In the Laboratory
Linking Laboratory Experiences to the Real World: The Extraction of Octylphenoxyacetic Acid from Water
W
Jorge E. Loyo-Rosales and Alba Torrents* Department of Civil and Environmental Engineering, University of Maryland, College Park, MD 20742; *
[email protected] Georgina C. Rosales-Rivera Area de Química, FES-Zaragoza, UNAM, Prol. Plutarco Elias Calles y Batalla del 5 de Mayo s/n, Mexico, 09239, DF, Mexico. Clifford P. Rice Environmental Quality Laboratory, ANRI, ARS/USDA, 10300 Baltimore Ave., Beltsville, MD 20705
A common topic of discussion in this and other education-related publications is the lack of connection between chemical concepts introduced in the classroom and their applications in the real world (1, 2); students usually complain about this and even dismiss laboratory exercises as “not real science” (3–5). In the particular case of nonchemistry majors (engineering or biology in our case), the problem is aggravated because, according to the students themselves, they are not interested in chemistry or fail to see the need for basic chemical knowledge in their fields. Therefore, in designing this laboratory experiment, we attempted to link several chemical concepts to the extraction of a water pollutant (octylphenoxyacetic acid, OPC, Figure 1) with the actual method used in our research. The necessary chemical concepts, such as stoichiometry, acid–base reactions, and polarity, are commonly part of basic chemistry courses and are covered in general chemistry textbooks, making this experiment suitable for advanced highschool or first-year college chemistry classes. We have used this experiment with both high school students and as part of an introductory environmental engineering course for civil engineering students that has an applied chemistry component. This experiment can be completed in a two-hour laboratory session (except for the last weighing, which takes place 24 hours later), and it can also be conducted as a demonstration if desired. The experience allows students to learn and apply basic experimental techniques such as the use of ordinary laboratory glassware (e.g., cylinders and pipets), analytical balance, separatory funnel, and rotary evaporator; proper handling and disposal of strong acids and solvents; and the application of concepts such as acid–base reactions, stoichiometry, and polarity. At the same time, it demonstrates a direct application of these concepts to a current research topic. Procedure This experiment requires the students to weigh a known quantity of the sodium salt of the OPC, dissolve it in water, transform it to the acid (insoluble) form, extract it using dichloromethane (DCM), and calculate the recovery. As an introduction to the laboratory experiment, a discussion on endocrine disrupters is conducted to familiarize the students with the background of the experiment and to explain the need for the extraction and quantitation of the OPC. This 248
Journal of Chemical Education
•
OH C8H17
O O
Figure 1. Chemical structure of the octylphenoxyacetic acid (OPC).
discussion is important because it provides the link between the real-world problem of toxic chemicals in the environment and the chemistry needed to solve it. In our experience, students were highly motivated by this discussion, which seemed to enhance their interest in the experiment. There is plenty of information on endocrine disrupters and their effects in biota in the scientific literature (6–8), and other divulgation forums (9). Before starting the laboratory, the students are required to answer a set of acid–base related questions: (i) the predominant form of OPC in natural waters given a pKa value of 4.0 and the pH ranges in natural waters, (ii) pH at which 99% of the OPC will be in the protonated form, and (iii) volume of 1 M HCl needed to take 200 mL of water to the pH calculated above. Answers to these questions are also discussed before the experimental work is started. Central to this discussion is the importance of the protonated and deprotonated forms of the OPC and how that affects its solubility in polar and nonpolar solvents. Once in the laboratory, 250 mg of OPC–Na are weighed and dissolved in distilled water and this solution transferred to a separatory funnel. This requires the students to learn the appropriate use of the analytical balance and weighing techniques, such as how to appropriately transfer a compound from one container to another. The next step is to add the quantity of HCl calculated above, causing the OPC to protonate and thus become insoluble. As a result, the originally transparent solution becomes opaque (cloudy). Next, 100 mL of DCM are added to the separatory funnel with the OPC solution. After mixing and allowing the phases to separate, the turbidity of the aqueous phase disappears because the OPC partitions into the DCM. The organic phase is collected and the water extracted twice with 100 mL of DCM each. These changes in turbidity are very conspicuous and students are required to explain them in terms of solvents’ polarity
