CHEM 121L General Chemistry Laboratory Revision 1.0 Absorbance Spectroscopy and Beer's Law Learn about the Interaction of Photons with Molecules and Ions in Solution. Learn about Spectroscopy. Learn about Beer’s Law. In this laboratory exercise, we will probe the behavior of electrons within molecules and ions using Absorbance Spectroscopy. In particular, we will examine the absorbance spectrum of Cupric Ions in an aqueous environment in the visible region of the electromagnetic spectrum. We will leverage the concentration dependence of the absorbance of a solution of Cupric Ions to determine the concentration of a solution of unknown concentration. This is a very common analytic technique for determining the concentration of absorbing species. Previously we have observed the emission spectrum of Hydrogen and Helium atoms; where the excited atoms relax and emit photons of energy equivalent to the atom's quantum state transition. Typical electronic transitions within atoms and molecules are such that the corresponding photons have energies in the Visible and Ultraviolet (UV-VIS) regions of the spectrum. Likewise, if a photon of the correct energy impinges on an atom, molecule or ion, such that this energy matches the energy required for a quantum state transition, the photon can be absorbed:
P a g e | 2 In these cases, the Energy difference between the participating quantum states is related to the Energy of the photons via: E = Ephoton= hc/ (Eq. 1)Thus, the photons absorbed or emitted by a sample of the atom, molecule or ion in question are a direct probe of the energy difference between quantum states of that atom, molecule or ion. A major complication occurs if an absorbing or emitting species is in a condensed phase; a liquid or a liquid solution. If the absorbing molecule/atom is in a solution, it is surrounded by constantly jostling solvent molecules. Thus, each molecule/atom finds itself in a slightly different environment than its brothers. This causes the energy gap between the quantum states responsible for the absorbance of photons to be slightly different for each molecule/atom. This means we will have a series of very, very closely spaced absorbance lines. Practically, this means the Absorbance Spectrum will be a broad band, rather than a sharp line. This is as diagramed below: In this case, we usually identify the absorbance band by the wavelength of maximal absorbance, max.