In the Laboratory
Do New Pennies Lose Their Shells? Hypothesis Testing in the Sophomore Analytical Chemistry Laboratory Richard J. Stolzberg Department of Chemistry and Biochemistry, University of Alaska Fairbanks, Fairbanks, AK 99775-6160
The penny has little buying power, but it has value in chemical instruction. Ira Remsen’s story of “nitric acid acting upon copper” is used in an “exocharmic” demonstration (1). The differences in composition, mass, and copper content of older (pre-1982) and newer (post-1982) pennies have been examined in introductory laboratory experiments (2– 4 ). An experiment for the determination of the shell thickness of copper-clad cents has been described (5). The “golden penny” demonstration has a long and chemically interesting history (6, 7 ). The Federal Reserve Bank of Minneapolis has a Web page for the penny (8). When we changed the sophomore analytical chemistry laboratory to a project-oriented group-activity format, the lure of penny chemistry was irresistible. We thought that the determination of copper content per penny for the post-1982 mint dates could be used as an introduction to the overall analytical process (9) and as an example of a dynamic chemical system. Our goals were to introduce students to techniques of quantitative chemical manipulations, experimental design (randomization, duplication, calibration), and statistical evaluation of trend data. We suggested that students test the following hypothesis: The copper shell of post-1982 pennies wears away at a measurable rate, so the copper content per penny decreases as a penny ages. Thus, the slope of the regression of mass of copper per penny versus year of minting would be positive. To test the hypothesis, students sampled, dissolved, and diluted ten pennies with a wide range of minting dates. They analyzed the resulting solutions by flame atomic absorption spectrometry. Students have reported two trends in their data that do not support the original hypothesis. Three sets of pennies sampled from a bag kept in the chemistry labs had a negative slope of the regression of mass of copper per penny versus year of minting. That is, the mass of copper per penny increases with the age of the penny in the sample. Two sets of pennies sampled from general circulation showed a strong positive correlation between copper content and penny mass. The third set taken from general circulation had a slope of the regression of mass of copper versus year of minting that was indistinguishable from zero. The original hypothesis was disproved, and two new hypotheses were suggested by students. Some students were taken aback. They did not believe their results because they did not get the “right answer”. One group wrote in the final paragraph of their report: “Therefore common sense dictates to reject our data and to conclude that although we found a quantifiable trend, the trend does not fit with logic and does not represent a real process.” Many chemical educators have commented on the need to change from “verification labs” to something more related to scientific investigation (10, 11). At least some of these students, who have just completed a year of general chemistry, give evidence that Ditzler, Ricci, and Lamba are correct.
Experimental Procedure For a prelab assignment, students were given three tasks. 1. Determine the mass of copper in a post-1982 penny using information available on the World Wide Web. A Boolean search for penny AND money AND small change gives a useful Web page (8). 2. Calculate the volume of the “initial penny solution” (see below) that should be taken and diluted to 100 mL to give a final copper concentration of 2.5 ppm. 3. Calculate the volumes of 100 ppm Cu standard that should be diluted to 100 mL to give working AA standards of 0, 1, 2, 3, 4, and 5 ppm Cu.
CAUTION: The strongly oxidizing nature of concentrated (16 M) nitric acid and 6 M nitric acid is stressed in the lab handout and prelab discussion. The toxicity of oxides of nitrogen is noted in the safety section of the lab handout. All work with nitric acid must be done in the hood while wearing goggles and rubber gloves. Students worked cooperatively in groups of four, with a “Supervisor”, an “Instrument Specialist”, and two “Bench Chemists”, as suggested by John Walters (12). A summary of the experimental procedure is given below. 1. Remove copper contamination from the interior of volumetric glassware by soaking for 10 minutes with 0.1 M HNO3. 2. Prepare primary standard 1000 ppm Cu using unoxidized Cu foil and 6M nitric acid. Do an accurate tenfold dilution to make 100 ppm Cu. 3. Prepare 100-mL portions of 0, 1, 2, 3, 4, and 5 ppm Cu by transferring 100 ppm Cu with a previously calibrated variable volume micropipet (Rainin P-5000) to Class A volumetric flasks and diluting with reverse osmosis (RO) water. 4. Choose, note the date, and weigh ten pennies at random. (In 1996, a bag of pennies kept by the stockroom clerk was used. In 1997, pennies were obtained from a commercial bank. Some preliminary culling of the abundant 1995 through 1997 cents was made so that there was a nearly equal distribution of minting dates.) 5. Work in a hood. Wear rubber gloves and goggles. Place each penny in a 125-mL Erlenmeyer flask, and add 10 mL of 16 M nitric acid. If the sample is not completely dissolved in 20 minutes, add another milliliter or two of concentrated nitric acid. Quantitatively transfer the solution to a 100-mL volumetric flask and dilute to volume with RO water. These are the “original penny solutions”. 6. Dilute each original penny solution to get approximately 2.5 ppm Cu by taking the appropriate-sized aliquot (0.41 mL) and diluting to a final volume of 100 mL in a class A volumetric flask. (Students use a variable volume micropipet [Rainin P-1000] and calibrate the delivered volume at a setting of 0.41 mL.) 7. Make duplicate dilutions of five (of the total of ten) of the original penny solutions using the same method as in the previous step.