Vol. 83 No. 2 February 2006
•
www.JCE.DivCHED.org
In the Laboratory
and the solubility of the protonated and nonprotonated form using the empirical rule “like dissolves like”. This step also serves as an example of liquid–liquid extraction, which was chosen as the extraction method in this case because it is commonly used in practice (10), and most educational laboratories are likely to own the materials required. Finally, the extract is dried in a Na2SO4 column, evaporated to approximately 5 mL in a rotary evaporator, transferred to a preweighed vial and evaporated to dryness under a gentle air flow. The vial is allowed to dry further in a desiccator under vacuum until constant weight is achieved (24 hours should suffice). After weighing the dry compound, students are required to calculate the recovery. The correct calculation needs to consider that the original compound was a sodium salt and the end product the carboxylic acid, that is, even if recovery is 100%, the 100 mg of starting material will not result in 100 mg of the product. This requires the students to use concepts of stoichiometry and acid–base reactions and understand the concept of recovery. As noted above, this experiment was conducted with a group of highschool students and a group of college undergraduates. Both groups obtained very similar recoveries, 95.1% and 94.8%, respectively, and commented positively on the experiment. Although we have not attempted it, this experiment could be further improved by using lower quantities of solvent and OPC. It is also possible to recycle the OPC by recrystallization from a NaOH solution, whereas the DCM could be distilled and reused. All residues should be disposed of properly. Hazards The hazards associated with this experiment include DCM and OPC toxicity and the handling of acid solutions and laboratory glassware. All work should be done in a fume hood to avoid inhalation of DCM vapors, and adequate eye and skin protection is recommended at all times to avoid contact with the chemicals, for example, goggles, gloves, lab coat. Additional information on the appropriate handling of these
www.JCE.DivCHED.org
•
compounds can be found in the corresponding Material Safety Data Sheets. W
Supplemental Material
Instructions for the students and notes for the instructors are available in this issue. Acknowledgments This work was partially supported by a Maryland WRRC grant and a RET-NSF. J. E. Loyo-Rosales is supported by a Conacyt-Fulbright/Garcia-Robles scholarship provided by the Mexican and American governments. We would also like to acknowledge discussion of this experiment with A. M. Lynch and her students at Northwest High School. Literature Cited 1. Perkins, D. Smart Schools. Better Thinking and Learning for Every Child; The Free Press: New York, 1992; pp 155–156. 2. de Vos, W.; van Berkel, B.; Verdonk, A. H. J. Chem. Educ. 1994, 71, 743. 3. Phelps, A. J.; Lee, C. J. Chem. Educ. 2003, 80, 829. 4. Wilkinson, S. Chem. Eng. News 2003, 81 (4), 51. 5. Habraken, C. L.; Buijs, W.; Borkent, H.; Ligeon, W.; Wender, H.; Meijer, M. J. Sci. Educ. Technol. 2001, 10, 249. 6. Crisp, T. M.; Clegg, E. D.; Cooper, R. L.; Wood, W. P.; Anderson, D. G.; Baetcke, K. P.; Hoffmann, J. L.; Morrow, M. S.; Rodier, D. J.; Schaeffer, J. E.; Touart, L. W.; Zeeman, M. G.; Patel, Y. M. Environ. Health Perspect. 1998, 106 (Supp. 1), 11. 7. Sonnenschein, C.; Soto, A. M. J. Steroid Biochem. Molec. Biol. 1998, 65, 143. 8. Nimrod, A. C.; Benson, W. H. Crit. Rev. Toxicol. 1996, 26, 335. 9. Endocrine Disrupting Substances in the Environment. http:// www.ec.gc.ca/eds/fact/broch_e.htm (accessed Nov 2005). 10. Dean, J. R. Extraction Methods for Environmental Analysis; John Wiley & Sons: New York, 1998; p 23.
Vol. 83 No. 2 February 2006
•
Journal of Chemical Education
249
