In the Laboratory
Applications of a U.S. EPA-Approved Method for Fluoride Determination in an Environmental Chemistry Laboratory: Fluoride Detection in Drinking Water
W
Gabrielle Rum, Wen-Yee Lee, and Jorge Gardea-Torresdey* Department of Chemistry & Environmental Science and Engineering, University of Texas at El Paso, El Paso, TX 79968; *
[email protected] Owing to the great awareness of hazardous wastes and contamination of natural resources, colleges and universities have increased the number and type of courses they offer in environmental chemistry. Teaching students the applicability of historical analytical techniques in an environmental setting raises the students’ awareness of critical environmental issues and offers a less traditional, more enticing way of learning the art of chemical analysis (1, 2). We felt the urgency to share our valuable experience with others, because of the numerous requests received regarding our application of United States Environmental Protection Agency (U.S. EPA) Method 340.2— Potentiometric, Ion Selective Electrode—for the analysis of fluoride in drinking water, in an environmental chemistry course. This method can be utilized in any environmental chemistry course at the college level. We made two changes to the method: (i) students were required to take a sample from their home water supply for analysis, and (ii) a single combination electrode was used for the analysis instead of a doubleelectrode system. Fluorine is the 13th most abundant element in the earth’s crust. Fresh waters in the USA have natural fluoride levels ranging from 0.1 to 12 ppm. In 1950, the American Dental Association adopted a resolution in support of fluoridation of public drinking water (3), and today more than half the
drinking water in the USA is fluoridated; the optimum range is between 0.7 and 1.2 ppm (4 ). Fluoride that is ingested and absorbed into the bloodstream strengthens teeth while they are growing by replacing hydroxyapatite with fluorapatite, which is much more resistant to decay (5). In recent years, much controversy has arisen in regard to the hazards of fluoridation, including fluorosis, a disease that blackens and weakens teeth and permanently curves the spine (6–8). The lethal dose (LD50) of fluoride in rats is 26.0 mg/kg body weight (intravenous) and 52.0 mg/kg body weight (oral) (9). Of the ingested fluoride that is absorbed through the stomach and intestine into the bloodstream, roughly 50% is excreted in the urine, 10% is excreted in the feces, and the remainder is deposited in the skeleton and other calcified tissues (10). This lab is related to the controversial toxicological problem of fluorosis, and as such was found to inspire students’ interest in chemistry and environmental issues. This method (11–13) is applicable for the measurement of fluoride concentrations from 0.1 to 1000 mg/L in finished waters, natural waters, brines, drinking waters, and surface waters, and in domestic and industrial wastes. It eliminates the need for distillation of the sample. Because many communities fluoridate their drinking water, monitoring of the water to ensure concentration levels between 1.4 and 2.4 mg/L is maintained (13). The fluoride concentration is determined by using an ion-selective fluoride combination electrode with a standard single-junction sleeve-type reference electrode and pH meter with an expanded millivolt (mV) scale. The measured potential corresponding to the level of fluoride ion in solution is described by the Nernst equation. When the background ionic strength is high and constant relative to the sensed ion concentration, the ionic activity is directly proportional to the concentration of the interested ion. The TISAB (t otal ionic strength adjuster buffer) provides a background of almost uniform ionic strength and maintains an appropriate pH range of 5 to 9 (extreme pH interferes in the measurement) (11, 12). The buffer solution also contains CDTA, cyclohexylenedinitrilotetraacetic acid, which will release free fluoride ions by complexing with the interfering cations (e.g., the most common interference, aluminum) up to 3.0 mg/L (14 ). Equipment and Chemicals
Figure 1. Map of the area sampled.
