In the Laboratory
Circuit Board Analysis for Lead by Atomic Absorption Spectroscopy in a Course for Nonscience Majors
W
Jeffrey D. Weidenhamer Department of Chemistry, Ashland University, Ashland, OH 44805;
[email protected] Recent reports have highlighted the growing problem of electronic waste disposal. A recent article in Chemical and Engineering News (1) noted that an estimated 100 million obsolete computers, monitors, and televisions are disposed of annually in the United States. Greenpeace (2) and the Basel Action Network (3) have drawn attention to the large quantity of old electronic components that are being shipped from developed nations to countries in East Asia and Africa where environmental regulations are less strict or enforcement much weaker. Minimal recycling of this waste takes place, and numerous instances of environmental contamination and hazards to workers have been documented. Electronic waste contains several hazardous components, including heavy metals such as lead and cadmium and organic compounds such as brominated flame retardants (1–3). With the elimination of lead from paint and gasoline in the United States, waste electronics have become an important source of environmental lead contamination. Students readily relate to the consumer products that generate electronic waste. By the time they reach college, most students have had one or two cell phones, one or more computers, and numerous other electronic products that they have had to replace. This experiment was developed for a nonscience majors course, though it could readily be adapted to general chemistry or analytical chemistry courses. Several years ago this university implemented a new general education curriculum emphasizing courses that are focused on particular topics rather than being broad surveys of knowledge areas. The chemistry department has developed several such courses, and a strong effort has been made to develop experiments in which students analyze samples that are of direct interest to them. Most experiments in this course involve the use of atomic absorption spectroscopy (AAS). Advanced techniques such as AAS are typically reserved for experiments in science majors courses, but some experiments for nonscience majors have been reported (4). Experiments that we have used include the analysis of Pb in paint samples by AAS (5 ), the determination of leachable Pb from ceramics by vinegar extraction of ceramic ware (6), and analysis of water samples by graphite furnace AAS. Other experiments reported in this Journal (7, 8) could be adapted for use in a nonmajors course with appropriate modification. This experiment was developed (i) to demonstrate the concept of atomic absorption (AA) by qualitative screening of circuit board materials and (ii) to demonstrate the use of AA for quantitative analysis of environmental samples. Qualitative Demonstration of Lead in Electronic Waste The screening technique reported by Brouwer (9) was used to demonstrate the presence of lead in electronic circuit board solders. For the initial experiment, a used cell phone circuit board and two used circuit boards from labowww.JCE.DivCHED.org
•
ratory instruments were used. With the AA (Varian 220 FS) set to monitor the lead lamp energy (giving a screen display of the intensity of light coming through the instrument), the looped tip of an 18-gauge nichrome wire inserted into a cork handle was placed at the base of the air-acetylene flame. Clean wire caused no effect on the signal, but wire that had been gently rubbed on pure lead strips or electronic components with lead-based solders gave an immediate and strong reduction of the lead lamp signal. This demonstration followed an experiment in which students conducted flame tests on various elements and provided an excellent way for students to see some of the principles involved in AAS. Quantitative Determination of Lead by AAS Pieces (approximately 5 × 5-cm, 10–15 g) of the same circuit boards that were screened qualitatively were cut with tin snips and analyzed following digestion in 90 mL of gently boiling aqua regia (3:1 HCl:HNO3) for two hours. After cooling, samples were filtered and diluted to an initial volume of 250 mL. To obtain solutions in the proper concentration range for AA analysis, each of these solutions was diluted 200-fold. A second extraction of each sample was performed to check on the completeness of the digestion procedure. These samples were filtered and diluted to a volume of 250 mL and analyzed directly. Samples were analyzed at 217.0 nm by (i) comparison to a calibration line generated by lead standards (0–10 mg/L) and (ii) by standard addition of 0.100 mg Pb (100 µL of a 1 µg兾µL Pb standard) to 20.0 mL of each solution. Each student received a printout of the absorbance readings of all samples. Data were analyzed following the lab using the spreadsheet program Excel. This lab required three, two-hour lab periods: (i) one for digestion of the samples and preparation of the solutions (students carry out the digestion and prepare the diluted standards; the instructor finishes filtration and dilution of the circuit board digests prior to the next period), (ii) one for qualitative and quantitative analysis of the samples (done in small groups with the instructor for 40–45 minutes), and (iii) a final period for review of the calculations and instruction in the use of Excel for data analysis. The value of the experiment could be enhanced by using an additional lab period for students to do the filtrations and all dilutions themselves. Hazards Aqua regia is extremely corrosive and all pouring and digestion on a hot plate must be done in a fume hood as heated aqua regia generates toxic nitrogen dioxide fumes. The acid digests generated in this experiment will contain heavy metals such as lead and cadmium. Dilute solutions of lead nitrate used to prepare standards are toxic and gloves should be worn to avoid contact with skin. Students should be reminded that dilute (1%) nitric acid used in the preparation
Vol. 84 No. 7 July 2007
•
Journal of Chemical Education
1165
In the Laboratory Table 1. Lead Content of Circuit Boards Based on Single Aqua Regia Extraction Pb (ppm x 104 ) Circuit Board Source
External Standard
Standard Addition
Cell phone
1.10 ± 0.03
1.15 ± 0.06
NMR
2.60 ± 0.03
2.81 ± 0.06
Ion chromatograph
1.79 ± 0.06
1.97 ± 0.25
NOTE: Values are the average of four replicate analyses with a 95% confidence interval.
