration [26] or a wall-jet system [43]. Fig.…”

“…Fig. 3 shows the time traces for a Pb 2+ -selective membrane in a rotating electrode set-up, with the membrane placed off-center from the rotating axis [26]. The thickness of the diffusion layer of the membrane phase may be increased as well, and this has indeed shown promise [19,42].…”

“…Direct comparisons of different methods are valid only if the same definition of the lower LOD is used. Experimental time traces, obtained by serial dilution of the sample, for a Pb 2+ -selective electrode based on a polymeric membrane containing an aqueous inner solution, in a rotating electrode set-up [26]. The numbers shown are logarithmic molar concentrations of the sample, and the LOD (DL1, see Fig.…”

Multispectral imaging of ion transport in neutral carrier‐based cation‐selective membranes

“…In solutions of submicromolar concentrations, even infinitesimally small amounts of ions, leaching from the membrane, may dominate the measured membrane potential (27). Once this leaching was eliminated, the detection limits of ISEs could be extended from micromolar to nano-and picomolar concentrations and simultaneously the experimentally determined selectivity coefficients were also dramatically improved (3,26,(28)(29)(30)(31). Our group introduced methods to eliminate ion leaching and to control ion transport through ISMs by galvanostatically controlled current (32,33).…”