Chemical Education Today
NSF Highlights
edited by Susan H. Hixson
Teaching Students to Use Electrochemistry as a Probe of Molecular Behavior
National Science Foundation Arlington, VA 22230
Richard F. Jones
Sinclair Community College Dayton, OH 45402-1460
by Grant N. Holder
Electrochemistry is typically not represented as strongly in the undergraduate curriculum as the other components of the analytical “triad”, spectroscopy and chromatography. The latest instrumentation for the latter techniques is considered indispensable for modern chemical education; by contrast, undergraduate laboratory electrochemical equipment can often be dated, rudimentary, or incomplete. The strengths and weaknesses of spectroscopy and chromatography for qualitative and quantitative work are repeatedly demonstrated—from sophomore-level Job’s Method experiments to senior- level exercises in ICP of natural waters or 2D-NMR of natural products. Beer’s Law is intertwined with the identification of structural groups starting at the introductory level. In like manner, quantitative HPLC, GC–MS experiments (utilizing isotopic dilution), and qualitative analyses of pesticide residues are numerous in pedagogical journals. Curiously, electrochemistry is not usually taught from a quantitative or qualitative perspective. Instruction at the introductory levels is archaic and obliged to support instruction in redox equations or free energy. Subsequently, pH meter manipulation may be accompanied by solution conductivity measurement in a third-year, inorganic lab. At senior levels, electrodeposition or anodic stripping voltammetry might be used for quantitative analysis of aqueous ions. A student can often obtain a B.S. degree without making a single voltammetric measurement, even as chemists use electrochemical techniques for novel studies of energy, electron-transfer (ET) mechanisms, surface processes, monolayers, chemical sensors, enzyme kinetics, synthesis, electropolymerization, and other avenues of research. The capabilities of modern electrochemical instrumentation allow for much more than trace metal-ion analysis. The goal of our program is to demonstrate to undergraduates that electrochemistry can be a powerful probe of molecular behavior. Experiments developed to this end should all be a) conceptually uncomplicated; b) unambiguous and precise; c) simple to execute; d) time- and cost-effective; and e) visually arresting, where possible. Our strategy is to incorporate appropriate electrochemical techniques as part of the overall theme of a course without disrupting the flow of instruction. Done properly, electrochemistry represents a logical choice to the student for his or her analyses. Good lab experiences come from good research. Therefore, our exercises are adapted directly from the scientific literature. Below are brief descriptions of some concepts, the experiments developed to showcase them, and the courses affected.
of Nicholson and Shain to calculate a first-order rate constant using CV (1). Reversible electrogeneration of polynuclear aromatic cation radicals is followed by exposure to various nucleophiles (pyridines or water) (2). Addition of the nucleophile gives rise to A1. 1 Nu → Z. Rotating-disk voltammetry allows evaluation of the kinetic parameters of enzyme-catalyzed reactions following a Koutecky–Levich/Michaelis–Menten data treatment (3). The enzymes utilized need have some redox-active cofactor (e.g., H2O2 or NAD(P)H) or be amenable to immobilization on a prepared surface so that ET can be direct (4 ). Alternately, chronocoulometry has been used to monitor the disappearance of a cofactor with time. Rates determined this way can be compared with spectroscopic results.
Rate Constants
Surface Studies
Reaction and ET rate constants are easily accessible through experiments in cyclic voltammetry (CV), rotatingdisk voltammetry, and chronocoulometry. These are incorporated into physical, inorganic, and biochemistry laboratories. A very effective one-pot experiment utilizes the method
We use double-potential-step chronocoulometry to measure the amount of surface coverage (ΓR) of an adsorbed compound for some sulfur-bearing transition-metal compounds. A plot of charge (Q) versus time1/2 for periods before and after reversal of the potential step (τ) allows for separation of
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Electron-Transfer Mechanisms Particular care was taken for realistic inorganic laboratory exercises. Experiments involve (a) scan rate (CV) studies of student syntheses; (b) calculation and rationalization of the HOMO–LUMO gap, and (c) observation of a cis–trans isomerization following ET (5). During the final weeks of the semester, students conduct individual synthetic projects, often using a single compound that is ligated to different metals. A stimulating example is diethyldithiocarbamate ligand. This low-cost, low-toxicity ligand in myriad formulations offers a wealth of exercises for students and is a boon to instructors due to its unusually facile preparation and purification schemes. By this time, participants have begun to think of voltammetry as part of their complete characterization. This is particularly satisfying since electrochemical studies appear in over 35% of the articles in the latest volume of Inorganic Chemistry. We select compounds for instrumental analysis that can be analyzed by various techniques throughout the semester. One successful and easily prepared candidate is 6,14dicyanodibenz[a,c]anthracene, which students manipulate at various times with HPLC, UV–Vis, IR, NMR, fluorescence, and spectroelectrochemical measurements. Other compounds, such as 5-nitro-2-chloro- and 3-nitro-2chloropyridine, exhibit visually arresting clues to help the student unravel the mechanism. The former exhibits reductive behavior that leads to the visible dissociation of NO2(g); the latter does not. Aromatic compounds containing nitro groups offer many entertaining possibilities, as do compounds exhibiting structural rearrangements following ET (6 ).
Journal of Chemical Education • Vol. 76 No. 11 November 1999 • JChemEd.chem.wisc.edu
Chemical Education Today
faradaic, capacitative, and surface components through preparation of an Anson plot. Acknowledgment This work was partially supported by a grant (DUE #9750519) from the National Science Foundation, Instrumentation and Laboratory Improvement Program. Thanks to Lawrence E. Brown, Cassandra T. Eagle, David G. Farrar, David M. Gooden, Ann E. Holder, Laurel L. McClure, and Thomas C. Rhyne for providing invaluable assistance within our ongoing program. Literature Cited 1. Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706. 2. Tolbert, L. M.; Khanna, R. K.; Popp, A. E.; Gelbaum, L.; and
Bottomley, L. A. J. Am. Chem. Soc. 1990, 112, 2372–2378. 3. (a)Scott, D. L.; Bowden, E. F. Anal. Chem. 1994, 66, 1217–1223. (b) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley & Sons: New York, 1980; pp 290– 293. (c) Mell, L. D.; Maloy, J. J. Anal. Chem. 1975, 47, 299– 307. (d) Kamin, R. A.; Wilson, G. S. Anal. Chem. 1980, 52, 1198–1205. 4. (a) Albery, W. J.; Cass, A. E. G.; Shu, Z. X. Biosens. Bioelectron. 1990, 5, 367–378. (b) Andrieux, C. P.; Dumas–Bouchiat, J. M.; Savéant, J. M. J. Electroanal. Chem. 1984, 169, 9–21. 5. Bag, R.; Lahiri, G. K.; Chakravorty, A. J. Chem. Soc., Dalton Trans. 1990, 5, 1557. 6. Geiger, W. E. Prog. Inorg. Chem. 1985, 33, 275–351.
Grant N. Holder is in the A. R. Smith Department of Chemistry, Appalachian State University, Boone, NC 286082036; email:
[email protected].
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