Materials Included In Kit

Additional Materials Required

Prelab Preparation

Plan carefully for the ozone test sites. Select areas where the test strips can be hung inconspicuously and undisturbed during the test time frame. Locations should be convenient to reach during class time or after school. Obtain permission or clearance for all sites as needed.

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly upon completion of this activity. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Excess starch/potassium iodide solution may be disposed of according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs, or for 15 groups of students. The activity can be completed in two class periods.

• This activity works best in areas of low humidity and high ambient ozone concentrations. In some parts of the country (especially rural areas) this activity may not produce very conclusive or interesting results.
• Check with local authorities to secure ozone readings for the days you conduct this activity. Have students compare their data with the scientific data. The actual ozone number is not critical. The relative amount of ozone is interesting and the relative comparisons of various locations (e.g., near freeways, copy machines, electrical outlets) can be very revealing.

Introduction

We have become all too familiar with “smog-alerts” and television reports asking for us to reduce vehicle traffic due to high ozone levels. How can we test for ozone? Why is it so bad?

Concepts

• Ozone chemistry

• Parts per million/billion
• Air pollution
• Troposphere
• Stratosphere

Background

Our atmosphere is divided roughly into two layers—the troposphere (between 0–9 kilometers above the Earth’s surface) and the stratosphere (9–15 kilometers above the Earth’s surface). About 90% of all natural ozone (O₃) gas exists in the upper stratosphere. This so-called “ozone layer” plays a key role in the Earth’s balance, by providing a protective shield for living things against harmful ultraviolet (UV) radiation from the sun. The effects of high levels of UV radiation include risks of cancers, cataracts, immune deficiencies, damage to plants and other genetic consequences. In the lower levels of the atmosphere (troposphere) ozone plays a destructive role as an irritant in smog.

In the stratosphere, ozone is usually found in concentrations of about 10–15 parts per million. Tropospheric ozone, usually occurs at about 120 parts per billion. Troposphere ozone is formed when hydrocarbons and nitrogen oxides from forests, industries and automobile exhaust react with heat and sunlight. In years past, tropospheric ozone didn’t seem to be affecting human health. But the quantity of ozone that has been recently produced by certain human activities has caused us to rethink acceptable ozone levels. The concentrations have increased to such high levels that ozone has become a real irritant.

While stratospheric ozone shields us from UV radiation, ozone in the lower troposphere is irritating and destructive to forests, crops, nylons, rubbers and other materials. High concentrations of ground level ozone injure or destroy living tissue and can be harmful to individuals with respiratory problems. Thus, we have a dual ozone problem—pollution or smog in the troposphere (“bad ozone”) and depletion of the ozone layer in the stratosphere (“good ozone”). These are two very different problems, both stemming from human industrial activities.

Since 1900, the amount of ozone near the Earth’s surface has more than doubled. In urban areas in the Northern hemisphere, high ozone levels usually occur during the warm, sunny summer months from May to September. Typically, ozone levels reach their peak late in the afternoon, after the Sun has had time to fully react with the exhaust fumes from cars. Tropospheric ozone is formed by the interaction of sunlight with hydrocarbons and nitrogen oxides which are emitted by automobiles and other industrial activities.

During the same time period (since 1900), industrial activity has released chemicals into the upper atmosphere that are simultaneously destroying upper level ozone molecules. See Figure 1 for one example of how ozone is destroyed.

This upper atmosphere destruction of ozone has created the very newsworthy “hole” in the atmosphere. This phenomenon continues to be monitored and studied rigorously. Understanding this upper level ozone phenomenon is potentially significant for climate, global warming, and perhaps even survival of life on Earth as we know it. The lower level “ozone” smog components have more immediate consequences and are easier to comprehend. Both problems are significant, interrelated, and continually studied. In this activity, the relative ozone concentration of the troposphere at ground level will be measured.

Christian Schoenbein discovered ozone in 1839. He established the presence of ozone in the air and demonstrated that it is a natural component of the air. He developed a way to measure ozone in the air using a mixture of starch and potassium iodide spread on filter paper. Schoenbein’s paper can be made and his original test for ozone duplicated.

The Schoenbein test is based upon the oxidizing ability of ozone—ozone is a stronger oxidizing agent than normal oxygen (O₂). Ozone in the air will oxidize potassium iodide on the test paper to produce iodine. The iodine in turn reacts with starch, staining the paper a shade of purple. The intensity of the purple color will depend upon the amount of ozone present in the air. The darker the color, the more ozone that is present. Two reactions are involved:

Materials

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly upon completion of this activity.

Procedure

Part I. Making Test Paper

1. Add 5 g of corn starch to 100 mL of distilled water in a 250-mL beaker.
2. Heat on a hot plate and stir the mixture with a stirring rod until it becomes evenly mixed and starts to gel. The mixture is gelled when it thickens and becomes somewhat translucent.
3. Carefully remove the beaker from the hot plate and add 1 g of potassium iodide. Stir well. Let the solution cool.
4. Place a piece of filter paper on a glass plate or other flat, cleanable surface. Using a small brush, brush the starch/potassium iodide solution evenly on one side of the filter paper. Turn the filter paper over and spread more of the solution on the other side of the paper. Apply the paste as uniformly as possible.

The paper can be used for testing at this point (proceed to step 7) or it can be readied for storage as described in steps 5–6.

5. Allow the paper to dry. Do not set it in direct sunlight. It can be air-dried overnight or the process can be sped up by placing the paper in a low-temperature drying oven.
6. When the paper is completely dried, cut the filter paper into 1" strips. Store the strips in an envelope inside a zipper-lock plastic bag or jar out of direct sunlight.

Part II. Testing for Ozone

7. If the test strip is wet from step 4, continue on to step 8. If the paper is dry (steps 5–6), dip a strip of the dried Schoenbein test paper in distilled water to reactivate.
8. Use an unfolded paper clip to make a hook to hang the wet test strip. Select a quiet place where you want to test for ozone. Be sure the test strip will be out of direct sunlight and that the strip can hang freely from the paper clip hook.
9. Expose the paper for approximately eight hours or overnight. When the strip is retrieved, either seal it in an airtight container for testing back at the lab or proceed to step 10.
10. To observe the test results, dip the paper in distilled water. Observe the color and determine the Schoenbein Number using the Schoenbein color scale on the Ozone Testing Worksheet.
11. Determine the relative humidity at your data collection site using a sling psychrometer or other humidity measuring device.
12. Follow the directions on the Ozone Testing Worksheet to determine the ozone concentration in parts per billion. Answer the discussion questions on the Ozone Testing Worksheet.
13. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

10416_Student1.pdf