Ultrasensitive sensing of pH and fluoride with enhanced constant potential coulometry at membrane electrodes

Abstract

Minute potential changes at membrane electrodes may be translated to easily detectable current spikes if the cell potential is held constant and the change is imposed onto a capacitive element placed in series. While this results in a very important increase in measurement precision, the current transient passing through the sensing membrane contributes to measurement uncertainty and may not be applicable to all sensing materials. This limitation is overcome by treating the ion-selective electrode as reference electrode, which allows one to avoid current passing through the ion-selective membrane. The potential stability of the Ag/AgCl element upon passage of current was confirmed to be excellent, as expected for an ideally non-polarizable electrode. This greatly increases the reproducibility and precision of the method. The approach also allows one to use the LaF3 crystal-based fluoride sensitive membrane, for which passage of current transients gives deleterious effects. An electronic circuit was built to automate the control of the capacitive element. An improvement of precision (from 829 to 177 μpH) and a reduction of signal drift (from 1.89% of total charge to 0.38%) was demonstrated with membrane-based pH electrodes while fluoride sensing was achieved for the first time with constant potential coulometry at high reproducibility and precision (RSD of just 0.026% activity change).