In the Laboratory
Using a Simulated Industrial Setting for the Development of an Improved Solvent System for the Recrystallization of Benzoic Acid: A Student-Centered Project
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Timothy R. Hightower* and Jay D. Heeren Department of Chemistry, Black Hills State University, Spearfish, SD 57799; *
[email protected] With the ever-growing need to provide industry with independent-thinking, problem-solving college graduates, there is also a need for increasing the number of laboratory exercises that promote development of this type of employee. It was the goal of this organic laboratory exercise to provide students with an opportunity to solve a real-life industrial problem. The students were informed that a train traveling through South Dakota derailed, spilling tons of benzoic acid onto the ground. The students were then designated as process-improvement chemists in charge of purifying the contaminated benzoic acid for resale. These novice chemists were given the responsibility to develop their own purification process and were not hindered by the instructor’s influence. The students were broken into groups where they elected a group leader, who assumed the role of middle management and reported all happenings to the instructor as well as a secretary, who captured all dialogue within the group. The job of the instructor was to serve only as their supervisor and not to lead any group. It was then left to each group to develop the purification process, with only the group leader reporting directly to the instructor with all questions and updates. The initial response of the students was one of bewilderment. With no direction or cookbook procedure provided, the students spent the first couple of hours somewhat dumbfounded with no progress being made. It was only after coming to the realization that the instructor had no intention of providing further information or assistance, that they began to settle on the task at hand. This type of approach was chosen to simulate an industrial setting in which the employees were expected to solve a problem quickly and with minimal cost. Upon analyzing different types of purification methods, for example, distillation and acid–base extraction, all groups moved toward utilizing the traditional water-recrystallization process. The students were elated they had found a cookbook procedure that they could follow to purify the benzoic acid (1). The group leader proposed this water recrystallization procedure to the instructor. The instructor however, wanting to ensure a high recovery of benzoic acid, sent the group leader back to his group in search of an improvement to the traditional procedure. While focusing on the major characteristics of an effective recrystallization solvent (1), the students began to investigate methods of increasing the solubility of benzoic acid. This investigation led the groups to examine different types of single recrystallization solvents in the literature (2), eventually leading them to consider a cosolvent system. Based on the solubility of benzoic acid in a variety of different solvents, the students projected that by combining two solvents, specifically water and ethanol, a better range of solubility could be obtained. This water兾ethanol system should be an improvement over each solvent independently owing to the fact that at lower temperatures benzoic acid is too soluble in ethanol; and at higher temperatures benzoic www.JCE.DivCHED.org
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acid is not soluble enough in water. It was at this juncture that the students began to take ownership of the project. The students constructed experiments to determine the solubility of benzoic acid in varying concentrations of water兾ethanol at temperatures ranging from approximately 0–70 ⬚C. The students chose 70 ⬚C because the ethanol兾water azeotrope boils at a temperature of 78.15 ⬚C (3). After generating the solubility curves, the students were able to determine the proper ratio of water兾ethanol that would dissolve a large quantity of benzoic acid at higher temperature but only dissolve a small quantity at lower temperature. From these results, the students proposed that a 25兾75 water兾ethanol solvent system would provide a significant improvement as compared to the traditional water solvent. The instructor again informed the group leader they had failed to provide enough evidence to prove their recommendation. From this discussion, the students realized the percent recovery of benzoic acid in these different solvent systems would provide useful information in choosing which of the solvent systems would be the best candidate. Upon completing the percent-recovery experiments, students determined percent recovery was increased by as much as 30% using ethanol or any of the ethanol兾water systems as compared to water alone. Based on the results of both the solubility curves and the percent recoveries, the students confirmed that the 25兾75 water兾ethanol solvent system would be an improvement to the water solvent. Upon conclusion of the lab portion of this exercise, each group of students was required to report their results in the form of an ACS-style article. This process included an internal peer review prior to submitting the article to the instructor. Experimental The project was presented verbally to the class in an office-type setting. No written instructions were provided. The students were given the opportunity to develop their own procedures and determine what equipment they deemed necessary to successfully complete the project with minimal guidance from the instructor. In addition, there were no guidelines as far as what resources could be used. The students had access to all desired books as well as the Internet. Solubility of benzoic acid was determined by preparing four mixtures of differing ratios of water to denatured ethanol (w兾w)—0兾100, 25兾75, 50兾50, 75兾25—and placing 10 mL of each in separate 25-mL test tubes. Solubility data were obtained by cooling the test tubes to approximately 0 ⬚C and adding small quantities of benzoic acid until the point of saturation. Using a hot water bath, each solvent mixture was heated in increments of approximately 10 ⬚C to a maximum of 70 ⬚C with the solubility of benzoic acid recorded at each increment. The procedure for determining the solubility of benzoic was developed by the students. The solubility for 100% water was obtained from the literature (2) and all solubility data points were graphed
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In the Laboratory
