Laboratory Experiment pubs.acs.org/jchemeduc
Supercritical Fluid Extraction versus Traditional Solvent Extraction of Caffeine from Tea Leaves: A Laboratory-Based Case Study for an Organic Chemistry Course Peter M. Schaber,†,* Judith E. Larkin,‡ Harvey A. Pines,‡ Kelly Berchou,† Elizabeth Wierchowski,† Andrew Marconi,† and Allison Suriani‡ †
Department of Chemistry and Biochemistry and ‡Department of Psychology, Canisius College, Buffalo, New York 14208, United States S Supporting Information *
ABSTRACT: In this case-based laboratory, an instrument sales person attempts to convince an analysis laboratory of the virtues of supercritical fluid extraction (SFE). The sales person deals directly with the laboratory technicians who will make the decision. Arrangements are made to have SFE instrumentation brought into the laboratory for a comparative study. The extraction and gravimetric determination of caffeine from finely cut tea leaves is chosen as the “test”. The technicians are divided into two groups. Group A performs a traditional solvent extraction using methylene chloride (CH2Cl2)/alkaline water and group B using a SFE method. Students play the role of laboratory technicians and use the data collected to determine which method they would choose considering several variables and issues. It puts the student into a real-world situation and requires them to apply their critical-thinking skills to make a decision. This case-based laboratory exposed second-year science students to a new technology, provided them with an opportunity to compare and contrast competitive methods, and promoted green chemistry initiatives. An unexpected instrument anomaly affected several students in Group B. How this anomaly was reflected in student assessment of this case-based laboratory is also presented. KEYWORDS: Second-Year Undergraduate, Laboratory Instruction, Organic Chemistry, Collaborative/Cooperative Learning, Hands-On Learning/Manipulatives, Gravimetric Analysis, Qualitative Analysis, Separation Science, Green Chemistry, Instrumental Methods
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laboratory technicians called upon to advise whether a new SFE method should replace a traditional liquid organic solvent extraction (LOSE) method. It puts the student into a situation that requires careful collection of data and the application of their critical-thinking skills to make a decision. Although the use of SFE techniques has been embraced by the research and industrial communities, thorough coverage of this technique in most undergraduate textbooks17 and laboratories is lacking. In addition, although several experimental applications designed to introduce undergraduate students to supercritical fluids (SFs) have appeared in this Journal,18−25 most are designed for upper-level analytical or physical chemistry laboratories and only a few23−25 focus on the SFE of foods (fats; using SF carbon dioxide, CO2). This laboratory introduces SFE to second-year organic chemistry students (exposing this technique to a much larger audience at an earlier career stage) in the context of a case-study, utilizes SF CO2 with a modifier (10% ethanol), and focuses on the extraction of caffeine, an alkaloid, from tea leaves.
everal innovative, nontraditional, instructional pedagogies have appeared in recent times.1−4 One of these approaches, case study teaching,5 has proved successful at this institution6−10 and elsewhere.11−16 Case-based laboratories are a variation on case study teaching that offer students a better approximation of real science than conventional undergraduate laboratory experiments. The laboratories are based upon realworld stories and ask students to deal with problems that challenge their creativity in ways conventional “cookbook” laboratory experiments do not. This approach provides students the opportunity to learn chemical principles in the context of relevant everyday problems, stimulate a sustaining interest, and develop higher cognitive skills and reasoning abilities. Science laboratory courses have been taught by this method but are still the exception rather than the rule. This case-based laboratory is specifically designed to demonstrate the power, efficiency, and safety of supercritical fluid extraction (SFE), while simultaneously exposing students to a more environmentally friendly (i.e., greener) extraction method. It requires small teams of students to conduct themselves in a manner as chemists would, faced with the authentic investigation of a real-world problem. Students role play as © 2012 American Chemical Society and Division of Chemical Education, Inc.