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In the Laboratory 8. Obtain an aqueous unknown and make duplicate 250fold dilutions. 9. Transfer portions of each of the 15 samples, 2 unknowns, and the 6 standards to labeled 15-mL plastic test tubes that fit the autosampler of the atomic absorption spectrometer (Perkin Elmer AS-90). Place the tubes into the autosampler in random order. 10. Adjust the atomic absorption spectrometer (Perkin Elmer 3000, WinLab software) according to supplied instructions and make duplicate absorbance measurements for each of the 23 tubes. Store the data on disk and reformat it so it can be read as an ASCII file. 11. Transfer the data set to the Excel spreadsheet, sort into standards and penny samples, and determine solution concentrations and uncertainties using a locally produced calibration template cal394a.xls (13). Then calculate the mass of copper per penny.
Student Results During 1996, when pennies were taken from a laboratory collection, all three groups saw the same unexpected results. The slope of the regression of mass of copper versus year of minting is negative (Table 1). This is opposite the trend predicted in the initial hypothesis. Regression of the combined data set, which I performed, corroborates this observation, with a more convincing level of probability. Figure 1 shows the entire data set graphically. The sampled pennies appear to gain approximately 2 mg of copper per year since minting. There was some suspected contamination in a modest fraction of the sample as the result of insufficient warning to rinse the reused volumetric flasks well. Nonetheless, the interpretation of the entire data set is the same whether all 44 data are used or only 40 data are used. For pennies stored in a bag for a number of years, the slope is ᎑2 mg Cu/year and highly significant. During 1997, when the pennies were taken from general circulation, student results were more varied, but equally interesting (Fig. 2). One group (B) decided that a simple linear model was inappropriate. Their 1983 and 1984 pennies were copper rich. Their 1987–1997 pennies showed no time trend. The second group (S) saw results similar to those observed in 1996, with a slope of ᎑1.7 mg Cu/year. Both groups made an unexpected observation. They reported a significant positive correlation between copper content and penny weight and suggested this as a good alternative hypothesis. When two years of copper content and penny weight data are combined, there is a strong positive correlation (Fig. 3). The third group (C) observed no correlation between minting year and copper content. Duplicate data (Table 2) indicate that a modest percentage of determinations is probably biased. In this case, two of 15 are outliers—one high and one low. The relative standard deviation is 3.5%, using 13 pairs of data (the two suspect data pairs have not been included.) Discussion The truth may be more complicated than a linear change in copper content. The analyses suggest that pennies minted from 1983 to 1984 may be considerably more variable than pennies minted after that time. They generally have a higher copper content than the newer pennies, and some are much heavier than expected. There are at least two possible causes for the observed results—minting procedures and transport of copper when pennies are circulated. Perhaps the U.S. Mint 1454
Figure 1. Observed mass of copper per penny vs year of minting, 1996 data. Pennies were taken from a storage bag. When duplicate dilutions were made, the average value is reported. The line is linear regression best fit for all 44 data.
Figure 2. Observed mass of copper per penny vs year of minting, 1997 data. Pennies were taken from general circulation. When duplicate dilutions were made, the average value is reported.
Mass Penny / g Figure 3. Observed mass of copper per penny (mg) vs penny weight (g), both years. The linear best fit slope is 340 mg Cu/g penny; standard error = 80 mg Cu/g penny.
improved the quality control of their minting process in the first two years of production. The result has been a smaller amount of variability among pennies and the ability to produce a thinner and lighter copper shell. Older, used pennies are visibly different from new pennies, indicative of the formation of copper compounds on the surface. Copper is relatively soft (2.5–3 on the Moh scale). Copper compounds that might be present on a penny are generally harder than elemental copper.
Journal of Chemical Education • Vol. 75 No. 11 November 1998 • JChemEd.chem.wisc.edu
In the Laboratory Table 1. Regression Parameters for Mass of Copper per Penny vs Year of Minting, 1996 Data Slope ± 95% Confidence No. of Interval/mg Cu year ᎑1 Measurements
Group AC
᎑1.3 ± 1.2
11a
G
᎑2.2 ± 2.2
14b
H
᎑2.7 ± 2.5
15
Combined data all groups
᎑2.2 ± 1.4
44c
NOTE: Pennies sampled from storage bag. aOne sample lost. Three data eliminated when calculating regression owing to possibly contaminated glassware. bOne datum eliminated owing to possibly contaminated glassware. cSuspect data were included in regression.
