The Determination of Carbon Dioxide in Exhaled Air A Calculator-Based Laboratory Experiment

Transcription

1 The Determination of Carbon Dioxide in Exhaled Air A Calculator-Based Laboratory Experiment James Gordon * and Nathaniel Barbe Division of Science and Mathematics, Central Methodist University, Fayette, MO 65248, jgordon@centralmethodist.edu Abstract A detection method using current technology was applied to a classic experiment in which gaseous carbon dioxide from exhaled air was collected in a flask and ultimately reacted with aqueous sodium hydroxide. A gas pressure sensor interfaced to a calculator-based data collection system was used to determine the percent of carbon dioxide in exhaled air. The average percent of carbon dioxide was found to be approximately 5 %. Key Words Laboratory, Introductory/High School Chemistry, Gases, Calculator-Based Learning

2 The Determination of Carbon Dioxide in Exhaled Air A Calculator-Based Laboratory Experiment James Gordon * and Nathaniel Barbe Division of Science and Mathematics, Central Methodist University, Fayette, MO, 65248, jgordon@centralmethodist.edu Introduction Carbon dioxide is a rather common but none-the-less important gas. It is an important atmospheric gas for the greenhouse effect and potentially for global warming. It is also an important product of respiration in humans. Carbon dioxide in the atmosphere amounts to about 0.03% (1). However, following respiration, there is a significantly larger amount of carbon dioxide exiting the lungs. Capnometry is the determination of carbon dioxide in exhaled or respired air. A related term capnography includes the continuous monitoring of carbon dioxide that provides a graphical output of the carbon dioxide levels (2). The amount of carbon dioxide in respired air can be biologically important in situations including monitoring the severity of pulmonary disease (3), monitoring inspired CO 2 during its therapeutic use (4), and measuring the volume of CO 2 eliminated to assess metabolic rate (5). Capnometric measurements can be made in several ways including using infrared light, mass spectrometry, Raman spectroscopy and/or a colorimetric ph indicator (6-8). While the experiment described in this work is slow by comparison to clinical methods and for safety reasons would not be used in a clinical setting, it offers a relatively rapid determination of carbon dioxide for the academic lab setting. In these experiments, the reaction between carbon dioxide and sodium hydroxide is taken advantage of: CO 2 (g) + H 2 O (l) H 2 CO 3 (aq) H 2 CO 3 (aq) + NaOH (aq) NaCO 3 (aq) + H 2 O (l) Sodium hydroxide in either the solid or aqueous form absorbs carbon dioxide from the surrounding atmosphere. As seen in the reactions above, carbon dioxide reacts with water to produce carbonic acid. The acid-base chemistry in the second reaction shifts the equilibrium in the first reaction so that more and more of the carbon dioxide is reacted ultimately producing sodium carbonate. In this work, exhaled air is trapped in a closed container, and a calculator controlled data collection system interfaced to a gas pressure sensor is used to determine the decrease in the total gas pressure inside the container as sodium hydroxide reacts with the gaseous carbon dioxide. Great use of the calculator-based lab technology is made in the science laboratories at Central Methodist University. Students with no previous experience with the technology comment on the ease of use of these systems, and they appreciated the ability to collect and analyze real-time data. Having developed this experiment as a part of

3 a student research program, it will be implemented into the first semester General Chemistry lab experience. This experiment is suitable for any college or high school introductory chemistry course. Procedure A simple diagram of the apparatus for the experiment can be seen in Figure 1. Two pieces of glass tubing (about 10 cm each) were place into a 2-hole #8 rubber stopper fitting into the mouth of a 500-mL flask. A small 20- ml glass weighing bottle was filled with approximately 10 ml of 6M NaOH. A plastic cap cut from a vinegar bottle was placed over the top of the weighing bottle and the covered container was placed into the flask. Any small container and cover resistant to sodium hydroxide is suitable. It is important to cover the container of NaOH until the experiment is started or the carbon dioxide being exhaled into the flask will be prematurely reacted. The student placed a straw onto one of the glass tubes and inhaled a normal breath and then exhaled through the straw and into the flask. This procedure was repeated twice more. It was assumed at this point that the gas in the flask was fully composed of exhaled air because the average tidal volume of an adult male or female is approximately 500 ml (9). The student quickly removed the straw and plugged the glass tube with a small cork. The second glass tube was connected to a Vernier gas pressure sensor through a short rubber tube and Luer lock connector. straw pressure sensor Figure 1 Experimental Apparatus A Vernier LabPro data collection system (Vernier Software & Technology, Beaverton, OR) controlled by a TI-84 Plus calculator running the Datamate software was setup to collect data in the time graph mode. Data was collected for 500 seconds at a rate of 1 point per 10 seconds. A few data points were taken at the start of the

4 experiment to measure a baseline value. Then the glass weighing bottle holding the NaOH was tipped over inside the flask releasing the NaOH. The flask was occasionally swirled until the data collection was finished. The percent difference between the initial pressure and ending steady state pressure was calculated and used as the percent of carbon dioxide. Hazards Sodium hydroxide is a corrosive material that can cause damage to eyes and skin. Great care should be taken when using the 6M NaOH including the use of gloves and eye protection. Any NaOH on the skin should be immediately washed with copious amounts of water. The resulting experimental solution should be neutralized and disposed of according to local regulations Results and Conclusions A typical graph of gas pressure versus time is shown in Figure 2. The flask remained undisturbed for approximately one minute after data collection was started so that the initial pressure could be recorded. Immediately after the glass weigh bottle was tipped over the aqueous NaOH began to react with the carbonic acid produced by the carbon dioxide in the exhaled air thus shifting the equilibrium of the CO 2 reaction to right causing the rapid decrease in pressure. The pressure inside the flask continued to decrease until the final steady state pressure was reached, indicating the depletion of carbon dioxide from the gas mixture. The percent difference (equation below) between the starting and ending pressure values was used to determine the amount of carbon dioxide because the only decrease in pressure was due to the decrease in carbon dioxide within the flask. Percent Difference = Initial Pressure Final Pressure x100 Initial Pressure The average value for the percent of carbon dioxide in the exhaled air was found to be 5.0 ± 0.7% (n=6). This value compares quite well to the accepted capnometric end-tidal values of CO 2 of about 5% or 38 mm Hg (10). Acknowledgements The authors would like to thank Angie Cornelius and Andrew Herbig for helpful discussions during the production of this manuscript and Thoren Maule and Andria Altman for the use of their breath. We would also like to thank Central Methodist University for funding this research project.

5 Pressure / atm % diff = 5.5% Time / sec Figure 2. A typical pressure versus time curve for the reaction.

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