Published: July 24, 2012 1327
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mentation at reasonable cost. However, this laboratory is also easily adopted to use with “homemade” instrumentation.23,29,30 All supercritical extractions were performed on an SFX System 2130 equipped with two syringe pumps, a 2.0 mL/min coaxial linear restrictor, and 10 mL sample cartridge. Carbon dioxide was of industrial grade with dip tube. Dynamic extractions were conducted for 45 min at 80 °C and 200 atm using 90% CO2 and 10% ethanol (95%) as modifier. Extracts were collected by bubbling through a small volume, a few milliliters, of ethanol. Specific experimental details of both extraction methods are available in the Supporting Information. As an extension of the experiment, the purified caffeine was used to qualitatively determine the components present in an Excedrin tablet using thin-layer chromatography (TLC) plates and a chloroform/95% ethanol mobile phase (10:1). The students crushed and powdered an Excedrin tablet and extracted the powdered tablet by adding 2−3 mL of methanol. The Rf values for each spot were recorded.
SFE is based on a simple principle; using temperature and pressure above its critical point a substance assumes the SF phase. This phase possesses physical properties intermediate between those of the gas and the liquid.26 Changing density, by pressure or temperature adjustment, or the addition of a chemical modifier, changes the solvating properties of a SF.27,28 Probably the most well-known use of SFE is the decaffeination of coffee using SF CO2. In this case-based laboratory, the instructor provides students with the case one week prior to the laboratory. In the meantime, students are given the task of looking up the structural formulas of several alkaloids (including caffeine) and toxicology information on methylene chloride, carbon dioxide, and so forth (see the Supporting Information). During a 1-h prelaboratory session, the instructor provides a general overview, reviews the theory underlying SFE, provides a demonstration of the SFE apparatus, and answers any questions. Students in the following 3-h laboratory session work in teams of two. One student from each team extracts caffeine from tea leaves using the classical methylene chloride (CH2Cl2)/alkaline water technique; the other uses SFE. The two versions of the same experiment differ only in the extraction method used; otherwise they are identical. At the next laboratory meeting, students submitted individual laboratory reports, a student survey comparing the two extraction methods, and completed a laboratory assessment survey to ascertain how well the laboratory achieved its objectives; the three documents are available in the Supporting Information. These objectives were (i) to enhance the students′ interest and enjoyment of the laboratory experience; (ii) to develop an appreciation for the realities and complexities of conducting such investigations in the context of solving realworld problems; (iii) to develop critical-thinking skills, specifically those used in data analysis and decision making; (iv) to develop student self-efficacy in use of the instrumentation, and (v) to develop an appreciation for SFE as a green extraction technique. The first year the experiment was introduced it was run in a total of three organic chemistry laboratory sections. An instrument error rendered the first section of SFE students (11 total) with little or no caffeine. This error was quickly rectified, and the next two sections of SFE students (14 total) did not experience such difficulties. (The experiment has been run in an additional six sections, over two consecutive years, without instrument error.) This unexpected situation provided a unique opportunity to study the impact of a laboratory anomaly on the students′ perception of the science.
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HAZARDS Safety goggles must be worn at all times during this experiment. All CH2Cl2 (narcotic at high concentrations) extractions, sample evaporations on steam baths, and development of TLC plates (chloroform is carcinogenic) should be done under a fume hood. Proper ventilation is required performing the SFE portion of this experiment (10% CO2 in air can causes loss of consciousness). A centrifuge is used in one version of this experiment. Students need to balance the instrument and not remove any vials from the turnstile until the centrifuge has completely stopped. Appropriate hand gear should be worn when dealing with hot water solutions and the hot sand baths (burns) and dry ice (frostbite) used during caffeine sublimation (see Supporting Information). Standard safety precautions should be exercised with respect to vacuum sublimation. Due care needs to be exercised when obtaining melting points utilizing sealed capillary tubes. Both methanol and ethanol are flammable substances; so no open flames should be present. All waste should be disposed of in an appropriate manner.
