Chemistry for Everyone edited by
Chemistry for Kids
John T. Moore Stephen F. Austin State University Nacogdoches, TX 75962
Lead-Testing Service to Elementary and Secondary Schools Using Anodic Stripping Voltammetry
David Tolar R. C Fisher School Athens, TX 75751
Amanda Goebel, Tracy Vos, Anne Louwagie, Laura Lundbohm, and Jay H. Brown* Science Department, Southwest Minnesota State University, Marshall, MN 56258,*
[email protected] The Environmental Protection Agency (EPA) estimates that 10–20% of total lead exposure in the United States occurs through the consumption of drinking water (1). The EPA notes that old copper plumbing and lead solder are likely sources of aqueous lead. Young children are particularly vulnerable to the toxic effects of lead exposure (1). Since 1998, our undergraduate chemistry club has provided a free leadtesting service to area elementary and secondary schools using anodic stripping voltammetry (ASV) (2–4). The main purpose of the project was to increase interaction between our university and various science classes throughout the region while providing a valuable community service. Sample kits containing two 100-mL Whirl-pak sample bags, an instruction sheet, and a postage-paid return box were sent to participating science teachers. Elementary school students were instructed to run two interior drinking water sources for about 5 min to flush out the lines, and then fill the bags with two cold drinking water samples (1). The water samples were returned to the chemistry club for analysis. Once the analyses were complete, chemistry club members generated final reports that included all voltammograms, standard addition calibration curves, linear regression analysis, error analysis, and concentration calculations. This information was sent to the science teachers for use in a variety of class projects. Currently, we have 28 science classes enrolled in the program. Based upon science teacher feedback, this has become a popular community service project for the region. Theory ASV is a sensitive electroanalytical technique with detection limits for aqueous lead in the low to sub-parts per billion (ppb) range (2, 3). The current EPA action limit for aqueous lead in drinking water is 15 ppb (1). Since typical ASV detection limits are below the EPA action limit, the method is suitable for this type of project (1–3). The sensitivity of ASV is obtained by preconcentrating the lead of a sample, in the form of a mercury amalgam, on the surface of a glassy-carbon working electrode using electroreduction: + Pb2 + 2 e−
Hg
Pb(Hg)
E ° vs Ag/AgCl = − 0.32 V
The amalgam is formed in situ by adding approximately 50 parts per million (ppm) aqueous mercury(II) nitrate to a sample prior to analysis (2). The concentration of lead in the resulting amalgam is much greater than in the bulk solution. The pre-concentration step is performed at a reduction po214
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tential that is well negative of E ⬚ to compensate for negative shifts owing to large concentrations of lead in the resulting amalgam and to overcome iR loss in the electrochemical cell (3). After a predetermined quantity of time, the plating process is stopped and the potential of the working electrode ramped in the positive direction. As the lead in the amalgam is re-oxidized, it is stripped back into solution resulting in current peak that is proportional to the quantity of lead in the bulk solution (2–4). Using the standard addition method, the concentration of lead in drinking water can be determined in the low to sub-ppb range (2, 3, 5). Experimental Our system consists of a Cypress Omni-101 potentiostat, a glassy-carbon 1.5-mm disk working electrode, a silver兾silver chloride reference electrode, and a platinum auxiliary electrode (Figure 1). Prior to each analysis, all electrode surfaces, a 3-mL electrochemical cell, and a magnetic stir bar were cleaned by soaking in 4 M nitric acid (Mallinckrodt; Lot 2704 KHPR) for 15 min and then rinsed with twice-distilled water. The surface of the glassy-carbon working electrode was polished using a small quantity of slurry made from aluminum oxide powder (Matheson, Coleman & Bell; 150 mesh; CB 938) and twice-distilled water. The slurry was prepared on a glass microscope slide and the electrode surface polished using a figure-eight motion for 3 min. The polished electrode was rinsed several times with twice-distilled water to remove the slurry that adhered to the electrode surface. A paper towel
power supply working electrode (1.5-mm glassycarbon disk)
cell
auxiliary electrode (Pt wire)
ammeter
reference electrode (Ag/AgCl) voltmeter Figure 1. Cypress three-electrode potentiostat system.
