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

Transport of ions across semipermeable membranes, like ion-exchange membranes, and salt bridges, like porous frits, each results in electric potential differences that are sensed in most electrochemical experiments. While often neglected, these electric potential differences can significantly impact the interpretation of experimental results. To assist in comprehension of these concepts, a one-to-two-hours-long hands-on laboratory activity, intended for students in electroanalytical chemistry courses, was designed, implemented, and evaluated. This activity requires that students construct a simple two-compartment electrochemical cell from inexpensive and readily available disposable plastic cuvettes, commercially available polymeric ion-exchange membranes, two nominally identical reference electrodes placed in liquid electrolytes that wet each side of the membrane, and a voltmeter or a potentiostat to measure the open-circuit potential between the reference electrodes, i.e. the cell potential. Using commercially available reference electrodes, each consisting of a wire (e.g. Ag/AgCl) immersed in a fritted tube containing an aqueous electrolyte, measured open-circuit potentials report on the magnitude of both Donnan and liquid-junction electric potential differences. These electric potentials are sensitive to the composition of ions in each electrolyte, through interactions with the ion-exchange membrane and porous frits at the tip of each reference electrode. When a cation-exchange membrane separates aqueous solutions consisting of different concentrations of hydrochloric acid (HCl) or potassium hydroxide (KOH), the cell responds like a pH or pK probe, respectively, because the membrane interfaces are more sensitive to the activity of H+ and K+ over Cl– and OH–, respectively. Analogously, when an anion-exchange membrane is used, the cell responds like a pCl or pOH probe, respectively, because the membrane interfaces are more sensitive to the respective activity of Cl– and OH– over H+ and K+. When the solutions contain large concentrations of multiple types of ions, measured open-circuit potentials typically deviate from these Donnan-dominated effects and are instead dominated by a liquid-junction-like effect where electric potentials within and across ion-exchange membranes form due to differences in the permeabilities of the ions. When reference electrodes consist of wires immersed directly into the aqueous electrolytes that wet the membranes, and not in fritted tubes, measured open-circuit potentials also include contributions from the chemical potential of redox-active species that are present in the aqueous electrolytes. While this hands-on laboratory activity is suitable for most upper-level undergraduate students in physical science and engineering disciplines, the concepts of Donnan electric potential, liquid-junction electric potential, and species chemical potential are advanced enough to educate graduate students and senior researchers alike. Our activity also helps instructors teach a topic that is pertinent to the electrochemistry of many aqueous chemical systems using simple components and a straightforward setup.