Laboratory Experiment pubs.acs.org/jchemeduc
Laboratory Experiment Investigating the Impact of Ocean Acidification on Calcareous Organisms Alokya P. Perera†,§ and A. M. R. P. Bopegedera*,‡ †
Bellarmine Preparatory School, Tacoma, Washington 98405, United States Department of Chemistry, The Evergreen State College, Olympia, Washington 98505, United States
‡
S Supporting Information *
ABSTRACT: The increase in ocean acidity since preindustrial times may have deleterious consequences for marine organisms, particularly those with calcareous structures. We present a laboratory experiment to investigate this impact with general, introductory, environmental, and nonmajors chemistry students. For simplicity and homogeneity, calcite was substituted for calcareous organisms and placed in buffer solutions of variable acidity. After 30 min, students quantified the percent mass loss of calcite in their buffer. Individual student data was then pooled into a class spreadsheet for further analysis. This experiment could enable a class discussion on ocean acidification supplemented with primary and secondary literature. KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate, Second-Year Undergraduate, Environmental Chemistry, Laboratory Instruction, Problem Solving/Decision Making, Acids/Bases, Applications of Chemistry, pH, Aqueous Solution Chemistry experience in using pH meters, an expectation of the first year chemistry laboratory curriculum.
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he intergovernmental Panel on Climate Change (IPCC 2013) states that “carbon dioxide concentrations have increased by 40% since pre-industrial times, primarily from fossil fuel emissions and secondarily from net land use change emissions. The ocean has absorbed about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidif ication”.1 The rate of this increase is unprecedented in Earth’s history. For hard evidence, one can look to data from Station ALOHA (A Long-term Oligotrophic Habitat Assessment), a circle of six mile radius in the Pacific Ocean (22° 45′N 158° W), established in 1988 for the purpose of oceanographic study.2 The oceans have become the major sink for this anthropogenic CO2,3 and it has manifested in a decrease in the ocean surface pH from 8.2 in preindustrial times to 8.1 today.4 Further decrease is expected unless significant measures are taken to prevent it.5 As such, ocean acidification may have deleterious consequences for marine organisms, especially those with calcareous structures. Such environmental concerns are not sufficiently presented in current college chemistry textbooks. Laboratory experiments for undergraduates to investigate these global issues are even harder to find because they must usually be conducted in the field for prolonged periods. In this article, we present a simple yet quantitative and effective experiment to help students grasp the impact of ocean acidification on calcareous organisms. The experiment can be completed within a 2−3 h lab period without the need for specialized equipment. It can be performed with general, introductory, environmental, and nonmajors chemistry students. During the course of this lab exercise, students will gain © XXXX American Chemical Society and Division of Chemical Education, Inc.
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EXPERIMENTAL DETAILS In order to collect measurable data within a single lab period, ocean water was mimicked with acidic buffer solutions6 of varying pH. For simplicity and homogeneity, calcite pieces were used to represent calcareous organisms. Each student was apportioned two different buffers and two pieces of calcite, from which they collected two data points during the lab period which were then contributed to a class data spreadsheet. An approximately 5 g piece of calcite was weighed accurately into a clean, dry, 600 mL or 1 L beaker. A pH meter was calibrated, and the pH of the assigned buffer solution was recorded. Then, about 250 mL of the buffer solution and a magnetic stir bar were added to the beaker, which was set on a magnetic stirrer. The contents of the beaker were mixed for 30 min while monitoring the pH. When a pH change in the beaker was observed, more buffer solution was added to maintain the original pH. This meant that the pH meter had to be closely monitored throughout the experiment. After 30 min, the stirring mechanism was turned off and the buffer was carefully decanted. The leftover calcite was carefully rinsed with two to three aliquots of deionized water. This rinsewater was decanted so as to not lose any of the solid calcite. The beaker with the leftover calcite was placed in an oven at 90 °C to completely dry the calcite. After cooling, the dry calcite was weighed using the same analytical balance. Each student repeated the
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dx.doi.org/10.1021/ed400873x | J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
Figure 1. Mass loss of the calcite as a function of the pH of the buffer solutions as generated (A) by the authors and (B) by the students.
