In the Laboratory
Minimizing Hazardous Waste in the Undergraduate Analytical Laboratory: A Microcell for Electrochemistry Eric J. Olson and Philippe Buhlmann* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 *
[email protected] Working toward greener experiments is imperative for responsible chemistry instructors (1, 2), but the reduction of waste in analytical teaching laboratories is often overlooked (3). For example, voltammetry experiments typically require tens of milliliters to accommodate the working, reference, and auxiliary electrodes (4, 5). This results in the consumption of significant volumes of solvent, analyte, and electrolyte, especially in large classes. Small-volume adapters designed for use in teaching laboratories are commercially available, but their use only reduces samples to approximately 2 mL (6). Moreover, typical microcells are either difficult to assemble or require the use of microfabrication techniques (7, 8), making electrode cleaning for repeated use complicated. Therefore, we developed the readily reusable but attractively simple microcell shown in Figure 1 and have used it in our laboratory for undergraduate research with great success. A schematic and a detailed description of the cell are available in the supporting information. The microcell consists of a 3 cm segment of 12 mm i.d. glass tubing and a poly(chlorotrifluoroethylene) top and bottom cap with four and one hole, respectively. Inserted through the top cap is a commercial 10 μm diameter platinum microelectrode, which serves as the working electrode.1 Also inserted into the sample solution through the top cap is a reference electrode consisting of a silver chloride-coated silver wire inside a Teflon tube that is plugged at the lower end with tightly packed cotton and filled
with 1 M KCl. The remaining two holes in the top cap are reserved for purging the sample of oxygen using a stream of nitrogen or argon gas. Purging is optional and is typically not necessary when analytes of medium or high concentrations are measured. A standard Au disk macroelectrode is used as the auxiliary electrode and inserted through the bottom cap of the cell. All junctions are sealed with Viton O-rings. Using this cell, voltammograms were readily obtained with sample volumes as small as 0.20 mL, at which point the working and auxiliary electrodes are separated by approximately 2 mm.2 Much smaller volumes were found to be possible but rather impractical, as the samples did not form a layer of uniform thickness at the bottom of the cell. As shown in the supporting information, a typical example of a voltammogram of aqueous potassium ferrocyanide with 100 mM potassium chloride as background electrolyte obtained with this cell exhibits exactly the same features as observed with large sample cells. A common example of a nonaqueous system often used in undergraduate laboratories to teach principles of voltammetry is ferrocene in acetonitrile (9). We tested our cell for use in this system and obtained ideal voltammograms (see the supporting information). Note, however, that measurements in organic solvents require the use of O-rings that will not swell in the chosen solvent. Conclusion Our cell design reduces the consumption of sample to approximately 0.20 mL from the several milliliters that is common for typical cells. This not only reduces the quantity of hazardous waste, but it also reduces the cost of reagents. The latter should be particularly useful for experiments involving enzymes and other biological materials. Acknowledgment The authors would like to thank Peter Ness of the University of Minnesota Physics Machine Shop for his assistance with designing the cell. This work was supported by the National Science Foundation (EXP-SA 0730437). Notes
Figure 1. Three photographs of the microcell: (A) Platinum working microelectrode; (B) Ag/AgCl reference electrode; (C) top cap; (D) glass cell body; (E) bottom cap; (F) Au disk auxiliary.
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1. A previous article in this Journal describes the construction and polishing of microelectrodes, along with an explanation of typical characteristics of the voltammograms that they provide (9). 2. No thin-layer effects are observed under these conditions. Note that theory predicts that, after application of a large overpotential, the analyte concentration at a distance of 0.5 mm
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Vol. 87 No. 11 November 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed100495k Published on Web 08/30/2010
In the Laboratory above a 10 μm diameter microelectrode only falls to 99% of the starting value even for very long experimental times (see the supporting information).
Literature Cited 1. Hjeresen, D. L.; Schutt, D. L.; Boese, J. M. J. Chem. Educ. 2000, 77, 1543–1547. 2. Marteel-Parrish, A. E. J. Chem. Educ. 2007, 84, 245–247. 3. Anastas, P. T.; Warner, J. C., Green Chemistry: Theory and Practice, New York: Oxford University Press, 1998. 4. Mabbott, G. A. J. Chem. Educ. 1983, 60, 697–702. 5. Van Benschoten, J. J.; Lewis, J. Y.; Heineman, W. R.; Roston, D. A.; Kissinger, P. T. J. Chem. Educ. 1983, 60, 702–706.
r 2010 American Chemical Society and Division of Chemical Education, Inc.
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6. Pine Research Instrumentation Home Page. http://www.pineinst. com/echem/index.asp (accessed September 17, 2009). 7. Ferreira, H. E. A.; Daniel, D.; Bertotti, M.; Richter, E. M. J. Braz. Chem. Soc. 2008, 19, 1538–1545. 8. Tur'yan, Y. I. Talanta 1997, 44, 1–13. 9. Ching, S.; Dudek, R.; Tabet, E. J. Chem. Educ. 1994, 71, 602–605.
Supporting Information Available A detailed discussion of the cell design including schematics and machining dimensions; a procedure for electrode polishing; hazards associated with the chemicals used in our experiments; representative cyclic voltammograms of ferrocyanide in water and of ferrocene in acetonitrile; a discussion of the current and concentration profiles at microelectrodes. This material is available via the Internet at http://pubs.acs.org.
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