In the Laboratory
Carbon Dioxide Absorbers: An Engaging Experiment for the General Chemistry Laboratory Thomas M. Ticich Department of Chemistry, Centenary College of Louisiana, Shreveport, Louisiana 71104, United States
[email protected] The toxic effects of carbon dioxide (CO2) require its continual removal from enclosed living spaces (1-3). The reaction between lithium hydroxide (LiOH) and carbon dioxide has been used for decades to remove the gas from spacecraft and submarines (4, 5). The overall reaction is exothermic, 2LiOHðsÞ þ CO2 ðgÞ f Li2 CO3 ðsÞ þ H2 OðgÞ and occurs in a two-step process: 2LiOHðsÞ þ 2H2 OðgÞ f 2LiOH 3 H2 OðsÞ
ð1Þ ð2Þ
2LiOH 3 H2 OðsÞ þ CO2 ðgÞ f Li2 CO3 ðsÞ þ 3H2 OðgÞ ð3Þ Given the importance of the reaction in life-support systems, its details have been well studied (6-9). An important figure of merit for a CO2 absorber is its absorption capacity, defined as the ratio of the mass of CO2 absorbed to the mass of absorber. LiOH provides a high absorption capacity due to its low molar mass relative to alternative compounds, a characteristic that has made it a particularly compelling choice aboard spacecraft where minimizing payload is critical. Despite the fact that general chemistry texts (10, 11) and articles in this Journal (4, 5, 12) describe the reaction and its applications, it has not been featured as a laboratory experiment in published manuals or in the literature. Methods for measuring the absorption capacity of LiOH typically involve flowing a gas mixture through the absorber and monitoring the presence of CO2 in the effluent gas by infrared absorption (6-9). This article describes a simple procedure for determining the absorption capacity adapted from a method for determining the atomic weight of Zn, a popular general chemistry experiment (13). The experiment, which is suitable for general chemistry and can be carried out in a 3-h laboratory period, utilizes the concepts of stoichiometry and gas laws and serves several learning goals. Students compare the theoretical predictions based on stoichiometric calculations with experimental data so they can understand why that latter must be used to determine the amount of absorber required for a spaceflight. In the process, they use the ideal gas equation to determine the mass of CO2 that reacts to compute the absorption capacity. They also connect qualitative observations to species in the chemical reaction to build their knowledge of descriptive chemistry while engaging in an important application. Ethanolamine, along with other alkanolamines, has been used to remove carbon dioxide from industrial waste gases (14-16) and offers the possibility of regeneration after use by mild heating. LiOH, on the other hand, cannot be readily regenerated so that sufficient material for the entire space mission must be on hand. Our general chemistry students have successfully applied
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the method to ethanolamine. By analyzing both absorbers, students can compare their absorption capacities as well as consider how reversibility can inform the choice of a material for a particular application. Experimental Procedure The experimental apparatus (Figure 1) consists of a 50 mL glass syringe with moveable piston (Popper #5058 micro-mate syringe with metal luer slip tip) attached through a metal adapter (Popper #6165), 12 cm of Tygon tubing (3/16 in. i.d. 5/16 in. o.d.), and a short piece of 5 mm glass tubing in a #2 rubber cork to a 10 mL vial that contains a small mass of the absorber (approximately 0.05 g of anhydrous LiOH or 0.30 g of ethanolamine). The values suggested for the mass of the absorber will result in a substantial change in CO2 volume without exhausting the contents of the syringe. The absorber is weighed in the vial to within 1 mg, sealed with a solid rubber stopper and set aside. Carbon dioxide gas is generated from the sublimation of a pea-sized piece of dry ice placed inside the syringe barrel. The apparatus is then flushed with CO2 vapor and filled to the 50 mL mark as described in the supporting information. As soon as the dry ice has completely sublimed, the rubber stopper connected to the tubing assembly should be loosely fitted into the sample vial. The syringe should be depressed to a volume of about 40 mL (the exact value should be recorded) to flush out the air in the vial, which should then be sealed tightly to the apparatus. There are several signs of chemical change that the students can observe. Most obvious is the movement of the syringe barrel as the CO2 is absorbed. The bottom of the vial will become quite warm, reaching 50 °C in less than 30 s. In the case of the LiOH reaction, droplets of water produced from the reaction will form inside the vial. Neither of the absorbers tested undergo any visible changes, although the Li2CO3 formed from the LiOH reaction can be detected afterward by its reaction with acid. Once the syringe barrel stops moving, the students should record the final volume and also note the pressure and temperature
Figure 1. Reaction apparatus for analysis of CO2 absorbers.