have been easy to help the struggling students collect “good” data, we chose to let them learn from working independently. Such challenges are not unusual in the first year chemistry laboratory. However, these challenges do not discount the validity of the experiment because Figure 1A clearly shows that reliable data can be collected by following the procedure outlined in this paper. It is clear from the data in Figure 1 that as pH decreases, the percent mass loss increases dramatically with the highest mass loss observed in the 1−2.5 pH range. It is unrealistic to postulate that the ocean pH would ever reach this level. However, it was necessary to use buffers with these low pH values to demonstrate the general trend in the impact of acidity on calcite given the limitation of a 3 h lab period. Although it was necessary during the experiment to add some quantity of buffer to reaction beakers to maintain initial pH values for all buffers, those with low pH values required more frequent additions. Therefore, when appropriating buffers to students, we ensured that each student was given one high pH and one low pH buffer so that lab time was fairly distributed among students. After completing this lab exercise, students were given two literature articles to read on this topic7,8 and worked in groups of three to read one additional primary literature article of their choosing on this topic. The follow-up discussion was lively as it was clear that the experiment had engaged the students. They paid particular attention to the evidence that some coccoliths increased their calcium carbonate production in response to increased acidity,9 some pteropod species could completely disappear from the polar regions due to ocean acidification,10 and all students were distressed by the impact of ocean acidification on corals and coral reefs.11 They were surprised to find information on another ocean acidification event in the Earth’s history12 and expressed concern that ocean acidification could one day tip the balance in the oceans to the detriment of many organisms that call it home.
experiment with a second buffer solution and a new piece of calcite. To collect multiple data points per buffer, each solution was appropriated to more than one student when possible. Students’ data were pooled into a class spreadsheet for analysis. Students determined the percent mass loss using the class data set and plotted the percent mass loss as a function of pH of the buffer solutions.
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HAZARDS Commercially purchased buffers are aqueous solutions of various acids and bases. Gloves must be worn when handling buffers. Oven mitts must be used when handling hot beakers from the oven.
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RESULTS AND ANALYSIS Figure 1 shows the impact of acidity on calcite. Figure 1A shows the data collected by the authors. A few basic buffers, especially near the current ocean pH range (8.0−8.5), were also tested but found to have no impact on calcite during the 30 min experiment time. About 10% of the data collected by students were excluded when generating Figure 1B (see discussion). These graphs depict a dramatic increase in the percent loss of calcite with increasing acidity. Raw data for Figure 1 are given in the Supporting Information.
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DISCUSSION This experiment was conducted with general chemistry students in the spring quarter after they had completed more than half of the general chemistry curriculum. Students had learned the concepts of pH and buffers, to calibrate and use pH meters, and to draw graphs using spreadsheet software prior to this lab. This experiment and subsequent data analysis helped further solidify these important skills. About 10% of students’ data points were excluded from Figure 1B. Reasons for this included loss of calcite and challenges using the pH probe. Calcite, if broken when the magnetic stir bar comes into contact with it, can be lost during the decanting process if one is not paying careful attention. In addition, a few students had difficulty with calibrating and using the pH meters. In their lab reports, students were required to explain reasons for discarding any data points from the class data set and the subsequent graph. Therefore, it was vital for students to assess where and why errors occurred and appreciate the value of collecting quality data because no one wished to have their data points discarded. Although it would
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ASSOCIATED CONTENT
S Supporting Information *
Complete lab instructions for students; notes for the instructors; data tables for Figure 1; and information on required chemicals, their CAS numbers, and proper disposal methods. This material is available via the Internet at http:// pubs.acs.org. B
dx.doi.org/10.1021/ed400873x | J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
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Laboratory Experiment
Caldeira, K.; Knowlton, N.; Eakin, C. M.; Iglesias-Prieto, R.; Muthiga, N.; Bradbury, R. H.; Dubi, A.; Hatziolos, M. E. Coral Reefs Under Rapid Climate Change and Ocean Acidification. Science 2007, 318, 1737−1742, DOI: 10.1126/science.1152509. (12) Zeebe, R. E.; Zachos, J. C.; Dickens, G. R. Carbon dioxide forcing alone insufficient to explain Palaeocene−Eocene Thermal Maximum warming. Nat. Geosci. 2009, 2 (8), 576−580.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address §
Colorado School of Mines, Golden, Colorado 80401, United States. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The Curriculum for the Bioregion Initiative on Sustainability of the Washington Center for Improving the Quality of Undergraduate Education provided a venue for discussing ideas that lead to the development of this laboratory exercise. The Evergreen State College Foundation provided funds that helped us refine the procedure into a first year chemistry laboratory as well as the data presented in Figure 1A. We are grateful to Dr. Gerardo Chin-Leo of The Evergreen State College for the assistance he provided us to obtain the scanning electron microscopy images that are given in the abstract.
