Project AIR - Teacher Area

Hands-on Lab

Schoenbein Paper

Note: Depending on age and ability level of students, for safety purposes, consider preparing the paper as a demonstration.

Objectives
Students will:

prepare Schoenbein ozone testing paper
use the paper to test ground level ozone in the student investigation

Materials

hot plate
250 ml beaker
100 ml distilled water
1/4 teaspoon corn starch
glass stirring rod
1/4 teaspoon Potassium iodide
filter paper
small paint brush
Pyrex plate (optional)
paper or rack for drying filter paper
distilled water in spray bottle

Background

Christian Friedrich Schoenbein discovered ozone in 1839 in Switzerland. He developed a way to measure ozone in the troposphere using a mixture of starch, potassium iodide, and water spread on filter paper. Ozone causes iodide to oxidize into iodine, causing Schoenbein paper, to change color when ozone is present. The iodine reacts with starch, staining the paper a shade of purple. The intensity of the purple color depends on the amount of ozone present in the air. The darker the color, the more ozone is present.

The reactions involved are:
2KI⁺⁺ --> 2KOH⁺⁺ + starch --> blue color

Note: this activity works best in areas of low humidity and high ambient ozone concentrations.

Procedure

1. Place 100 ml of distilled water in a 250 ml beaker.

2. Add 1 1/4 teaspoon of corn starch.

3. Heat and stir mixture until it gels (thickens) and becomes somewhat translucent.

4. Remove the beaker from the heat source and add 1/4 teaspoon potassium iodide. Stir well. Allow the solution to cool before applying to the filter paper.

5. Lay a piece of filter paper on a glass plate, or hold it in the air, and carefully brush the paste onto the filter paper. Turn the filter paper over and do the same on the other side. Try to apply the paste as uniformly as possible.

6. Wash hands after applying the potassium iodide mixture. (Although potassium iodide is not toxic, it can cause mild skin irritation.)

7. Place the paper out of direct sunlight and allow it to dry. To save time, place the paper on a microwave-safe plate and microwave for one minute.

8. Cut the filter paper into 1-inch wide strips.

9. To store the paper, place the strips in a sealed plastic bag or glass jar out of direct sunlight.

Testing Procedure

1. Spray a strip of test paper with distilled water and hang it at a data collection site out of direct sunlight. Make sure the strip can hang freely.

Note: Because relative humidity affects results, Schoenbein paper should not be left outside during periods of high humidity.

2. Expose the paper for approximately eight hours.

3. Observe and record test results immediately. Use the Schoenbein Color Scale and Relative Humidity Chart to obtain an ozone reading. If the color change of the paper was not uniform, determine the Schoenbein Number by the color in an area with the most noticeable change.

Schoenbein Color Scale

                                   0 - 3
Little to no change

                                  4 - 6
Lavender hue

                                    7 - 10
Blue or Purple

4. Obtain the relative humidity for the local area. Round off the relative humidity reading to the nearest 10 percent. (Higher relative humidity makes the paper more sensitive to ozone, and a higher Schoenbein Number could be observed. To correct for this, the relative humidity must be determined and figured into the calculation of ozone concentration.) Refer to the Relative Humidity Schoenbein Number Chart. Along the bottom of the chart, find the point that corresponds to the Schoenbein number recorded from the paper. From that point, draw a line straight upward until it intersects with the curve that corresponds to your relative humidity reading. To find the ozone concentration in parts per billion, draw a perpendicular line from the Schoenbein number/relative humidity point of intersection to the left side of the chart.

Note: Please be aware of false positive test results that may occur from NO_x in heavy traffic areas.

This lesson was adapted from the University Corporation for Atmospheric Research, Cycles of the Earth and Atmosphere module and the Environmental Resource Guide from the Air and Waste Management Association.