In the Laboratory
The Dog Ate My Homework: A Cooperative Learning Project W for Instrumental Analysis Leanna C. Giancarlo* and Kelli M. Slunt Department of Chemistry, Mary Washington College, Fredericksburg, VA 22401; *
[email protected] Much attention has recently been devoted to curriculum reform in the instructional chemistry laboratory, including the incorporation of project-based laboratories and cooperative learning experiences (1–5). These laboratories center around a cleverly designed problem or situation and encourage students, working within a group, to formulate and then implement a proposed solution. While many of these laboratories are being written for general chemistry, fewer have been reported for more advanced laboratories including analytical chemistry and instrumental methods of analysis (1–5). In addition, students frequently complain that the laboratory experiments even in these more advanced courses do not have real-life applications or that the experiments are not derived from “real” situations. Here, we describe a real-life experience that provides an ideal opportunity to combine problem-solving skills, critical thinking, and collaborative learning in the context of a realistic instrumental analysis laboratory. The situation (based on actual events) for this instrumental methods project is the following: A group of chemists is approached for consultation. A puppy has swallowed the metal knob from a kitchen cabinet, and the owner is uncertain as to the toxicity of the partially digested, yet recovered, object. The group is informed that there is a significant possibility that the metal is a lead alloy, which may be toxic to the young dog if left untreated; the problem lies in the fact that chelation therapy is very expensive and the owner would like to know quickly if it is, in fact, necessary. The group must devise two means (for confirmation) to identify the major components of the knob both qualitatively and semiquantitatively. Materials This experiment utilizes an odd-shaped metal piece that conceivably could have been ingested by a large dog. The instructor can obtain odd-shaped shot pieces from a local hardware store for this purpose. It is important to have an irregular size so that volume determination is not obvious based on geometry. It is equally important that the metal piece be a fairly large size, such that the piece does not readily fit into a graduated cylinder smaller than 500 mL. The remaining equipment required for the experiment is standard glassware, an analytical balance, deionized water, concentrated nitric acid, and an inductively coupled plasma (ICP) or atomic absorption/emission (AA/AE) spectrometer. Hazards Appropriate safety procedures for handling and using concentrated acids (here, 15.7 M HNO3; CAS #7697-37-2) for the dissolution of the metal object must be followed.
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Proper precautions include, but are not limited to, the use of safety goggles and nitrile gloves; the dissolution reaction should also be conducted in a fume hood. In this experiment the reaction of the metal knob (a Zn–Al alloy) with concentrated nitric acid occurred vigorously, producing NO2 fumes and heat. Methodology One method that the students can use to elucidate the components of the metal knob is to determine the density of the metal object. The mass can be easily obtained via an analytic balance to a high degree of precision (⫾0.0001 g). Since the object has an irregular shape, however, computation of the volume by the students using a mathematical calculation will be difficult, imprecise, and tedious. In addition, the object is too large to fit in a graduated cylinder small enough to provide precision similar to the balance. Instead, the students may use the mass of water displaced by insertion of the pre-weighed object into a beaker full of water to obtain the volume and therefore the density. A simple method to accomplish this successfully is the use of nested beakers. The water is displaced from a central beaker into a massed outer beaker. In order to avoid further spillage of the water from the center beaker into the massed beaker during separation prior to weighing, some of the water in the center beaker is carefully removed using a Pasteur pipet. A second method to accomplish the volume determination is to cut the object into smaller pieces that will easily fit into a more precise volumetric measuring device; this method also catalyzes a discussion with the students of proper sampling techniques. Once the density of the metal object is calculated, the students can compare the density to the densities of known alloys and pure metals in the CRC Handbook, as shown in Table 1 (6, 7). Another method to determine the composition of the metal knob is to use an analytical spectroscopic technique such as ICP or AA/AE. The students should carefully dissolve the metal sample in concentrated nitric acid in a vented fume hood (due to the release of NO2 (g) and vigorous exothermic reaction). After running standard metal samples of known concentration, the unknown metal–HNO3 solution is diluted and analyzed. The students can then compare their findings with the alloy data from the density experiment. Results A knob weighing 43.2491 g was analyzed using the above-mentioned methods. This metal object displaced 7.3901 g (error of ⫹0.05 g) of water from the central nested beaker. The volume of the knob was calculated using the density of water at the measured temperature, and the mass and this vol-
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In the Laboratory
Nickel (ASTM B160, 161, 162)
8.89
composed of approximately 56% Zn and 44% Al with some other trace metals. Comparison of these percent compositions with the results of the density experiments reveals good agreement: using the density of the pure metals and the percentages derived from ICP spectroscopy, one obtains a density of the sample of approximately 5.2 g cm᎑3 (0.56 × 7.14 g cm᎑3 ⫹ 0.44 × 2.7 g cm᎑3). Based on these results, it was concluded that the dog did not require chelation therapy due to the negligible lead content in the metal object. This practical experiment can be used in an instrumental analysis or analytical course. The students can work cooperatively in groups of three or four students to determine the identity of the metal using two different methods. This paper only describes two possible ways to solve the problem. Students may develop other more creative methods for determining the composition of the metal sample. Comparison between results using assorted shapes (derived from the same materials) and analyzed with varied methods can act as a starting point for a discussion of the advantages and disadvantages of different analytical or instrumental techniques.
