Lab: Field testing for Ozone
adapted from Project Learn, National Center for Atmospheric Research, Boulder, Colorado;
April 2004
Ozone in the air will oxidize with potassium iodide on “Schoenbein paper” to produce iodine. The iodine will react with
the starch to produce a purple color. The intensity of purple is directly proportional to the amount of ozone in the air. The
two reactions are:
2 KI + O3 + H2O  2 KOH + O2 + I2
then
I2 + starch  blue or purplish color
After touching the Schoenbein paper, wash your hands thoroughly with soap and
water. KI is mildly poisonous.
PROCEDURE:
Obtain a test strip of Schoenbein paper and place it into an air-tight ziplock baggie or into a glass jar. Keep it
out of the sunlight.
Choose a test site for your Schoenbein paper. Spray the test paper with distilled water and hang it at the data
collection site out of direct sunlight. Make sure the test strip can flutter freely in the air.
Expose the test strip to the air for approximately 8 hours during which time it should dry out. (This type of test
strip won’t work when the humidity is high.) After the 8 hours, drop the dry test strip back into the baggie and
seal the baggie, trying to remove as much air as possible. When you’re ready, remove the test strip from the
baggie and spray it with distilled water. Compare the color on the test strip with the Schoenbein color scale.
Record your data as a Schoenbein Number.
Schoenbein Number
0
1
3
5
7
9 10
---------------------------------------------------------------------yellow/white
pale
dark
lavender
blue or purple
High humidity makes the test paper more sensitive to ozone and you will get an inflated Schoenbein Number.
To correct for the humidity factor, check the relative humidity of your test site at the time you collect your test
strip.
Determine the relative humidity (to the nearest 10%) then use the Relative Humidity Correction Chart to find
the correct ozone level.
Start at the bottom of the Relative Humidity Correction Chart. Find the point that corresponds to the
Schoenbein Number from your test strip. From that point draw a line upward until it intersects with the curve
that represents the relative humidity of your test site. To find the ozone concentration in parts per billion, draw a
perpendicular line to the left of the Schoenbein Number/relative humidity intersection point.
Lab Field Testing for Ozone
C. Schneider
page 1
RESULTS
1. Where was your test site?
During which hours did you test for ozone?
What was the Schoenbein Number of your test strip?
What was the relative humidity of your test site?
Using the Relative Humidity Correction Chart, what was the ozone level at your test site?
2. Any problems in your testing procedure? If so, describe.
3. How did your results compare with those obtained by other students? Similar? Different? Were
your testing sites comparable?
4. Was your ozone level high, medium or low? What do you think caused/causes this level? e.g.
heavy auto traffic or lack of auto traffic? sunlight or lack of sunlight?
Lab Field Testing for Ozone
C. Schneider
page 2
SUMMARY QUESTIONS:
5. Three students tested for ozone with Schoenbein test strips at Lindbergh Field (San Diego Airport)
and all three recorded a Schoenbein Number of 5. However Teddy tested with a final relative humidity
of 80%. Tina tested with a final relative humidity of 60%. And Tim tested with a final relative
humidity of 40%. What were the corrected ozone readings for these three students?
6. Teddy, Tina and Tim tested for ozone at the same site, yet their ozone concentrations were
different. Explain how that could have happened.
7. We used the Schoenbein method to test for ozone in the troposphere. TxDOT (our state transit
authority) doesn’t use this method to test for ozone. Why not?
8. How are Ozone levels tested in the Stratosphere?
9. Describe how Ozone is a good thing in the Stratosphere (show the formulas…), and a bad thing in
the Troposphere.
Lab Field Testing for Ozone
C. Schneider
page 3