Ion-selective electrodes are among the oldest and most important potentiometric sensing methods. In this experiment, students assemble their own reusable ion-selective electrode from glass tubes with an attached PTFE membrane, [(CH[subscript 3]CH[subscript 2]CH[subscript 2]CH[subscript 2]CH[subscript 2]CH[subscript 2]CH[subscript 2]CH[subscript 2])[subscript 3]NCH[subscript 3]]X ion exchanger, and both internal and external Ag/AgCl reference electrodes. Students then evaluate the electrode response as a function of specific-ion activity, measure the activity of an unknown-concentration solution, and determine the electrode selectivity by repeating the analysis in the presence of a fixed amount of interfering ion. With a choice of chloride, nitrate, acetate, and sulfate specific-ion electrodes and a variety of interfering ions, each student in a lab section can do a different experiment while keeping both the lab cost for consumables and the setup time low. The obtained selectivity values are in agreement with the Hofmeister series. Students practice analytical laboratory skills by designing and preparing a calibration curve, see benefits and limitations of ion-selective electrodes, and make use of the Nernst equation.