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 88 No. 2 February 2011 10.1021/ed100826p Published on Web 11/23/2010
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In the Laboratory Table 1. Student Data for CO2 Absorber Experiment Absorber
Number of Trials Average Absorber Mass/g Average CO2Volume/mL Absorption Capacity/(g CO2 absorbed/g absorber)
LiOH
80
0.051
17.4
0.61 ( 0.14
Ethanolamine
38
0.330
20.5
0.115 ( 0.036
in the laboratory. The mass of CO2 absorbed is readily computed from these data using the ideal gas equation and, together with the mass of the absorber, the absorption capacity can be determined. The Li2CO3(s) is easily disposed of by reaction with 2 M HCl, which also confirms its presence. The products of the ethanolamine reaction are diluted with water and sequestered for subsequent disposal. Hazards LiOH causes burns and is toxic if swallowed or inhaled. Ethanolamine is a corrosive liquid that causes burns. It is harmful by inhalation and irritating to the eyes, respiratory system, and skin. To minimize handling of this substance by students, it is recommended that the instructor or laboratory assistant dispense the appropriate volume of the liquid for them (∼0.3 mL) using an automatic pipet. The students will still need to accurately weigh the liquid dispensed in a stoppered vial. Dry ice can cause frostbite upon contact with skin and should be handled with tongs or forceps. The syringe barrel in the apparatus should move freely to prevent a build-up of pressure. Results and Discussion The raw data from five general chemistry sections are shown in Table 1 as well as the average absorption capacity obtained and the standard deviation of the aggregate set. The 1-2 s required to flush the vial and seal the stopper introduces error into the method. Direct observation shows that the reaction with LiOH proceeds at a rate of about 10 mL CO2 absorbed/min, so that the corresponding error in the CO2 volume is less than 0.5 mL. The reaction with ethanolamine proceeds at approximately twice the rate of the reaction with LiOH, which increases the error in the measured CO2 volume to nearly 1.0 mL. Thus, one would expect greater variability in the results for ethanolamine than for LiOH. The observed standard deviations, however, do exceed the percent variation expected solely due to flushing errors. The author has obtained results with far less variability than those of the general chemistry students. Thus, the greater range of values the students obtain are likely due to procedural errors such as failing to maintain a sealed system during the absorption process, failing to check that the syringe barrel moves freely along its entire path, or initiating the absorption experiment before all of the dry ice has sublimed. Less experienced experimentalists are also prone to misreading the volume scale on the syringe as well as other recording errors. The value for LiOH deviates from its theoretical maximum of 0.92 that assumes a complete reaction. The actual value of the absorption capacity of LiOH can depend on a variety of factors, including particle size, humidity, temperature, and CO2 concentration. For flowing systems, the flow velocity and absorber bed geometry can also play a role. Although the reaction is diffusion controlled, the 170 mesh particle size of the LiOH is too small to affect the absorption capacity (9). The best-fit curve to published data of the absorption capacity at various temperatures, 190
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obtained using a flowing-gas method, gives a value of 0.67 at 35 °C (7). This result compares favorably with values obtained by our students at room temperature. The absorption capacity of ethanolamine depends on whether it is pure or in solution. Data in refs 17 and 18 predict an absorption capacity for pure ethanolamine of 0.12, which likewise shows excellent agreement with the average of the student data. At the beginning of the laboratory period, we show short clips from the film Apollo 13 (19) that portray engineers solving the problem of retrofitting LiOH canisters. Students use stoichiometry to predict the mass and volume of CO2 that react as well as the absorption capacity for each absorber. Each pair of students is required to complete two trials on LiOH and two trials on ethanolamine. Class data are compiled and shared during