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Hazards The Material Safety Data Sheet (MSDS) that accompanied the certified lead standard listed the material as an acidic www.JCE.DivCHED.org
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Table 1. Anodic Stripping Voltammetry Experimental Parameters Parameter
Value
Deposit potential
᎑1000 mV
Beginning potential
᎑1000 mV
Ending potential
0 mV
Scan rate
+100 mV/sec
Sensitivity
10 µA/V
Noise filter
1 msec
Number of scans
1
4
Absolute Current / µA
0.90 µA 3
0.73 µA 2
0.49 µA
0.16 µA
1
0 -700
-600
-500
-400
-300
-200
-100
Potential vs Ag/AgCl / mV Figure 2. Experimental voltammograms. The concentrations of added lead (reading from the bottom up) were 0, 10, 20, and 30 ppb. Peak heights (in µA) are indicated to the right of each peak. The voltammograms are stacked for clarity.
least-squares fit: slope (m) = 2.5 ± 0.2 × 10ⴚ2 µA/ppb
1.0
y intercept (b) = 2.0 ± 0.6 × 10ⴚ1 µA x intercept (Xf) = −8 ± 2 ppb
0.8
Peak Height / µA
was used to dry the outside edge of the polished working electrode (6). A 1-ppm lead nitrate standard was prepared daily by dilution of a 1000-ppm certified stock (Spex Certiprep; certified at 996 ppm; Lot 7-117PB) using a 1-mL class-A transfer pipet and a 1-L class-A volumetric flask. All solutions were prepared from in-house, twice-distilled water using a glass still. ASV experiments indicated that the lead concentration of our twice-distilled water was below our detection limit (10 ppb) (7). Samples were prepared by adding 1 mL of the water submitted by the elementary or secondary school and 1 mL of an electrolyte-plating solution to the cleaned electrochemical cell containing a cleaned magnetic stir bar. The volumes were accurately measured using 1-mL class-A transfer pipets. The electrolyte-plating solution contained 100 ppm mercury(II) nitrate monohydrate (Mallinckrodt; Lot 6853N13H04), 0.2 M potassium nitrate (J. T. Baker; Lot 644120), and 10% (v兾v) nitric acid (Mallinckrodt; Lot 2704 KHPR). The polished glassy-carbon working electrode, silver兾silver chloride reference electrode, and platinum auxiliary electrode were placed in the electrochemical cell and connected to the potentiostat (Figure 1). Dissolved oxygen was removed by bubbling high-purity nitrogen gas (Praxair; Lot 4099035) that was presaturated with 0.1 M potassium nitrate (J. T. Baker; Lot 644120) through the sample for 5 min with rapid stirring. The experiment was conducted under a nitrogen blanket. This was done to avoid problems associated with dissolved oxygen (3). The sample was preconcentrated for 3 min with rapid stirring at the noted deposit potential (Table 1). The magnetic stirrer was then turned off and the plating process continued in a quiet solution for an additional 30 s. This “rest period” ensures a homogenous concentration of lead throughout the amalgam (3). The potential was then ramped in the positive direction (Table 1) and the resulting voltammogram acquired. Using a microsyringe (Hamilton; 100 µL; Model 710), 20-µL of the 1-ppm lead nitrate standard was then added to the sample. The sample was then magnetically stirred for 1 min to ensure proper mixing. A second voltammogram was then acquired using the procedure noted above. This was followed by two more 20-µL standard additions of lead nitrate to provide four voltammograms (Figure 2). Baselines were drawn with a ruler and peak heights (in µA) measured from the hand-drawn baselines up to the top of each peak (Figure 2). The peak heights were then used to construct a standard addition curve (Figure 3) (5). Since the sample contains 1-mL of drinking water and 1-mL of the electrolyte-plating solution, the concentration of lead in the tap water is twice the absolute value of the x intercept (5). Thus, the concentration of lead in the drinking water sample was 16 ppb. Propagation of uncertainty provides a relative error of 25% (5). The greatest source of error was the uncertainties associated with the measured peak heights.
