Teaching the Concepts of Donnan Potential and Liquid-Junction Potential by Evaluating Homemade pH and Ion-Sensitive Probes Constructed Using Commercial Ion-Exchange Membranes and Reference Electrodes

Abstract Transport of ions across membranes or separators can result in electric potential differences that are sensed in most electrochemical experiments and impact the interpretation of the experimental results. To assist in comprehending the underlying causes and expected values of these electric potential differences, we designed, implemented, and evaluated hands-on laboratory experiments that were performed by undergraduate and graduate students in electroanalytical chemistry courses. The experiments require that students construct a simple two-compartment electrochemical cell using plastic cuvettes, a polymeric ion-exchange membrane, two reference electrodes placed in aqueous electrolytes that wet each side of the membrane, and a voltmeter or a potentiostat to measure the open-circuit potential between the electrodes. Open-circuit potentials measured between commercially available Ag/AgCl reference electrodes report on total Donnan and liquid-junction potentials. These electric potential differences are dictated by the composition of ions in each liquid electrolyte through interactions with the ion-exchange membrane and porous frits at the tip of each reference electrode. When reference electrodes consist of wires immersed directly in the liquid electrolytes, measured open-circuit potentials include contributions from Nernst potentials, which arise from electric double layer charging at each electrode interface driven by chemical potentials of redox-active species in the liquid electrolytes. While these experiments are suitable for most upper-level undergraduate students in STEM, the concepts of Donnan and liquid-junction electric potential differences and chemical potentials are advanced enough to educate graduate students and senior researchers alike. Student feedback obtained using surveys administered before and after the laboratory experiments provides additional guidance to instructors who plan to implement these experiments.