In the Classroom
edited by
highlights
Susan H. Hixson
projects supported by the NSF division of undergraduate education
National Science Foundation Arlington, VA 22230
Curtis T. Sears, Jr. Georgia State University Atlanta, GA 30303
Electrochemical Measurements in the Undergraduate Curriculum John F. Wheeler,* Sandra K. Wheeler, and Laura L. Wright Department of Chemistry, Furman University, Greenville, SC 29613 Instrumental analysis in the undergraduate curriculum is often divided into three primary areas: (i) spectroscopy, (ii) separations, and (iii) electrochemistry. At many institutions, demonstrations of electrochemical principles are limited to static measurements [e.g., pH, ion selective electrode (ISE), or potentiometric titration] and a polarographic determination using a dropping mercury electrode (DME). While the underlying principles of dynamic electrochemistry are illustrated using polarography, practical difficulties of DME use, hazards associated with mercury pools, and the outdated manner in which data are typically recorded and analyzed often leave students disillusioned with modern electrochemistry. While most programs are well-equipped with spectroscopic and chromatographic instruments, far fewer have placed similar priority in acquiring modern electroanalytical instruments. The last 25 years have witnessed a revolution in voltammetric analyses, including the routine use of solid electrodes and solid state sensors, amperometric detection for high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE), newly developed electrochemical waveforms, software simulation for mechanistic interpretation, and the implementation of computer-controlled workstations. In the fall of 1994 we purchased two voltammetric workstations, a low-current amplifier for microelectrode use, and two amperometric detectors. Here we describe several ways in which we have initially utilized this equipment in our undergraduate curriculum and research programs. Cyclic Voltammetry in Chemistry 23: Techniques of Chemistry I This second-year course is a unique discovery-type lab in which chemistry majors perform multistep syntheses of organic and inorganic compounds strictly from literature references. Structural and quantitative characterization of the products of each synthetic step is accomplished using 1H and 13C NMR, IR, UV-vis, GC-MS, thermal analysis (TGA), and cyclic voltammetry (CV). Although extensive coverage of CV theory is not provided, students can identify oxidative and reductive waves for reversible electrochemical couples and gain an appreciation for the speed and simplicity with which this information is obtained. One of the syntheses carried out each year involves the monoacetylation of ferrocene (1), which results in a significant shift (+240 mV) in the oxi-
*Corresponding author. Phone: 864/294-3371. FAX: 864/2943559. email:
[email protected].
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dation potential of ferrocene as shown in Figure 1. By comparing CV data for their lab preps with those of authentic acetylferrocene and ferrocene standards, students can identify any residual ferrocene in their product and estimate purity. The straightforward operation of the voltammetric workstations using a Windows™ software platform permits students to modify acquisition parameters (e.g., scan rate, initial potential) easily, thus generating discussion regarding the nature of the electrochemical reactions occurring at the electrode/solution interface. Oxidation/reduction potentials and peak currents are conveniently displayed using instrument software, which we find to be especially helpful for introductory electrochemical experiments. Square-Wave Voltammetry and LCEC in Chemistry 33: Analytical Chemistry Chemistry 33 may be regarded as an advanced instrumental analysis course with substantial lecture coverage of electrochemical theory, methods, and instrumentation. One of the most popular labs each year includes an assay for acetaminophen (N-acetyl-p-aminophenol) in over-the-counter tablets using Osteryoung square-wave voltammetry (OSWV). After a thorough mechanistic study of the phenol oxidation using cyclic voltammetry (2), students quantitate the mass of acetaminophen in a tablet based on conventional calibration procedures using OSWV at a glassy carbon electrode. Several other electrochemical studies are included in this lab, including a measurement of electrode area, a demonstration of steady-state response due to nonplanar diffusion at a microelectrode, and observation of the selective adsorption of hydrogen on platinum (3). A second experiment developed for this course utilizes amperometric detection for the analysis of phenolic acids in natural beverages. Following their extraction using a disposable C18 cartridge (4), phenolic acids are separated using RPLC and detected at a glassy carbon electrode (0.800 V vs. Ag/AgCl). Injection of standards (gentisic, caffeic, sinapinic, cinnamic, vanillic, and ferulic acids) facilitates identification of the acids in actual beverages, and the gains in sensitivity and selectivity associated with EC detection are easily recognized by using a UV detector in-line. Electrochemical Measurements in the Undergraduate Research Program Several of our faculty have already made substantial use of the voltammetric instrumentation in their research programs. For example, those with an interest
Journal of Chemical Education • Vol. 74 No. 1 January 1997
In the Classroom
can be electrochemically excited to luminesce. Characterization of the redox potentials of complexes that produce such fluorescence has been greatly accelerated using the voltammetric systems. In the separations area, electrochemical detection can be applied to capillary electrophoresis. Fiber microelectrodes are ideally suited to CE, but sensitive potentiostats can easily be damaged to due grounding problems. To circumvent these difficulties, attempts are underway to couple an amperometric detector equipped with factory-installed sacrificial computer chips to a CE system constructed in-house. Acknowledgment This work was partially supported by a grant (DUE 9451216) from the National Science Foundation Division of Undergraduate Education Instrumentation and Laboratory Improvement program. Literature Cited Figure 1. Cyclic voltammograms of (A) 20 mM (mono )acetylferrocene and (B) 20 mM ferrocene in 50 mM tetrabutylammonium perchlorate/acetonitrile. Pt disk working electrode (2.5 mm2), scan rate 50 mV/s.
in developing conducting polymer films in which organometallic substituents comprise the backbone of the organic polymer use the workstations to electrochemically deposit and characterize new films (5). Some complexes
1. Angelici, R. J. Synthesis and Technique in Inorganic Chemistry, 2nd ed.; W. B. Saunders: Philadelphia, 1977; pp 157–168. 2. Van Benschoten, J. J.; Lewis, J. Y.; Heineman, W. R.; Roston, D. A. J. Chem. Educ. 1983, 60, 772–776. 3. Koppang, M. D.; Holme, T. A. J. Chem. Educ. 1992, 69, 770–773. 4. Lunte, C. E.; Wheeler, J. F.; Heineman, W. R. Analyst 1988, 113, 95–98. 5. Martin, K.; Dotson, M.; Litterer, M.; Hanks, T.; Veas, C. Synth. Metals 1996, 78, 161–168.
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