0.6
0.4
0.2
0 -10
0
10
20
Concentration Added Lead
30
/ ppb
Figure 3. Standard addition curve. The solid line represents a leastsquares fit of the experimental data. The dashed line represents extrapolation to the x intercept. Each circle ( 䊉) represents a single measurement at the noted concentration.
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Chemistry for Everyone Table 2. Sample Results from the 2002–2003 Academic Years School Site
Lead Concentration (ppb)
1
12 ± 36%
2
10 ± 25%
3
16 ± 25%
4
14 ± 12%
5
11 ± 11%
lead salt solution. The noted composition was 2–5% HNO3, 0.16–1.6% Pb(NO3)2, 94–97% H2O. For reactivity, the material was listed as corrosive. The first aid section noted that the material was harmful by ingestion and may cause cancer or reproductive harm. For skin contact, rinse the affected areas with water for at least 15 min. For ingestion, get immediate medical help. This material should not be poured down the drain—dispose as heavy metal waste. The MSDS for 70% nitric acid listed the material as a concentrated acid solution. For reactivity, the material was considered a corrosive strong oxidizer. The first aid section noted that 70% nitric acid is an extreme poison. For inhalation, provide fresh air. For skin contact, thoroughly wash affected areas with water for at least 15 min. For ingestion, get immediate medical help. The fire hazard section noted that the material may react explosively with readily oxidized materials. The MSDS for potassium nitrate listed the material as a potassium salt. The noted composition was 99–100%. For reactivity, the material was listed as a severe oxidizer. The first aid section noted that for inhalation, provide fresh air. For skin contact, thoroughly wash affected areas with water for at least 15 min. For ingestion, induce vomiting. The fire hazard section listed potassium nitrate as strong oxidizer that reacts with reducing agents and may cause ignition. The MSDS for mercury(II) nitrate monohydrate listed the material as a mercury salt hydrate. The noted composition was 99–100%. For reactivity, material was listed as an oxidizer. The first aid section noted that mercury(II) nitrate monohydrate is very toxic by inhalation and may cause death. You should provide oxygen if necessary, but do not perform mouth-to-mouth. Mercury(II) nitrate monohydrate can be absorbed through the skin. For skin contact, thoroughly wash affected areas for at least 15 min with soap and water then get medical help. The material may cause kidney damage through ingestion. The fire hazard section notes that the material may produce toxic gases through thermal decomposition. Mercury(II) nitrate monohydrate reacts with reducing agents. Solutions should not be poured down the drain, but disposed as heavy metal waste. Results In a given year, most of our samples were below our detection limit of 10 ppb (7). Table 2 contains sample results that were at or above our detection limit. Public water utilities throughout our state report 90th percentile lead concentrations in the range of 2–26 ppb (8). In light of these reports, our results (Table 2) appear reasonable. A few samples may 216
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have exceeded the EPA action limit of 15 ppb (1). Possible sources of positive bias include incorrect sampling procedures, carry over from previous experiments, lead impurities in our reagents, and incorrect standard concentrations. To address the question of lead impurities in our reagents, we tested all electrolyte-plating stock solutions using ASV. This was done by performing ASV experiments using twice-distilled water as the sample. The concentrations of lead in these experiments were found to be below our detection limit (10 ppb) (7). It is unlikely that our electrolyte-plating solutions were a source of positive bias. Our cleaning procedures were also tested in a similar manner. Lead standards were prepared daily by dilution of a certified stock using classA volumetric glassware. All volume measurements associated with sample preparation and standard additions were made using glassware with tolerances of 5% or less. The instructions provided with the sampling kits were consistent with fully-flushed sampling procedures as described by the University of North Carolina Extension Service (1). We did not adopt first-draw sampling methods recommended by the EPA (1) because of cost issues and we did not conduct random multiple sampling at each site throughout the day as suggested by the EPA (1) because of time constraints. The lead concentrations in Table 2 were the result of analyzing each water sample once. The percent relative error associated with each concentration (Table 2) was calculated by propagating the uncertainty associated with the x intercept (as determined by linear regression) and the volume tolerances of the glassware used to prepare the sample (5). Conclusions Since we began this project in 1998, the enrollment has steadily increased to its current membership of 28 science classes. Based upon science teacher feedback, this has become a popular community service project for the region. Science teachers have used the results to discuss water quality issues with their students. Several teachers have used the data for in-class graphing exercises and to discuss the equation of a straight line. One faculty member used the results to define parts per billion (ppb)—a concentration unit rarely encountered in elementary or secondary school science classes. Finally, two faculty members brought their results to annual meetings with administration. Chemistry club members presented their work as part of our annual Environmental Chemistry Conference. Over the years, attendance at this conference has steadily increased to approximately 70 regional science teachers, secondary school students, undergraduate students, faculty, and staff. We believe our chemistry club members learned valuable skills including voltammetry, the standard addition method, quality control, concentration calculations, detection limit calculations, and error propagation. With the streamlined procedure, a single analysis took about 30 min. Analyzing two water samples and generating a final report took a little longer than 1 h. With appropriate precautions, the analytical procedure is relatively safe and easily applied by well-trained undergraduate research students. Because of its growing popularity, our group would like to encourage other universities to engage in similar outreach programs.
