In the Laboratory edited by
NSF Highlights
Susan H. Hixson
Projects Supported by the NSF Division of Undergraduate Education
National Science Foundation Arlington, VA 2230
Curtis T. Sears, Jr. Georgia State University Atlanta, GA 30303
Using Microwave Sample Decomposition in Undergraduate Analytical Chemistry R. Griff Freeman and David L. McCurdy Division of Science, Truman State University, Kirksville, MO 63501
A shortcoming of many undergraduate analytical chemistry classes is that students receive little exposure to sample preparation. Considered the most error-prone and difficult part of an analysis, sample preparation typically requires up to 61% of the analysis time, but is often not emphasized in academic settings precisely because of time requirements (1). Microwave-induced heating can shorten the time of sample preparation and can also be used with less operator intervention, improved safety, greater control over the reaction conditions, minimal sample contamination and loss, and reduced reagent consumption. The objective of this project is to introduce this modern method of sample preparation into the analytical chemistry curriculum at Truman State University; primarily in the sophomore-level quantitative analysis class, but also as part of advanced classes and undergraduate research. In this paper, we highlight three experiments in quantitative analysis in which microwave sample decomposition has been incorporated. They are the gravimetric determination of nickel, the determination of sodium in snack foods using flame atomic emission spectrometry, and the Kjeldahl nitrogen determination. The gravimetric determination of nickel by precipitation with dimethylglyoxime (DMG) is a common experiment in many teaching laboratories. In this determination, nickel ore samples are dried, weighed, and decomposed by reaction with strong acids. The Ni 2+ in the resultant solution is then reacted with DMG to form an insoluble complex that is filtered, dried, and weighed. Our students have three 4-hour lab periods to complete this experiment. Approximately one class period is spent decomposing the sample on a hot-plate. The amount of time spent on this experiment is substantial. However, in addition to learning the principles of gravimetry, our students learn the importance of careful sample manipulation and patience in the laboratory. Thus, keeping a gravimetric experiment in the curriculum is beneficial, but we would prefer that the experiment take less time. Microwave ovens serve two roles in the redesigned experiment. First, an inexpensive home microwave is used to dry the filtering crucibles (both empty and with filtered precipitate). Second, a CEM MDS-2000 Microwave Decomposition System (CEM Corporation, Matthews, NC) replaces the traditional open-vessel hot-plate decomposition with a closed-vessel microwave decomposition. Both steps increase the speed of these laborious operations, reducing the time of the experiment by one full laboratory period. A microwave
method for sample preparation in the nickel gravimetric experiment was recently published in this Journal (2). While ours is similar in many ways, there are differences that will be highlighted in a forthcoming paper (Freeman, R. G.; Morgan, R.; Sallee, D. M.; Engel, B.; McWilliams, G.; McCurdy, D. L., manuscript in preparation). A second experiment performed in our quantitative analysis lab class is the determination of sodium using flame atomic emission spectrometry. The experiment originally included the development of a calibration curve, illustrating calibration linear dynamic range, and culminated with the determination of sodium in a sample prepared by the instructor. We have developed an experiment that retains the important concepts of the existing experiment, but adds components of sampling, sample decomposition, and aspects of Quality Assurance, without exceeding one laboratory period. We also chose to heighten interest by assigning samples with which the students were intimately familiar: snack foods such as potato chips. These foods, though complex, are relatively easy to decompose and sodium is readily determined via flame atomic emission spectrometry. Since the introduction of closed-vessel microwave decomposition, sample preparation can be completed in about onehalf of a laboratory period. While samples are decomposing unattended, students can prepare working standards (learning about serial dilution), familiarize themselves with the operation of the FAES instrument, and prepare for the final dilution of the sample into the proper working range. After the FAES measurement, they compare the results of their sodium determination to Na values stated on the package and discuss what could have been done to further assure the quality of their results. The Kjeldahl nitrogen determination is the third experiment where microwave decomposition is being incorporated. It is a common analysis that many students in quantitative analysis will encounter again in their careers (3). In the standard Kjeldahl determination, the time-limiting step is a lengthy open-vessel decomposition of the protein-containing sample. Microwave decomposition provides us with a vehicle for a more rapid Kjeldahl determination. It is possible to perform the Kjeldahl decomposition step using an open-vessel microwave method in less than 11/2 hours, a considerable improvement over the usual 3–4 hours. With this increased speed, the actual nitrogen determination can be accomplished in 1 to 11/2 laboratory periods with accuracy that is equivalent to that of the classical decomposition.
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Besides the clear benefit of increased emphasis on sample preparation, the availability of microwave sample decomposition helps modernize our students’ views of “real-world” analytical chemistry and allows them to perform experiments that are too difficult, unsafe, or time-demanding if classical methods of sample preparation are used. These benefits are also being seen in formal upper-level class projects and as part of independent undergraduate research.
Literature Cited 1. Majors, R. E. In Handbook of Instrumental Techniques for Analytical Chemistry; Settle, F. A., Ed.; Prentice Hall PTR; Upper Saddle River, NJ, 1997; pp 17–54. 2. Carmosini, N.; Ghoreshy, S.; Koether, M. C. J. Chem. Educ. 1997, 74, 986–987. 3. Harris, D. C. Quantitative Chemical Analysis, 4th ed.; Freeman: New York, 1995.
Acknowledgments This work was supported by the National Science Foundation, Division of Undergraduate Education, Instrumentation and Laboratory Improvement Program, award no. 9651463 and the Truman State University Undergraduate Student Research Program. In addition, we thank undergraduate research students Rachel Morgan, David Sallee, Justin Hettick, Brian Engel and Grant McWilliams for their dedicated contributions.
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