Table 2. Duplicate Analysis Results Based on Same Original Copper Solution, 1997 Data Group
Mint Year
Replicate 1/ mg Cu penny᎑1
Replicate 2/ mg Cu penny᎑1
B
1983
97
102
B
1987
71
73
B
1992
63
68
B
1995
76
75
B
1997
74
74
C
1983
65
66
C
1988
69
62
C
1991
63
48a
C
1994
72
72
C
1996
66
69
S
1985
96
91
S
1990
77
76
S
1992
84
124b
S
1993
78
79
S
1995
64
59
aSuspect
biased low. bSuspect biased high.
Malachite (Cu2(OH)2CO3) and azurite (Cu3(OH)2(CO3)2) have a hardness of 3.5–4 and atacamite (Cu2(OH)3Cl) has a hardness of 3–3.5 (14 ). Harder, older pennies may erode the copper surface of newer pennies, when pennies rub against one another in pockets, bags, and purses. The erosion process could remove copper from new, untarnished pennies. Some of the eroded copper could be incorporated into the older pennies. These observations are consistent with recently published measurements of the mass of circulated pennies minted from 1982 to 1992 (15). The linear regression of the mass of penny versus age of penny was reported to be ᎑2.8 mg/ year (standard deviation 0.4 mg/year). That is, older pennies are heavier than newer pennies, owing to the formation of copper compounds and the physical transport of copper from new pennies to older pennies. These student results appear to be accurate. Although the Treasury Department Web page (16 ) indicates that there should be 62.5 mg of copper in each penny, a chemical analysis (5) reports 68 mg. The median content of the pennies in this study is 71 mg. Furthermore, during 1997, two of the groups analyzed an aqueous unknown containing 1002 ppm Cu. They reported 958 ppm (4% low) and 1012 ppm (1% high), indicating acceptable accuracy in dilutions, standard preparation, and AA measurements.
The proposed hypothesis is entirely reasonable, yet experiments show it to be untrue. Such is the case in many scientific investigations, particularly in the initial stages of a study. When experienced scientists test hypotheses, they expect that some will be found invalid and will be disproved. For many students, some unlearning is required before they are comfortable with experimental results that are contrary to their perception of the true state of nature. This experiment gives students an opportunity to observe an unexpected result and to propose an alternative hypothesis. Some students face the challenge well by being excited about the unexpected result and by suggesting an alternative hypothesis. We should maximize the chances that our students will observe unexpected, yet interpretable, results. We must give them the opportunity to realize that “although the conjectured state of nature may be false or at least inexact, the data themselves are generated by the true state of nature” (17). Future Studies The population studied in this experiment, post-1982 cents, will allow iterative experiments for successive classes. Next year, weighed (60–80 mg) pieces of clean copper wire will be used to assess the accuracy of the overall process. Students will compare 1983 and 1984 pennies with newer pennies and they will compare pennies from storage with pennies from general circulation. They will determine the copper content of pennies that are especially heavy. This population of pennies is an excellent example of a dynamic chemical system. Acknowledgments The atomic absorption spectrometer was purchased with a matching grant from the National Science Foundation Instrumentation and Laboratory Improvement Grant, DUE-9452248. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
Ramette, R. W. J. Chem. Educ. 1980, 57, 68. Richardson, T. H. J. Chem. Educ. 1991, 68, 310. Ricci, R. W.; Ditzler, M. A. J. Chem. Educ. 1991, 68, 228. Mauldin, R. F. J. Chem. Educ. 1997, 74, 952 Vanselow, C. H.; Forrester, S. R. J. Chem. Educ. 1993, 70, 1023. Szczepankiewicz, S. H.; Bieron, J. F.; Kozik, M. J. Chem. Educ. 1995, 72, 386. Dominic, S. D. J. Chem. Educ. 1995, 72, 389. Small Change; http://woodrow.mpls.frb.fed.us/pubs/region/ reg923c.html (accessed June 1998). Atkinson, G. F. J. Chem. Educ. 1982, 59, 201. Ditzler, M. A.; Ricci, R. W. J. Chem. Educ. 1994, 71, 685. Lamba, R. S. J. Chem. Educ. 1994, 71, 1073. Walters, J. P.; Anal. Chem. 1991, 63, 971A; Anal. Chem. 1991, 63, 1077A; Anal. Chem. 1991, 63, 1179A; The template is “cal394a.xls” at URL http://www.uaf.edu/chem/ 394A/software.html (accessed June 1998). CRC Handbook of Chemistry and Physics, 66th ed.; Weast, C. R., Ed.; CRC: Boca Raton, FL, 1985; p B199. Harris, D. C. Exploring Chemical Analysis; Freeman: New York, 1997; p 385. Circulating Coins: Specifications; URL http://www.usmint.gov/ circulating/specifications.cfm (accessed Aug 1998). Box, G. E. P.; Hunter, W. G.; Hunter, J. S. Statistics for Experimenters: An Introduction to Design, Data Analysis, and Model Building, Wiley: New York, 1978; p 5.
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