the post-laboratory discussion so each team can assess its results. They compare theoretical and experimental values and discuss which to use for planning a space mission. Students are then asked to compute the mass of LiOH required for a 10 day space shuttle mission with a crew of seven astronauts and for a 3 month trip to Mars for a crew of three. They will need to obtain information on the amount of CO2 expired per day per person. Finally, they compare the performance of both absorbers and consider how reversibility enters into these comparisons. The learning goals were assessed through a question on the final laboratory exam as well as the laboratory reports. The laboratory exam question requires students to apply their knowledge of why the experimental absorption capacity should be used to determine the amount of absorber required for a space flight. That question is given in the supporting information. The average score on the question for three classes (a total of 51 students) was 7.6 (out of 10 points) with 71% of the students earning 8 or more points on the question. Lab reports were used to assess the other learning goals. These reports show that 100% of the students are able to determine the mass of CO2 absorbed from the ideal gas equation. Of course, all students must perform this calculation for the post-laboratory discussion to commence, so some receive assistance to accomplish the task. The reports also show that 83% of the students who attempted to connect their qualitative observations to species in the reaction did so correctly. (Some students omitted this from their reports, however.) Finally, 95% of the students correctly used the class data to compare the absorption capacities of the absorbers. That performance was likely bolstered by the post-laboratory discussion of the question that precedes report submission. Literature Cited 1. Documentation for Immediately Dangerous to Life or Health Considerations ( IDLH); NTIS #PB-94-195047; National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention, U.S. Government Printing Office: Washington, DC, 1994. 2. Carbon Dioxide as a Fire Suppressant: Examining the Risks; EPA #430R-00-002; Environmental Protection Agency, U.S. Government Printing Office: Washington, DC, 2000.
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3. Carbon Dioxide, Industrial Exposure and Control Technologies for OSHA Regulated Hazardous Substances, Volume I of II, Substance A-1; Occupational Safety and Health Administration, U.S. Department of Labor, U.S. Government Printing Office: Washington, DC, 1989. 4. Bowman, W. H.; Lawrence, R. M. J. Chem. Educ. 1971, 48, 260– 262. 5. Kelter, P. B.; Snyder, W. E.; Buchar, C. S. J. Chem. Educ. 1987, 64, 60–62. 6. Boryta, D. A.; Maas, A. J. Ind. Eng. Chem. Process Des. Dev. 1971, 10, 489–494. 7. Wang, T. C. Ind. Eng. Chem. Process. Des. Dev. 1975, 14, 191–193. 8. Wang, T. C. Aviat., Space Environ. Med. 1981, 52, 104–108. 9. Boryta, D. A.; Maas, A. J. Carbon Dioxide Absorption Dynamics of Lithium Hydroxide. In The Characteristics of Carbon Dioxide Absorbing Agents for Life Support Equipment, presented at the Winter Annual Meeting of the American Society of Mechanical Engineers 1982; American Society of Mechanical Engineers: New York, 1982; pp 83-101. 10. Kelter, P.; Mosher, M.; Scott, A. Chemistry: The Practical Science; Houghton Mifflin: New York, 2008; p 113.
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11. Ebbing, D. D.; Gammon, S. D. General Chemistry, 8th ed.; Houghton Mifflin: New York, 2008; p 905. 12. Goll, J. G.; Woods, B. J. J. Chem. Educ. 1999, 76, 506–508. 13. Kildahl, N.; Varco-Shea, T. Explorations in Chemistry: A Manual for Discovery; John Wiley and Sons, Inc.: New York; 1996; pp 63-66. 14. Park, S. H.; Lee, K. B.; Hyun, J. C.; Kim, S. H. Ind. Eng. Chem. Res. 2002, 41, 1658–1665. 15. Aroonwilas, A.; Veawab, A.. Ind. Eng. Chem. Res. 2004, 43, 2228– 2237. 16. Ma'mum, S.; Nilsen, R.; Svendsen, H. F. J. Chem. Eng. Data 2005, 50, 630–634. 17. Yeh, A. C.; Bai, H. Sci. Total Environ. 1999, 228, 121–133. 18. Medinsky, M. A. J. Anal. Toxicol. 1986, 10, 24–27. 19. Apollo 13, Dir. Ron Howard, Universal Pictures, 1995.
Supporting Information Available Student handout and notes for the instructor. This material is available via the Internet at http://pubs.acs.org.
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