1604
Apparatus Orion 420A Electrometer (pH meter) Fluoride/combination fluoride electrode, Orion 96-09 Nichiryo Oxford Benchmate micropipet (10–100 µL) Magnetic mixer; Teflon-coated stirring bar 50-mL beakers (quantity depends on number of samples)
Journal of Chemical Education • Vol. 77 No. 12 December 2000 • JChemEd.chem.wisc.edu
In the Laboratory
Reagents NaF [7681-49-4] stock solution, 100 ppm F TISAB buffer solution: TISAB, pH 5.0–5.5, containing 1,2-cyclohexylenedinitrilotetraacetic acid, CDTA Discussion of Method Three 50-mL fluoride standards were prepared (0.1, 1.0, and 10.0 ppm). To each standard 25 mL of TISAB buffer was added and the total volume was made to 50 mL with deionized (DI) water. A blank consisting of 25 mL of TISAB and 25 mL of DI water was also prepared. Seven samples were collected for analysis (an unknown provided by the instructor and 6 additional water samples from neighboring cities and towns [Fig. 1], or students could bring in water from different parts of town). All samples were to be diluted 1:1 with TISAB (25 mL sample/25 mL TISAB). The electrode was immersed into the solution, and when it stabilized (usually after ca. 3 min) the readings were recorded. After each sample reading, the electrode was rinsed with DI water to prevent any solution carry-over (15). Hazards Students need to be aware of the danger of the chemicals used in the experiment. If inhaled or swallowed, NaF (sodium fluoride) can cause fluoride poisoning. Early symptoms include nausea, vomiting, diarrhea, and weakness. Later effects include Table 1. Electrode Readings and Fluoride Concentrations F ᎑ Concn
Solution
Potential/ mV
Blank
198.4
—
—
Fluoride standards 0.1 ppm F ᎑ 1.0 ppm F ᎑ 10.0 ppm F ᎑
145.1 83.4 23.3
— — —
— — —
Unknown
25.9
Sample taken from Deming Visitor’s Center, Deming, NM Texaco Station, Lordsburg, NM Wendy’s restaurant, Benson, AZ Northwest Baymont Inn, Tucson, AZ Home, Resler Dr. & I-10, West El Paso, TX Chemistry Dept. UT–El Paso, El Paso, TX
104.8 57.8 a 119.1 128.0 105.1 105.1
log
ppm
᎑ 0.952862 17.91 ᎑ 0.342676 ᎑ 0.430706 ᎑ 0.723620 ᎑ 0.577482 ᎑ 0.347602 ᎑ 0.347602
0.90 5.30 0.38 0.52 0.90 0.90
a Because of the low reading, a second sample was analyzed, with the same result.
Figure 2. Calibration curve for the determination of fluoride.
central nervous system effects, cardiovascular effects, and death. In addition, TISAB is corrosive and can cause eye and skin burns as well as digestive and respiratory tract burns. Results and Discussion The Nernst equation gives the relationship between potential and concentration of an ion, in this case F ᎑ : E = A – B log[F ᎑]. The concentration of fluoride in an unknown sample, therefore, can be obtained from the equation [F ᎑] = 10[᎑(E–A)/B]. The concentration of fluoride in drinking water can thus be determined by plotting the potential reading for each standard vs the standard concentration on semilog paper, or by the use of a statistical computer program. From this plot, the concentration of fluoride in the samples can be extrapolated (or when using a computer program, the potential readings for each sample should be inserted into the equation resulting from the least squares line between the standards). (NOTE: Samples were diluted 1:1 with TISAB; therefore the actual concentration of fluoride in the sample will be twice the value obtained by the above methods.) The results of the electrode readings and fluoride concentrations for the blank, standards, unknown, and samples analyzed in our lab are shown in Table 1. The three standards were correlated with the JMP Statistical program (16 ) using least-squares analysis (Fig. 2). The potential and the log of the concentration of fluoride in the sample are described by the following equation: log concn F ᎑ (ppm) = 1.37814 – 0.01642 (mV) The least-squares analysis showed a squared correlation coefficient, R 2, of .999942, an excellent correlation of the data. This means that 99.9942% of the data could be predicted using this equation, and more than fulfills the U.S. EPA regulation of a 98% standard correlation. Because of the low reading for Lordsburg (which indicated a high concentration of fluoride) another test sample was made and a second reading was taken. It differed from the first sample by only 1/10 of a millivolt. The fluoride concentration at Lordsburg was 5.3 ppm, almost 6 times that of its neighboring city Deming, approximately 60 miles away, and in violation of the U.S. EPA’s maximum concentration level of 4 ppm. The unknown, prepared by the teaching assistant, showed a fluoride concentration of 17.91 ppm. The actual concentration was 18 ppm. This is a recovery of 99.5%, which corroborates that the analytical response and technique are excellent. If one considers that the tap water contained 0.9 ppm of fluoride, the recovery is then 94.5%, which is acceptable according to U.S. EPA standards. The El Paso water gave a measurement of 0.9 ppm of fluoride; El Paso Water Utilities test results average between 0.6 and 1.1 ppm fluoride in water sampled from that area of the city (17). West El Paso well water (an average value for 19 wells) is reported as about 0.8 ppm fluoride. Therefore, the fluoride concentration of the sample collected was found to be in the expected range. Students found this laboratory to be an exciting avenue by which to not only gain experience in analytical techniques, but also to see the practicality and applicability to environmental chemistry analyses. The inclusion of students’ own water samples was an added attraction, keeping those who were usually quickly distracted both captivated and intrigued.