using Microsoft Excel. Percent recovery was determined for each of the differing ratios of water兾ethanol by heating 3 grams of benzoic acid to 70–75 ⬚C in the appropriate quantity of solvent as determined by the students’ solubility curve at 70 ⬚C. If the benzoic acid did not dissolve in the original quantity of solvent suggested, increments of 10% of the original quantity of solvent were added until all the benzoic acid was dissolved. The mixture was cooled to approximately 4 ⬚C, filtered, washed three times with 10 mL of cold water (approximately 4 ⬚C), dried, and the mass of the recovered benzoic acid recorded. Percent recovery was also established for water using the same procedure as the water兾ethanol solvent system with the exception that water was heated to 96 ⬚C, the boiling point of water in the Black Hills of South Dakota. Hazards Benzoic acid may be harmful if swallowed, act as an eye or respiratory irritant, and cause allergic respiratory or skin reaction. Ethanol is flammable and has a low enough flash point that spills on hot plates represent a hazard. Do not seal the test tubes while they are heated or in the process of being heated. This could lead to an explosion due to increased pressure. Results and Discussion In this set of experiments, students examined a variety of ratios of water兾ethanol and initially determined that the 25兾75 water兾ethanol system provided the best solvent system for the recrystallization of benzoic acid. This system allowed for larger quantities of benzoic acid to be dissolved at 70 ⬚C than the water system allowed at 100 ⬚C and also decreased the quantity of benzoic acid dissolved at lower temperatures as compared to ethanol as a single solvent. These findings dramatically decreased the heating and cooling time initially required and increased the quantity of benzoic acid purified in a single run. This conclusion was complemented by the students’ percent-recovery data, which determined that the percent recovery from water was much less than the percent recovery from the water兾ethanol cosolvent systems: approximately 60% recovery from water as compared to a minimum of 88% from any of the water兾ethanol cosolvent systems. Further analysis of the cosolvent systems, as well as ethanol alone, provided a percent-recovery range of 88–95%, indicating little difference in the percent recovery between the differing ratios of water兾ethanol. Although this was an improvement over the traditional water recrystallization process, students found that ethanol alone would be a better solvent than the 25兾75 water兾ethanol system, provided the purification process required many batches to purify all the material and the ethanol could be recycled. This conclusion was obtained by not only realizing that the solubility of benzoic acid was increased in ethanol, but also that the ethanol would be saturated with benzoic acid during the initial run and that the loss due to solubility would then be minimized upon additional runs. Although results varied from group to group, the overall solubility trend did not. Students found as expected, that as the percentage of ethanol to water increased, the solubility of benzoic acid also increased. Conclusion In addition to students gaining the knowledge of purification of benzoic acids, a common undergraduate experiment, 1664
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students were able to determine for themselves that a new and improved solvent system could be developed as compared to the traditional water system. Although most groups did not obtain identical results, all groups determined that higher ratios of ethanol兾water dissolved higher quantities of benzoic acid; but not all agreed on the same ratio. From these results, students were able to examine their experimental errors and provide input for improving the purification process. Upon completion of the exercise, a survey was given to further analyze the learning experience of the students. The survey was broken into three parts; the first part of the survey focused on the laboratory experience, the second on the team environment, and the third addressed only those in middle management. Of the twelve students who took the survey, one considered it a “poor use of time” while other comments included: “great lab helped gain a practical as well as technical understanding of standard organic chemistry”, “the experiment was a great way to both challenge yourself and learn better group skills”, “this lab actively related real situation with lab procedure, which up until now I have not obtained in other labs” and “definitely recommend”. From the questionnaire portion of the survey, it was found that students felt they had ownership of the project and the environment simulated an industrial-type setting. All students, including the student who considered it a “poor use of time”, recommended this laboratory exercise to future classes and felt that the laboratory experiment provided a well-rounded learning experience as well as an improvement over the traditional textbook laboratory experiments. As a result of this exercise, students indicated they were able to see better the applications of the textbook procedures. When asked whether their problem-solving skills increased as a result of this experience, all students were confident their problem-solving capabilities did improve. When addressing the importance of the team approach, all students enjoyed working in a team setting and appreciated its effectiveness. In addition, they also enjoyed being directed by a peer and agreed that peer leadership was effective. All middle management stated that even though they desired the position initially, they found it more difficult to lead the group and to report to a boss than originally thought; however, they would yet again accept the position. In conclusion, we will continue to implement this laboratory exercise at our university and search for ways to improve it. WSupplemental
Material Figures and graphs compiled from the student-generated data are available in this issue of JCE Online. Acknowledgments The organic chemistry students at Black Hills State University who participated in the development of this laboratory exercise are acknowledged. Literature Cited 1. Williamson, K. L. Organic Experiments, 9th ed.; HoughtonMifflin Company: New York, 2004; pp 32–48. 2. Budavari, S. The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals, 12th ed.; Merck: Whitehouse, NJ, 1996; p 183. 3. Williamson, K. L. Organic Experiments, 9th ed.; HoughtonMifflin Company: New York, 2004; pp 68–69.
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