Measuring Carbon Dioxide in Breath

Transcription

1 Measuring Carbon Dioxide in Breath OBJECTIVES 1. Measure the partial pressure of carbon dioxide in your breath 2. Estimate the volume of air you exhale per day 3. Estimate the volume and mass of CO2 you produce in 24 hours INTRODUCTION Cellular respiration is the process that provides energy for the reactions of life. In most aerobic organisms, the overall process is fueled by the systematic reaction of glucose with oxygen to form carbon dioxide and water, although other fuels can also be used as well. C6H12O6 + 6 O2 (g) à 6 CO2 (g) + 6 H2O (l) + energy As an aerobic organism, you are always producing carbon dioxide by this process. In this experiment, we will try to estimate just how much CO2 you produce in 24 hours. In Part I, you will measure the partial pressure of CO2 in your breath. Dalton s Law of Partial Pressures says that the total pressure of a mixture of gases is the sum of the pressure exerted by each component: Ptot = P1 + P2 + P3 + The main gases present in your breath are nitrogen (N2), oxygen (O2), water vapor (H2O) and carbon dioxide (CO2), but we will focus on the last one. To measure the pressure exerted by just CO2, you will fill a container with your breath, and then remove the CO2 by this reaction: NaOH (aq) + CO2 (g) à NaHCO3 (aq) Notice that the carbon atoms are no longer part of the gas at the end of the reaction, but are dissolved in solution as bicarbonate ions. This means that the CO2 will cease to exert pressure in the flask, and the total pressure will drop. You can measure this pressure change to find how much CO2 was in your breath before the reaction. In Part II of the experiment, you will estimate the total volume of air you breathe in a day. Using data from both parts of the experiment, it is possible to calculate the volume and mass of CO2 you produce in 24 hours. Measuring CO2 in Breath Page 1

2 PROCEDURE Part I. Measuring partial pressure of carbon dioxide in your breath 1. Assemble the flask and tubing as shown in the figure. Push the stopper firmly into the flask so it will not leak. Moisten the end of the long glass tube and carefully slip the rubber tubing over the end. (You will add the vial and stir bar later.) Figure 1. Setup for measuring partial pressure of CO2 2. Attach your plastic straw to the open end of the rubber tubing, and use your mouth to draw water at least halfway up into the long (50-60 cm) glass tube. Clamp the tube with a pinch clamp as shown in the figure, and wait a minute or two, watching to make sure there are no leaks. (How would you know if there was a leak in your setup?) Measuring CO2 in Breath Page 2

3 3. Once you are confident that there are no leaks in your setup, remove the stopper from the flask and place a small (4 ml) vial on the end of your thistle tube. Using the tube, press the vial up onto the bottom of the inverted Florence flask. Then turn the whole flask, vial and tube upright. Your instructor will show you how to do this. 4. While keeping the thistle tube down in the vial so the vial remains upright, clamp the flask to a ring stand while it sits on a stir plate. Carefully add about 0.5 ml of silicone oil and 2 ml of 6 mol/l NaOH to your thistle tube/vial. The oil should coat the surface of the base to prevent it from reacting with air. Caution: the NaOH solution is very corrosive to your skin and eyes. Wear goggles, and wash your hands soon after using it. 5. Add a stir bar to the Florence flask, and position the flask on your stir plate so that the stir bar is not touching the vial. Position the vial so that it will stay upright on its own. Carefully remove the thistle tube, and replace the rubber stopper in the vial. 6. If you have been able to do all this and your vial is still upright with NaOH solution and oil in it, you have completed the most difficult part of the lab. Good job! Otherwise, go back and do it again. You will need to wash out your Florence flask if you spilled the vial too soon. 7. Purge the air in the flask with your breath by blowing through your straw for at least 2 minutes. Lots of bubbles should come out the end of the glass tube. 8. Place the pinch clamp on the rubber tubing as before so the system is sealed, and turn on the stir plate. Tip the vial over, and position the flask so that the stir bar is mixing or splashing the NaOH around a little. 9. Note the time, and use a dry erase marker to show the water level every five minutes. When five minutes goes by without a change in the water level, measure the height of the water column in mm with a meter stick. 10. The height of a water column is a simple way to measure the pressure difference between the two surfaces of the water exposed to gas. Use the equation below to calculate the pressure decrease in millimeters of mercury (mm Hg). This is the same unit that is commonly used to express blood pressures. P!"! mm Hg = h mm H!O When you are all done, wash out your Florence flask and thistle tube to remove the NaOH for the next user. Measuring CO2 in Breath Page 3

4 Part II. Determining your daily air output In order to determine the amount of CO2 you produce in 24 hours, you need to know not only the partial pressure of CO2 in your breath, but also how much air you breathe out in a given time period. There are a number of different measures of lung capacity (see figure). Notice that a typical breath at rest is much smaller than a deep breath. Together with your lab partner, you will measure tidal volume. There are spirometers available in the lab for measuring volumes. Get a clean mouthpiece for your own use, and note that they work by inhaling through them, not exhaling as you did with the Florence flask. It is helpful to nimbly adjust the movable plastic marker, as the volumes can otherwise be difficult to read with much precision. Make 5 measurements of tidal volume (to the nearest 100 ml), and average your results before continuing with your calculations. Figure 2. Measures of lung capacity You will also need to find the number of breaths you take per minute. It is hard to do this while you are thinking about it, so have your lab partner can observe your breathing for 60 seconds when you are not paying attention and record the number of breaths you take. Measuring CO2 in Breath Page 4

5 Name: Pre- lab Questions: Measuring CO2 in Breath 1. What gases (give their chemical formulas) make up air and breath? 2. What is the difference between the air you exhale and the air you inhale? 3. Look at Figure 2. If you inhale deeply, which volume on this chart does that correspond to? 4. According to Figure 2, is it possible to exhale all the air from your lungs? 5. If you breathe 15 times a minute while sleeping, and each breath is about 500 ml, how many L of air do you breathe in 2.00 hours? Measuring CO2 in Breath Page 5

6 Name: Lab Report Sheet: Measuring CO2 in Breath Part I. Partial Pressure of CO2 Name of test subject Height of water column (mm H2O): 1. Calculate the partial pressure of CO2 in your breath in mm Hg. 2. The total pressure at sea level is around 760 mm Hg. What percentage of the air pressure you breathe out is carbon dioxide? Remember (% = part / total X 100) 3. For gases, percent by pressure and percent by volume are the same. Rewrite your percentage of CO2 as a volume conversion factor. (# L CO2 / 100 L breath) Part II. Respiratory Volumes (Same test subject as part I) Trial # Average Volume (ml) Number of breaths in 60 seconds: 4. Write these two results as conversion factors below. (What are the units?) Measuring CO2 in Breath Page 6

7 5. For this question, you will calculate the mass of CO2 produced in 24 hours. Show your calculations and use conversion factors for each part. a. How many breaths would be taken in 24 hours? b. How many L of breath are exhaled during this time? c. Remember that not all the air you breathe out is CO2. Using the percentage of CO2 you found for breath, calculate how many L of carbon dioxide gas you exhale per 24 hour day. d. The density of pure CO2 at the conditions of this experiment (20 C and normal atmospheric pressure) is 1.83 g/l. Use this density to find the mass (in grams) of CO2 you produce in a 24 hour day. Measuring CO2 in Breath Page 7