JChemEd.chem.wisc.edu • Vol. 77 No. 12 December 2000 • Journal of Chemical Education
1605
In the Laboratory
Acknowledgments We would like to acknowledge the General Electric Faculty of the Future Program, the Environmental Science and Engineering Program at the University of Texas at El Paso, and the Department of Chemistry at the University of Texas at El Paso. W
Supplemental Material
Supplemental material for this article is available in this issue of JCE Online. It includes more information on the toxicity of fluoride, practical and theoretical details of the method, and a handout for students with a data sheet.
9.
10. 11.
12.
Literature Cited 1. Kegley, S. E.; Hanse, K. J.; Cunningham, K. L. J. Chem. Educ. 1996, 73, 558–562. 2. Kegley, S. E.; Stacy, A. M. J. Chem. Educ. 1993, 70, 151–152. 3. American Dental Association. Fluorides and Fluoridation; http://www.ada.org/public/topics/fluoride/fluoride.html (accessed Oct 2000). 4. Weil, A. Ask Dr. Weil—To Fluoridate or Not? http://www.pathfinder. com/drweil/qa_answer/0,3189,136,00.html (accessed Oct 2000). 5. Van Meter, F.; Hayes, M. Scientific American; Ask the Experts; http://www.sciam.com/askexpert/chemistry/chemistry4.html (accessed Oct 2000). 6. Newbrun, E.; Horowitz, H. Perspect. Biol. Med. 1999, 42, 526–543. 7. Colquhoun, J. Perspect. Biol. Med. 1997, 41, 29–44. 8. National Institute of Environmental Health and Safety, NIH.
1606
13.
14.
15. 16. 17.
TR-393: Toxicology and Carcinogenesis Studies of Sodium Fluoride (CAS No. 7681-49-4) in F344/N Rats and B6C3F1 Mice (Drinking Water Studies); http://ntp-server.niehs.nih.gov/htdocs/ LT-studies/tr393.html (accessed Aug 2000). Ramamoorthy, S.; Ramamoorthy, S.; Baddaloo, E. G. Handbook of Chemical Toxicity Profiles of Biological Species. Vol. 2: Avian and Mammalian Species; CRC/Lewis: Boca Raton, FL, 1995. Whitford, G. J. Dent. Res. 1990, 69 (Spec. Issue), 539–549. U.S. Environmental Protection Agency. Methods for Chemical Analysis of Water and Wastes, Method 340.2: Fluoride; Storet No. 00950; Environmental Monitory and Support Laboratory, Office of Research and Development, U.S. EPA: Cincinnati, OH, 1971. U.S. Environmental Protection Agency. Methods for Chemical Analysis of Water and Wastes, Method 340.2: Fluoride; Storet No. 00950; Environmental Monitory and Support Laboratory, Office of Research and Development, U.S. EPA: Cincinnati, OH, 1991. American Society for Testing & Materials. 1991 Annual Book of ASTM Standards, Section 2: Water and Environmental Technology, Vol. 11.01:Water (II); ASTM: West Conshohocken, PA, 1991; pp D1179–D1188. Standard Methods for the Examination of Water and Wastewater, 18th ed.; Greenberg, A., Ed.; American Public Health Association: Washington, DC, 1992; pp 4–59. Orion Fluoride Electrode Instruction Sheet; Orion Research: Beverly, MA, 1991. JMP Statistical Modelling Program, version 3.1; SAS Institute: Cary, NC, 1995. El Paso Water Utilities. City of El Paso—Chemical Analysis: City Water; El Paso Water Utilities: El Paso, TX, 1994.
Journal of Chemical Education • Vol. 77 No. 12 December 2000 • JChemEd.chem.wisc.edu
