The laboratory experiment presented here describes the use of a calcium ion-selective electrode (Ca-ISE) to determine the concentration of zinc in a supplement tablet. This approach was developed to show students how to expand the range of applications of Ca-ISE to enable the determination of metal ions other than Ca 2+ . To determine the concentration of Zn 2+ , students dissolved the sample in a Ca-EDTA buffer solution and used EDTA as titrant. We discuss with students the importance of a fixed pH and ionic strength for the accurate analytical quantification. Students also used colorimetric titration of the same sample with murexide as an indicator to determine the concentration of Zn 2+ . They used statistical tools to confirm that the two approaches provide similar results. Students concluded that both titrations overestimate the amount of zinc in a tablet by 3−5% in comparison with the known value reported by the manufacturer and discussed factors that affect the accuracy of Zn 2+ determination.