In the Laboratory
Reduction of Calcium Concentrations by the Brita® Water Filtration System: A Practical Experiment in Titrimetry and Atomic Absorption Spectroscopy
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Kimberly G. Olsen* and Lisa J. Ulicny Department of Chemistry, Loyola College in Maryland, Baltimore, MD 21210-2699; *
[email protected] Titrimetric determinations have long been a standard in the sophomore-level quantitative analysis curriculum (1–3). Traditionally, one of the first experiments encountered in the quantitative laboratory is an acid–base titration with the possible addition of an ion-exchange component. Another experiment often utilized is the chelation of bivalent ions by EDTA. However, we have seen that as the semester progresses, the repetition of titrations with little differentiation between experimental methods often results in excellent technique, but little theoretical retention and even less excitement for this very important procedure. One way to encourage more interest on the part of the students would be to relate the titration experiment to an area of significant importance to them. We have examined the ability of a popular water filtration system to decrease water hardness by removing calcium ions (4). The students’ interest in environmental and medical issues, combined with the scientific examination of a household item often used by the students and their families, arouses significant curiosity in the participants. The Experiment The experiment is framed around a classical Ca2+-EDTA complexometric titration (5–7) and has three main sections. In the first week, the students prepare calcium carbonate solutions to standardize a solution of ethylenediaminetetraacetic acid (EDTA). After the EDTA solution is standardized, the students pass tap water through the Brita filter to simulate filter use over time. Since the total daily capacity of the Brita filter is 7.5 L, the students must work as a team to filter and collect calcium standard samples outside of the scheduled laboratory period over a one-week span. In the second week, the samples are tested for calcium content and the data are analyzed for changes in the amount of calcium removed by the filter. The final section in this laboratory is the regeneration of the filter with 6 M HCl, not unlike the protocol used for traditional cation-exchange resins. This part of the experiment links the chemical concepts of ion-exchange resins to a household item. An additional instrumental analysis module, included in the Notes to the Instructor,W brings to light some limitations of complexometric titrations, the most important being the nonspecificity of chelating molecules. This module utilizes atomic absorption spectroscopy to examine the concentration of calcium ions in the filtered standard samples. However, the module does not easily fit into a two-week lab-time schedule. If an instructor is interested in this portion of the laboratory, it could be included in place of the filter regeneration section. Alternatively, it could be added to the senior instrumental analysis lab. This option would be an interesting twist on the
students’ continuing education, as a lab from their sophomore year reenters their curriculum two years later! Materials The materials in this laboratory are readily available to most chemistry programs. They include a Brita filtration pitcher with filter cartridges and the common chemicals necessary for EDTA titrations, and tap water. The instrumental analysis module requires an atomic absorption spectrometer, but detection occurs at a single wavelength in absorption mode, so the experiment does not require complex instrumentation. Hazards This experiment presents no significant hazards. Conclusion This experiment has been a successful addition to our quantitative analysis course. The team building and data sharing within this lab were very popular with the students, and the exercise brings the laboratory to life with a “real” examination of chemistry in action. Acknowledgment We wish to acknowledge the Loyola Hauber Fellowship Program for funding for LJU. W
Supplemental Material
The laboratory protocol, instructions to the student, and notes for the instructor are available in this issue of JCE Online or by contacting the corresponding author. Literature Cited 1. Ceretti, H.; Hughes, E. A.; Zalts, A. J. Chem. Educ. 1999, 76, 1420. 2. Yappert, M. C.; DuPre, D. B. J. Chem. Educ. 1997, 74, 1422. 3. Soriano, D. S.; Draeger, J. A. J. Chem. Educ. 1993, 70, 414. 4. American Public Health Association, American Water Works Association, and Water Environment Federation. Standard Methods for the Examination of Water and Wastewater, 15th ed.; APHA: Denver, 1980; Method 2340. 5. Day, R. A.; Underwood, A. L. Quantitative Analysis, 6th ed.; Prentice-Hall: Upper Saddle River, NJ: 1991; pp 622–623. 6. Harris, D. C. Quantitative Chemical Analysis, 5th ed.; Freeman: New York, 1999; Chapter 13. 7. Rubinson, J. F.; Rubinson, K. A. Contemporary Chemical Analysis; Prentice-Hall: Upper Saddle River, NJ, 1998; Chapter 8.
JChemEd.chem.wisc.edu • Vol. 78 No. 7 July 2001 • Journal of Chemical Education
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