Unconditioned Symmetric Solid-Contact Electrodes for Potentiometric Sensing.

In potentiometric sensing, the preparation of the electrodes preceding a measurement is often the most time-consuming step. Eliminating the conditioning process can significantly speed up the preparation procedure, but it can also compromise the need for proper pre-equilibration of the membrane. We propose here a symmetric setup to address this challenge with an identical indicator and reference elements measured against each other, thereby compensating for potential drift. This strategy allows one to achieve potentiometric measurements using non-conditioned all-solid-state ion-selective electrodes for the detection of nitrate and potassium ions with Nernstian response slopes and detection ranges identical to those of conventional systems. To establish symmetry, a set of solid-contact ion-selective electrodes placed in a reference cell is measured against a set of identical electrodes in a sample cell. By subtracting the potentials between the two cells, potential instabilities not directly relevant to the measuring sample are eliminated, giving minimal potential drifts and stable 5-day potential responses. The value of the nitrate-selective electrodes in the symmetric setup had a standard deviation of just 3 mV for the 5-day period in contrast to 19 mV in the asymmetric system, clearly demonstrating the influence of the conditioning step which is almost eliminated in the former system. During the 20 h potential monitoring experiments, the drift dropped to below 0.3 mV/min in less than 6 min, as opposed to an average time of 35 min for the asymmetric system. The applicability of the proposed setup was successfully demonstrated with the measurement of nitrate in a river water sample, where a potential drift lower than 0.1 mV/min was reached in less than 5 min of first contact with solution.