This laboratory exercise involves the measuring the UV-Vis absorbance spectra of the same set of samples analyzed in Experiment 1. Students learn the principle of the analytical method (Beeršs Law, atomic energy configurations and transitions) and instrumentation (UV-Vis Spectrophotometer). Students analyze the spectra of solutions of a single compound to determine the relationship between distinct spectral patterns and structural features of the compound. At certain wavelengths, the light absorbance of a sample is proportional to the concentration of the absorbing species. Although the direct determination of organic carbon is possible, as demonstrated in Experiment 1, this measurement is usually made in a laboratory and is not particularly rapid to complete. Consequently, the use of a surrogate measurement, such as UV absorbance, with which to quantify organic carbon concentration, has many advantages. Given the portability of spectrophotometers, this analysis can be made in the field and at contaminated sites, and can be installed on line for process control. In this exercise, students compare the spectra of known, single component solutions to that of unknown, multi-component mixtures in order to illustrate exactly what is measured by this method and what are the limitations of this approach. Students investigate various methods of calibration and compare their results to a variety of examples in the literature.