Operational Limits of Controlled Current Coulometry with Ion‐Selective Polymeric Membranes

Abstract

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 ionselective 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 Bucks 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.

Interpretation of chronopotentiometric transients of ion-selective membranes with two transition times

“…By minimizing or eliminating these fluxes, sub-nanomolar detection limits have been achieved [11, 13–17]. In addition to the CP determination of the breakpoint times, understanding the mechanism of processes inside the membrane during applied currents is also important in other CP applications, e.g., (i) in pulsed potentiometric methods that allowed the detection of polyions [18]; (ii) in pulsed potentiometric applications that improved the selectivity of calcium sensors [19]; and (iii) during the coulometric generation of ions from the ion-selective membranes [20, 21]. In these CP experiments the membranes are commonly loaded with a high concentration of lipophilic background electrolyte to reduce membrane resistance and avoid migration effects.…”

“…In these CP experiments the membranes are commonly loaded with a high concentration of lipophilic background electrolyte to reduce membrane resistance and avoid migration effects. Consequently, the CP transients of such membranes were studied most frequently [20]. In this paper, we focus on the voltage transients of ion-selective membranes cast without a background electrolyte, and propose a new interpretation for multiple transition times that can emerge in CP experiments.…”

Opportunities and challenges of using ion-selective electrodes in environmental monitoring and wearable sensors

“…Recent work give guidance on the sensor design highlighting that the level of interferences is more severe than in direct potentiometry, and that sample volumes need to be 11 precisely reduced but also suggesting solutions for the limitation due to non-Faradaic processes [124,125,127]. With this approach silver and calcium could be detected in water solution at μM levels.…”

“…Controlled potential thin-layer coulometry using ISEs seems a very intriguing and powerful approach to achieve calibration free analysis and independence from the reference electrode potential, temperature changes and other processes which may alter the diffusion kinetics [42,[124][125][126]. Recent work give guidance on the sensor design highlighting that the level of interferences is more severe than in direct potentiometry, and that sample volumes need to be 11 precisely reduced but also suggesting solutions for the limitation due to non-Faradaic processes [124,125,127].…”

Polymeric Membrane Ion‐Selective Electrode for Butyrylcholinesterase Based on Controlled Release of Substrate

“…the fluxes in the direction of sample solution, has been shown to be very promising. Bakkers group reported on the selective coulometric reagent delivery of ions from ionophore based polymeric membranes for calibration-free titrations [17] and investigated the operational limits of controlled current coulometry with these ion-selective membranes [18]. The same group also developed an anion electrode based on precipitate equilibrium at the sample-membrane phase boundary with released silver ions from the sensing membrane [19].…”