Ground-level Ozone Testing
Background Information:
Ozone (O3) is a form of oxygen that is normally found in Earth’s stratosphere where it functions to absorb harmful ultraviolet radiation from the sun. However, at ground level in the troposphere, ozone is a pollutant. Ozone is formed at ground-level through a photochemical reaction involving other pollutants. The pollutants that react to form ozone are nitrogen oxides (NOx) and volatile organic compounds (VOCs). Motor vehicle exhaust, industrial plant pollution, gasoline vapors, and chemical solvents release nitrogen oxides and VOCs. Sunlight and warm weather cause these pollutants to react and form ozone. Ozone is a respiratory irritant and can be harmful to people when levels are high. Exposure to ozone can cause coughing, wheezing, and pain when breathing. Ozone aggravates the symptoms of asthma and other lung diseases such as emphysema and, with repeated exposure, permanent lung damage can result. Ground-level ozone also damages trees and crop plants. The EPA monitors ozone in many parts of the county. The air quality index for ozone ranges from good (green) to very unhealthy (purple). Ozone action days may be called when ozone levels are forecast to reach unhealthy levels. These programs encourage voluntary actions to reduce emissions of pollutants that contribute to ground-level ozone formation.
In this lab we will conduct a test to measure ground-level ozone developed by Dr. Schoenbein. This test is based on the oxidation capability of ozone. Ozone in the air will oxidize potassium iodide (KI) on a piece of paper to produce iodine. The iodine reacts with starch to produce a purple color. The darker or more purple the color, the higher the amount of ozone present in the air. This reaction is shown by the following equation:
2KI + O3 + H2O à 2KOH + O2 + I2
For this experiment, each group will need to decide on a variable to test. This variable will determine the purpose of the lab. Your group’s hypothesis should also be based on this variable.
Materials:
Potassium iodide filter paper corn starch
Glass stirring rod small paint brush beaker
Hot plate distilled water Schoenbein color scale
Methods:
Preparation of Schoenbein paper:
1. Place 50 ml of distilled water in a beaker will 2.5 grams of cornstarch
2. Heat and stir mixture until it gels (mixture will become translucent)
3. Remove the beaker from the heat and add 0.5 grams potassium iodide. Stir well. Allow the solution to cool.
4. Carefully brush the paste onto a piece of filter paper. It is important to apply the paste as uniformly as possible. Turn the filter paper over and brush the paste onto the other side.
5. Allow the filter paper to dry. You may microwave the paper for 30-45 seconds.
6. Place the paper in a Ziploc bag until time of exposure.
Testing Procedure:
1. Wet the test paper with distilled water and hang it at the collection site. IT MUST BE OUT OF DIRECT SUNLIGHT. Make sure the paper hangs freely.
2. Expose the paper for approximately 8 hours. Immediately after exposure, return the paper to a Ziploc bag.
3. To observe and record test results, wet the paper with distilled water. Observe the color and determine the number on the Schoenbein scale.
4. Determine the local humidity by using a bulb psychrometer or local weather data. Round the relative humidity to the nearest 10%. High relative humidity makes the paper more sensitive to ozone and a higher Schoenbein number is observed. To correct for this, the relative humidity must be determined and factored in. Use the chart provided to correct for relative humidity and determine the ozone concentration in ppb.
Observations:
(Your variable) - / Schoenbein number / Ozone (ppb)Analysis:
1. How did the test papers change after exposure? Why did this change take place?
2. What was the relative humidity for your test day? Is this considered “high” or “low”?
3. How does relative humidity of 30% vs. 70% affect a Schoenbein number of 4?
4. Explain any differences in the individual test papers. WHY do you think each group member got the results that they did?
5. Did you accept or reject your hypothesis? Explain.
6. What factors affect the reliability of the Schoenbein test papers?
7. What limitations exist in using the Schoenbein paper to measure ozone?
8. How could you increase the accuracy of measuring ozone?