The concentration of Fe 3+ was measured to determine the extent to which the Fenton reaction occurred. Ferric ions achieve maximum absorption at wavelengths between 300 and 305 nm , but it complexes with SCN -, producing FeSCN 2+ . The ferric thiocyanate complex absorbs strongly in the visible range, allowing the use of cost-efficient and readily available Vernier Spectrophotometers to characterize the absorption spectrum. By adding an excess of Potassium Thiocyanate, KSCN, virtually all of the ferric ions in solutions form ferric thiocyanate. Then, the concentration of Fe 3+ produced by the Fenton reaction can be determined by referencing a calibration curve.