14.1: What is Molar Absorptivity? Beer's law, as we learned in Chapter 13, gives the relationship between the amount of light absorbed by a sample, the concentration of the species absorbing light, the distance (path length) the light travels through the sample, and the molar absorptivity of the species absorbing light. 14.2: Absorbing Species There are two general requirements for an analyte’s absorption of electromagnetic radiation. First, there must be a mechanism by which the radiation’s electric field or magnetic field can interact with the analyte. For ultraviolet and visible radiation, absorption of a photon changes the energy of the analyte’s valence electrons. The second requirement is that the photon’s energy must exactly equal the difference in energy between two of the analyte’s quantized energy states. 14.3: Qualitative and Characterization Applications UV/Vis absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, and the intensity of that absorption, is determined by the chemical environment of the absorbing moiety. UV/Vis spectroscopy also provides ways for studying chemical reactivity. 14.4: Quantitative Applications The determination of an analyte’s concentration based on its absorption of ultraviolet or visible radiation is one of the most common quantitative analytical methods. In addition, if an analyte does not absorb UV/Vis radiation—or if its absorbance is too weak—we often can react it with another species that is strongly absorbing. 14.5: Photometric Titrations If at least one species in a titration absorbs electromagnetic radiation, then we can identify the end point by monitoring the titrand’s absorbance at a carefully selected wavelength.