of standards is still corrosive to the eyes, and safety glasses must be worn. The AA spectrophotometer generates exhaust gases that should be properly vented. UV-absorbing safety glasses should be used. Results The solutions obtained after acid digestion of the circuit boards were varying shades of green owing to the high copper content (as much as 10–40% by weight; ref 10) of the electronics. This was helpful in explaining to students the importance of checking the analysis by standard addition owing to the possible interference by other elements present in the digests. The visual appearance of the circuit boards before and after the aqua regia digestion was quite striking. Results of the analysis are shown in Table 1. Circuit boards were found to contain 11–26,000 ppm Pb by comparison to the external calibration line and a slightly higher quantity of Pb by standard addition. The quantities of Pb found are in agreement with values reported by Ernst and co-workers (10) for circuit board materials. Based on the total quantity of lead obtained by two successive extractions, a single digestion was effective in recovering the majority (97%) of Pb from the cell phone circuit board. Similar results were obtained for the NMR circuit board, while the first extraction of the ion chromatograph circuit board recovered only 73% of the total Pb obtained. Conclusions This experiment provides an effective demonstration of the lead content in consumer products that students can readily identify with. For nonmajors, the lab provides a significant introduction to major quantitative techniques of environmental analysis. While sophisticated instrumentation is
not usually a part of nonmajors laboratories, use of techniques such as AAS can help students to not only understand the scientific method, but also understand how scientists use equipment to answer environmental questions. Students who go on to become policymakers or company executives may need to make decisions based on sophisticated analyses later on in their life and having an appreciation for how this equipment is used and potential errors could be invaluable. The experiment can readily be extended to projects for science majors in the general chemistry or analytical chemistry courses. Circuit boards contain a number of other metals (Cd, Cr, Cu, Ni, Sb) that could also analyzed by AA. The experiment can also easily be connected to discussions of the potential environmental hazards of unsafe disposal of used electronic equipment and discussion of the various technical solutions, such as the elimination of heavy metals in circuit boards, and public policy options being considered to deal with this growing problem. W
Supplemental Material
Instructions for the students and detailed notes for the instructor are available in this issue of JCE Online. Acknowledgment The National Science Foundation (DUE 9952552) and Ashland University provided financial support for the acquisition of the Varian 220 AA spectrophotometer used to develop this experiment. Literature Cited 1. Hileman, B. Chem. Eng. News 2006, 84 (1), 18–21. 2. Greenpeace International. Hi-Tech: Highly toxic. http:// www.greenpeace.org/seasia/en/campaigns/toxics/hi-tech-highlytoxic (accessed Feb 2007). 3. Basel Action Network. The Digital Dump. http://www.ban.org/ BANreports/10-24-05/documents/TheDigitalDump.pdf (accessed Feb 2007). 4. Kostecka, K. J. Chem. Educ. 2000, 77, 1321–1323. 5. Markow, P. J. Chem. Educ. 1996, 73, 178–179. 6. Sheets, R. Sci. Total Environ. 1997, 197, 167–175. 7. Mabury, S.; Mathers, D.; Ellis, D.; Lee, P.; Marsella, A. J. Chem. Educ. 2000, 77, 1611–1612. 8. Correia, P.; Oliviera, P. J. Chem. Educ. 2004, 81, 1174–1176. 9. Brouwer, B. J. Chem. Educ. 2005, 82, 611–612. 10. Ernst, T.; Popp, R.; Wolf, M.; van Eldik, R. Anal. Bioanal. Chem. 2003, 375, 805–814.
Stages in this experiment are featured on the cover of this issue. See page 1075 of the table of contents for a description of the cover.
1166
Journal of Chemical Education
•
Vol. 84 No. 7 July 2007
•
www.JCE.DivCHED.org
