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| - Dynamic Diffusion Model for Tracing the Real-TimePotential Response of Polymeric MembraneIon-Selective Electrodes
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| - A numerical solution for the prediction of the time-dependent potential response of a polymeric-based ion-selective electrode (ISE) is presented. The model addresses short- and middle-term potential drifts that aredependent on changes in concentration gradients in theaqueous sample and organic membrane phase. This workhas important implications for the understanding of thereal-time response behavior of potentiometric sensorswith low detection limits and with nonclassical super-Nernstian response slopes. As a model system, the initialexposure of membranes containing the well-examinedsilver ionophore O,O‘ ‘-bis[2-(methylthio)ethyl]-tert-butylcalix[4]arene was monitored, and the large observedpotential drifts were compared to theoretical predictions.The model is based on an approximate solution of thediffusion equation for both aqueous and organic diffusionlayers using a numerical scheme (finite difference in timeand finite elements in space). The model may be evaluatedon the basis of experimentally available parameters andgives time-dependent information previously inaccessiblewith a simpler steady-state diffusion model. For the casesstudied, the model gave a very good correlation withexperimental data, albeit with lower than expected diffusion coefficients for the organic phase. This model mayaddress numerous open questions regarding the responsetime and memory effects of low-detection-limit ion-selective electrodes and for other membrane electrodes whereion fluxes are relevant.
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