The cyclic voltammograms for the incremental addition of 50 mM FeSO4•7H2O to the digested iron supplement tablets are shown in Figure . The chemical reversibility of a redox reaction at the surface of a working electrode is established if the anodic to cathodic peak current ratio is 1. As shown in the sample student results, the anodic to cathodic peak ratio of 0.9682 confirms chemical reversibility of the Fe 2+ /Fe 3+ redox couple at the electrode surface (Table ). The cathodic and anodic peak potentials occurred at 440 and 540 mV respectively. A sample standard addition calibration curve with an R 2 value of 0.9985 is shown in Figure . The choice of which peak current to use for the calibration curve is left at the discretion of each student. The concentration of iron in the digested samples is obtained using the calibration curve, and the associated uncertainty ( ) is calculated with the aid of the Excel LINEST function. Each student submits a note card containing the amount of iron calculated in mg and the associated relative error. Our collated students' results for the mass of iron in these over-the-counter ferrous sulfate supplements was 60.1 ± 3.0 mg compared to 65.0 mg shown on the ferrous sulfate ion supplement label.