Portable instrument and current polarization limitations of high sensitivity constant-potential capacitive readout with polymeric ion-selective membranes

Abstract

In this work, the development of a portable device integrated with electronic circuits, termed PotentioCap, is described for the constant-potential coulometric readout of ion-selective membranes. A range of capacitors (22–220µF) included in the device can be automatically chosen by the control software. The device was evaluated in standard pH solutions and stabilized seawater samples using a hydrogen-selective electrode placed in series with one capacitor. The transient current and integrated charge over time correlate well with that from capacitive readout using a benchtop potentiostat using an external electronic circuit. The capacitor of 47µF is sufficient to amplify the current signal of 0.01 pH unit change with slopes of 2.68µC/decade and 2.49µC/decade in standard pH solutions and stabilized seawater samples, respectively. Unfortunately, however, instrumental control of the measurement protocol, as opposed to the chemical reconditioning reported earlier, results in an order of magnitude larger measurement error (precision of 0.6mpH). To understand this the Nernst-Planck equation is used to describe the effects of constant and exponential decay currents with ion-selective membranes. Numerical simulations show that the passage of 0.2µC of charge across the membrane causes a ∼1 % change in membrane concentration. The resulting phase boundary potential is shown to drift in the same manner as that observed experimentally with current polarized membranes. Furthermore, an improved capacitive model using an RC time constant is proposed by additionally considering the potential change with time. The model results compare favorably with the experimental data. Electric migration is shown not to be a significant contribution to the transient current. We discuss solutions to overcome the observed limitation of potential stability, which currently holds back the realization of ultra-high sensitivity measurements with ion-selective membranes.