Symmetric cell for improving solid-contact pH electrodes

Abstract

Traditional pH glass electrodes are designed in a symmetrical manner to guarantee the most reliable and reproducible potentiometric measurements possible. Solid-contact and other pH probes not based on glass membranes are desirable because they allow for new types of applications, may be mass fabricated and less prone to breakage. Unfortunately, however, they introduce electrochemical asymmetry because the reference element used in the reference electrode compartment is now different. This work shows how symmetry may be restored with solid-contact pH probes, using a H+-selective ionophore-based polymeric membrane deposited on top of a conductive polymer (PEDOT-C14) as a transducer layer. The new cell implements a reference element that is composed of a similarly formulated pH probe immersed into a buffer solution and an Ag/AgCl element directly connected to a single-junction Ag/AgCl/3.0 M KCl reference electrode that is placed in contact with the sample solution. By implementing this design, the zero point of the solid-contact pH sensing system may be shifted to the conventional value of pH 7.0. The value of the zero point was experimentally confirmed as 6.96 ± 0.02 pH units at three different temperatures in the range from 5 to 25 °C. This symmetric solid-contact potentiometric cell gave a long-term potential drift of 48 ± 16 μV h−1, comparable to that of a combination pH glass electrode.