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REFERENCES
(1) Intergovernmental Panel on Climate Change. Climate Change 2013: The Physical Science Basis. Working Group 1 Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Summary for Policy Makers; Stocker, T. F., et al., Eds.; IPCC: Switzerland, 2013; pp 11. Electronically available at http://www. climate2013.org/images/uploads/WGI_AR5_SPM_brochure.pdf (accessed May 2014). (2) Station Aloha. http://aco-ssds.soest.hawaii.edu/ALOHA/ (accessed May 2014). (3) Sabine, C. L.; Feely, R. A.; Gruber, N.; Key, R. M.; Lee, K.; Bullister, J. L.; Wanninkhof, R.; Wong, C. S.; Wallace, D. W. R.; Tilbrook, B.; Millero, F. J.; Peng, T.-H.; Kozyr, A.; Ono, T.; Rios, A. F. The Oceanic Sink for Anthropogenic CO2. Science 2004, 305, 367− 371. (4) Brewer, P. Direct observation of the oceanic CO2 increase. Geophys. Res. Lett. 1978, 5 (12), 997−1000. (5) Caldeira, K.; Wickett, M. E. Anthropogenic Carbon and Ocean pH. Nature 2003, 425, 365. (6) The cost of all the buffer solutions was about $300 and they have a shelf life of about two years. We used the same batch of buffer solutions for two successive years, thereby cutting down the cost of the experiment. Information on composition of the buffers and appropriate disposal methods are provided in the Supporting Information. (7) Doney, Scott C. The Dangers of Ocean Acidification. Sci. Am. 2006, 294 (3), 58−65. (8) Hardt, Marah J.; Carl, Safina Threatening Ocean Life from Inside Out. Sci. Am. 2010, 303 (2), 66−73. (9) Iglesias-Rodriguez, M. D.; Halloran, P. R.; Rickaby, R. E. M.; Hall, I. R.; Colmenero-Hidalgo, E.; Gittins, J. R.; Green, D. R. H.; Tyrrell, T.; Gibbs, S. J.; Von Dassow, P.; Rehm, E.; Armbrust, E. V.; Boessenkool, K. P. Phytoplankton Calcification in a High-CO2 World. Science 2008, 320, 336−340, DOI: 10.1126/science.1154122. (10) Orr, J. C.; Fabry, V. J.; Aumont, O.; Bopp, L.; Doney, S. C.; Feely, R. A.; Gnanadesikan, A.; Gruber, N.; Ishida, A.; Joos, F.; Key, R. M.; Lindsay, K.; Maier-Reimer, E.; Matear, R.; Monfray, P.; Mouchet, A.; Najjar, R. G.; Plattner, G.-K.; Rodgers, K. B.; Sabine, C. L.; Sarmiento, J. L.; Schlitzer, R.; Slater, R. D.; Totterdell, I. J.; Weirig, M.F.; Yamanaka, Y.; Yool, A. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 2005, 437, 681−686, DOI: 10.1038/nature04095. (11) Hoegh-Guldberg, O.; Mumby, P. J.; Hooten, A. J.; Steneck, R. S.; Greenfield, P.; Gomez, E.; Harvell, C. D.; Sale, P. F.; Edwards, A. J.; C
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