Operational limits of controlled current coulometry with ion-selective polymeric membranes

Plasticized polymeric membrane electrodes containing selective chemical receptors (ionophores) that are used in potentiometric and optical sensors have recently been shown to be attractive in controlled current coulometry for calibration free reagent delivery. For this purpose, a galvanostatic pulse of fixed duration is applied across an ion-selective membrane containing added ion-exchanger sites, resulting in the release of a calculated amount of ions from the membrane into the sample phase. This paper evaluates the operational limits of such coulometric actuators with chronopotentiometry, using silver-selective membranes as model systems. Diffusion theory predicts the depletion of ionophore and ion-exchanger at one of the two interfaces in a matter of minutes, owing to the relatively small diffusion coefficients in such membranes. Chronopotentiometry on membranes containing lipophilic cations at either side of the membrane, rather than silver ions, confirms that the ionophore depletes at the inner side of the membrane. At the sample side, ionophore is expected to increase in concentration and does not result in a loss of selectivity. Chronopotentiometric responses show a drastic transition at long times, typically 30 min, which cannot plausibly be explained by the depletion of added lipophilic ion-exchanger at the sample side since the diffusion coefficients are similar to that of ionophore. It is postulated that the intrinsic anion-exchanger sites of the PVC matrix are relatively immobile and do not easily concentration polarize upon application of a transmembrane current pulse, in agreement with Buck's earlier work on fixed site membranes. Indeed, silver and calcium-selective membranes fabricated with increased concentrations of such fixed sites, by using carboxylated PVC, exhibited chronopotentiometric breakdown times larger than 60 min and no loss in coulometric efficiency. The results obtained here will help in designing coulometric actuators with improved characteristics on the basis of hydrophobic polymeric ion-selective membranes.