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RESULTS AND DISCUSSION Similar conditions to those described above have been found appropriate for the efficient SFE of caffeine from tea leaves.31,32 It should be noted that although methanol can and has been used as a modifier for the extraction of caffeine,33 the use of ethanol simultaneously provides for efficient extraction conditions and limits student exposure to a more toxic substance. The SFE solvent used in this experiment thus provides for a greener extraction approach as it reduces the use of more toxic substances, results in the generation of fewer waste products, and is composed of naturally occurring substances. This experiment presents a “teaching moment” where students can be made aware of the environmental and economic benefits of green chemistry in general, the use of greener solvents in particular, and the responsibility chemists have to society with respect to sustainability issues. All students were able to complete the laboratory exercise in the allotted time period and isolated caffeine purity was qualified by melting point results (234.0−236.5 °C). Typical percent recovery of caffeine averaged approximately 44 ± 10% for the SFE method, and 22 ± 8% for the LOSE method. These results indicate that SFE of caffeine from tea leaves is a more complete method (verified via t test). Using purified caffeine
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THE CASE STUDY EXPERIMENT An instrument sales-person attempts to convince an analysis laboratory of the virtues of SFE. Arrangements are made to have SFE apparatus brought into the laboratory for a comparative study. The extraction and gravimetric determination of caffeine from tea leaves is chosen as the “test”. Students, playing the role of technicians, are broken up into two major groups: group A performs a traditional solvent extraction and group B uses a SFE method. A student from group A is paired with a student from group B so that the results from the two extraction methods can be compared. The crude caffeine isolated from both methods is sublimed and tested for purity via melting point. A commercial SFE unit was used in this version of the laboratory, and vendors are available providing SFE instru1328
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from either the SFE or LOSE technique and standards of aspirin and p-acetamidophenol (acetaminophen), all students were successfully able to qualitatively identify the components of the Excedrin tablet using Rf values obtained from TLC.
LOSE and SFE laboratory experiences (see the Supporting Information). In addition, students in the SFE-without-error subgroup, compared to LOSE students, more strongly agreed with seven survey statements that “the case study helped me better understand phenomena in the real world”, “doing the case study helped me understand how chemists solve problems”, “the case study stimulated my interest in chemistry”, they would “remember more from the case study approach than from traditional teaching”, “I learned more doing the SFE lab than other labs I did this semester in this course”, “I would like to use SFE technology in future science labs that I take”, and “experience with SFE increased my awareness that chemistry can be done in an environmentally friendly manner”. The SFEwith-error subgroup only differed significantly from the LOSE group on one of the 15 questionnaire ratings: they reported enjoying “the SFE lab more than other labs I did in the course this year”.
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ASSESSMENT OVERVIEW The student survey comparing the extraction methods was designed to have students determine the superior extraction technique based on their laboratory experience. The laboratory assessment survey was designed to ascertain how well the laboratory achieved its educational objectives. Part I of the laboratory assessment survey consisted of fifteen questions, Part II consisted of two open-ended questions asking students which extraction method they would choose on both a high- and lowvolume basis, and why (see Supporting Information). Fortythree students were available to complete the laboratory assessment instrument. The results were analyzed in two different ways. In the first analysis, a simple comparison was made between the survey responses of the LOSE and SFE team members. For the second analysis, the SFE students were divided into two subgroups: one group of SFE students (SFEwithout-error) who completed the extraction process according to protocol and the second group of students (SFE-with-error) who were unable to complete the SFE protocol because of an equipment anomaly. The survey results indicate that this case-based laboratory was successful in meeting its objectives, when the laboratory protocol was experienced by the students as had been intended. They also reveal unexpected and unintended consequences of an anomaly in the laboratory protocol on students′ attitudes about the case, the SFE equipment, and the laboratory content and team work. Students′ responses showed that the class as a whole recognized the costs and benefits of the LOSE and SFE methods and were able to use that knowledge in the context of a real-world problem. A large majority of the students (95%) recommended the SFE method for a high-volume task environment (>1000 extractions/yr). However, when asked to recommend a method under low-volume conditions (