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Acknowledgments The authors would like to thank the following chemistry club members for their contributions to the project: Adam Beers, Mandy Faber, Sara Gossman, Jennifer Hoffman, Dan Walsh, Jared Peterson, Mitch Schultz, Chris Theisen, Suzanne Bergren, and Stacy Sadlo. We are also grateful to Lynn Barteck and Laren Barker for their financial support. Additional funds were obtained through the Minnesota State Colleges and Universities (MnSCU) Instrumentation Fund and the Southwest State University Faculty Improvement Grant (FIG) program. We would also like to thank participating science teachers for their support and encouragement. Finally, we are grateful to Robert Eliason for the idea of this successful community service project and John Hansen for his technical assistance. Literature Cited 1. (a) Lead and Copper Rule Minor Revisions: Fact Sheet; EPA 815F-99-010, December 1999, http://www.epa.gov/safewater/standard/leadfs.html (accessed Nov 2003). (b) Environmental Protection Agency Federal Register, 2000, 65, 1955–1956. (c) Sampling procedure in accordance with fully-flushed conditions as described by the North Carolina Cooperative Extension Service document he-395/WQWM-8, June 1995, http:// www.ces.ncsu.edu/depts/fcs/housing/pubs/fcs395.html (accessed Nov 2003).
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2. Wang, J. J. Chem. Educ. 1983, 60, 1074–1075. 3. (a) Wang, J. Stripping Analysis: Principles, Instrumentation, and Applications; VHC: Deerfield Beach, FL, 1985. (b) Bard, A.; Faulkner, L. Electrochemical Methods: Fundamentals and Applications; Wiley & Sons: New York, 1980. 4. (a) Schiewe, J.; Oldham, K. B.; Myland, J. C.; Bond, A. M.; Vicente-Beckett, V.; Fletcher, S. Anal. Chem. 1997, 69, 2673– 2681. (b) Roe, D. K.; Toni, J. E. Anal. Chem. 1965, 37, 1503– 1506. (c) Ball, J. C.; Compton, R. G. Electroanalysis 1997, 9 (17), 1305–1310. (d) Wu, H. P. Anal. Chem. 1996, 68, 1639– 1645. 5. (a) Harris, D. C. Quantitative Chemical Analysis, 5th ed.; W. H. Freeman: New York, 1999. (b) Kolthoff, I. M.; Sandell, E. B.; Meehan, E. J.; Bruckenstein, S. Quantitative Chemical Analysis, 4th ed.; Macmillan Company: London, 1969. 6. Polishing method provided by Cypress Systems Inc., 2500 West 31st Street, Suite D, Lawrence, Kansas 66047. 7. For detection limit calculations see Miller, J. C.; Miller, J. N. Statistics for Analytical Chemistry, 2nd ed.; Ellis Horwood: Chichester, United Kingdom, 1998. 8. The 90th percentiles were calculated by disregarding 10% of the samples with the highest lead concentrations. Of the 100 community water supplies tested, 10 had 90th percentiles exceeding 15 ppb. See the 2002 lead–copper monitoring results of community water supplies published by the Minnesota Department of Health. http://www.ci.woodbury.mn.us/govt/ waterquality03.html (accessed Nov 2003).
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