Cast gray iron (ASTM A48-48, Class 25)
7.2
w
Malleable Fe (ASTM A47)
7.32
91% Al, 9% Zn
2.80
95% Zn, 5% Al
6.80
67% Pb, 33% Sn
9.4
Table 1. Densities of Commercial Metals, Elements, and Alloysa Description of Metal, Element, or Alloy
Density (g/cm3)
Stainless steel (type 304)
8.04
Al alloy, 3003 rolled (ASTM B221)
2.73
Al alloy, 2017 annealed (ASTM B221)
2.8
Al alloy, 380 (ASTM SC84B)
2.7
Cu
8.91
Zn (ASTM B69)
7.14
Chemical lead: 99.93% Pb, 0.08% Cu
11.35
Antimonial lead (hard lead): 92–94% Pb, remainder Sb
10.9
a
Supplemental Material
A student handout and instructor’s notes are available in this issue of JCE Online. Acknowledgments The authors would like to thank Judith Kyle, DVM for her assistance in providing background details regarding the laboratory exercise. We would also like to thank Thomas G. Digges, Jr. for presenting us with this interesting problem.
Data from refs 6 and 7.
Literature Cited ume (7.41 cm3) yielded a density of 5.83 g/cm3. According to the CRC Handbook (6, 7), the knob is not composed of a pure metal; for example, Al, Zn, Pb, and Fe have densities of approximately 2.7, 7.14, 11, and 7.2 g cm᎑3, respectively (Table 1). One can therefore conclude that the object is an alloy, composed of a “heavy” (e.g., Zn, Fe, Pb) and a “light” metal (e.g., Al), and further analysis must be performed to identify the metallic components. The knob was then partially dissolved in concentrated nitric acid; 33.5266 grams remained. A portion of the resulting acidic solution was diluted in a 1:10 ratio using deionized water. Analysis using an argon ICP spectrometer determined that the knob contained ~0 ppm Pb, 1101 ppm Al, 1440 ppm Zn, and less than 10 ppm Fe. Therefore, the metal object is
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1. Emry, R.; Curtright, R. D.; Wright, J.; Markwell, J. J. Chem. Educ. 2000, 77, 1323–1324. 2. Juhl, L.; Yearsley, K.; Silva, A. J. J. Chem. Educ. 1997, 74, 1431–1433. 3. Cooper, M. Cooperative Chemistry Laboratory Manual, 2nd ed.; McGraw-Hill: New York, 2003. 4. O’Hara, P. B.; Sanborn, J. A.; Howard, M. J. Chem. Educ. 1999, 76, 1673–1677. 5. Wright, J. C. J. Chem. Educ. 1996, 73, 828–832. 6. Weast, R., Ed. CRC Handbook of Chemistry and Physics, 68th ed.; Chemical Rubber Co.: Cleveland, OH, 1987. 7. Hodgman, C. D., Ed. Handbook of Chemistry and Physics, 38th ed.; Chemical Rubber Publishing Co.: Cleveland, OH, 1956.
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