Mathematical model of current-polarized ionophore-based ion-selective membranes: Large current chronopotentiometry


The properties of ionophore-based ion-selective membranes were investigated theoretically and experimentally for the first time in large-current chronopotentiometric experiments. In these experiments, when either the free ionophore or the ion-ionophore complex concentration drops to zero at one of the membrane boundaries, a characteristic drop in voltage (breakpoint) appears in the chronopotentiometric transients at a transition time τ. Based on a mathematical model of current polarized membranes, theoretical equations were derived and tested. These equations describe the correlation between the transition time τ and the concentration of free ionophore, the concentration of ion-ionophore complex, and the diffusion coefficients of species in the membrane. The simulated concentration profiles correlated well with concentration profiles recorded experimentally prior to the transition time using spectroelectrochemical microscopy (SpECM). Diffusion coefficients calculated from (i) the transition times, (ii) curve fitting, and (iii) the initial ohmic resistance of the studied membranes were also compared. Similar to chronoamperometry, the chronopotentiometric transition times provided possibilities for assessing the loss of free ionophore and lipophilic anion from cation selective electrode membranes during continuous use. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

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