STEM Education: Measuring Ground Level Ozone
Deborah Carlisle
Preparing Ozone Test Strips (aka Schoenbein Papers)
Background Information:
In this lab, you will prepare test strips that are chemically sensitive to ozone and use
them to measure the concentration of ground-level ozone. Test papers similar to ours
were first prepared over 100 years ago by Christian Friedrich Schoenbein (1799-1868),
a professor at the University of Basel, Switzerland. To prepare the ozone sensitive
paper, filter paper will be coated with a gelled paste made of starch and water,
containing a small amount of potassium iodide. To use the Schoenbein paper, you will
hang it, in the air, out of direct sunlight, for 2 hours. During this time a chemical reaction
will take place if there is ozone present in the air. Ozone is a very reactive molecule that
will oxidize the potassium iodide salt (KI) on the Schoenbein paper forming elemental
iodine (I2). The iodine then reacts with the starch on the paper to produce a purple color.
The intensity of the purple color on the exposed Schoenbein paper correlates with the
concentration of ozone present in the air at the test site. The two chemical reactions
follow:
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2)
2KI + 03 + H2O  2KOH + O2 + I2
I2 + I-  I3- + starch (Yields a Blue /Purple color)
Learning Goals
By the end of the investigation students will be able to:
 Use their test results and interpret the relative amount of ozone in
the air.
 Understand that ozone is found near the earth’s surface(ground
level) in the troposphere as well as the stratosphere.
 Analyze different relative amounts of ozone found in various
locations.
 Understand that ozone test strips/Schoenbien paper detect ozone
gas through a chemical reaction that is caused by ozone in the air.
 Use technology to more precisely analyze relative color changes
obtained with ozone test strips.
National Science Education Standards:
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Science as Inquiry: Grades 5-8 and 9-12
Ability to perform scientific inquiry, understanding about scientific
inquiry
Science and Technology: Grades 5-8 and 9-12
Understanding about science and technology
Science in Personal and Social Perspectives: Grades 9-12
Environmental quality, changes in environments
Key Questions:
Where is ozone gas found?
Does the amount of ozone gas vary from day to day?
Do some areas/locations have more ozone than others? Can we identify them?
What conclusions can you draw about ozone gas based on your test results?
Materials:
1 Batch makes 12,15.0 cm filter papers (enough for 1 class of 30)
Potassium Iodide (solid) 1/4 teaspoon
Distilled water
Filter paper
Hot plate or stove top (heat source)
Corn Starch (1 tablespoon is plenty)
Glass stirring rod
Artists paint brush (medium size)
250 mL beaker
Pyrex pie plate
Pot holder of mitt for removing beaker from heat source
5-6 Plastic dinner plates w/dividers (for drying prepared filter papers)
Rubber/or laytex gloves (optional)
Scotch tape
Index cards
Procedure:
1. Measure 90 mL of distilled water in a 250 mL beaker.
2. Add 1 ¼ teaspoons of corn starch to a small beaker or ceramic cup, add
10 mL of distilled water, and mix with the glass stirring rod.
3. Add the corn starch mixture to the 90 mL of water in the beaker.
Pre-mixing the starch with a small amount of water helps it to mix and evenly
distribute in the larger volume of water. (The starch does not dissolve, but is
suspended.)
4. Place the beaker containing the starch on the hot plate. Turn the hot plate to a
med-high setting. Stir the solution continuously, using the glass stirring rod, until
it gels. The solution will become translucent and thicken when it is gelled.
If using a stove, add 2-3” of water to a small sauce pan and heat the water to a
boil. Remove pan from heat and place the beaker in the water (similar to a
double boiler). Continue to heat the starch and water solution on a medium
setting. Stir starch solution continuously until it gels.
5. Once solution is gelled remove from the heat and add ¼ of teaspoon of
potassium iodide(KI) solid. Stir well until all of the potassium iodide is dissolved.
Allow the solution to cool for 5 minutes before applying to the filter paper. Stir
again right before application.
6. Lay a piece of filter paper on the pyrex plate. Hold onto one edge and lift the
filter paper up, just a little. Dip your brush into the starch/KI solution and begin to
apply the starch mixture to the filter paper. The application comes out the best
when you apply the mixture with the grain of the filter paper. Continue to apply
the starch evenly across the side of the filter paper, going back over areas that
have too much or too little to smooth them out.
7. Flip over the filter paper in your glass plate. It may gently stick to the bottom,
so lift it slowly. Still holding the edge, apply the starch/KI mixture to the opposite
side of the filter paper. Both sides should have an even application of the
starch/KI mixture.
8. Lay the prepared filter paper on a plastic dinner plate so that the dividers
support it (lift it) off the bottom. The papers dry better and stick less when less of
the gelled starch is in contact with the bottom of the plate.
9. If not wearing gloves, wash your hands thoroughly after applying the starch/KI
mixture. Potassium iodide is not toxic, but can cause skin irritation.
10. Allow the paper to dry out of direct sunlight. Drying requires approximately 2
hours under normal indoor conditions. If it is a humid day a low temperature
drying oven is helpful. Helpful tip: After about 30 minutes gently lift the papers off
the plate to be sure they are not sticking too much. If they are this will separate
the areas that are adhering.
11. Place the dry filter papers in a double zip-lock,TM bag for storage. Remove
excess air from the zip lock bag by flattening it with your hand. Filter papers
should be stored inside a brown paper bag to minimize exposure to sunlight.
12. Before use cut filter papers into ½ or ¾ inch strips. Hold one section of the
filter paper near the edge and try to avoid touching the surface of the paper. Lots
of fingerprints may decrease the sensitivity of the paper.
Use of Ozone Test Strips:
Students may carry out the testing procedure in groups of 3-4, or individually as
time and materials allow.
Teacher note: Test strips do not yield conclusive results during periods of high
humidity.
1. Each student group should use 4 test strips (two for controls and two for
testing).
2. Students should select one or two test sites to monitor with their test strips.
Students may test two different sites or place replicate strips at one location.
Teacher note: It is a good idea to have more than one student group test
the same location. This provides a useful comparison and replicate of
results.
3. Prepare index cards with the test site location name, the date, time of
exposure (both initial and final), and student name(s). Test strips should be
attached to the index card using scotch tape. Be sure to include a control that will
be placed indoors, in a convenient location, out of direct sunlight.
4. Students should hang test strips at the chosen test site using tape. Test strips
need to be attached so that they are able to move freely and have maximum
exposure to the moving air. Place out of direct sunlight.
Note: Direct sunlight causes chemical changes that increase the oxidation of the
iodide ion interfering with conclusive ozone monitoring.
5. Students should hang their control strips indoors immediately after placing
strips at their test site. It is important that the control strips are exposed to air for
about the same amount of time as the strips used at the test site. Hang the
control strips so that they can move freely in the air. Select a location that will not
be disturbed, and out of direct sunlight.
Note: Do not place near a photocopy machine as these actually produce ozone
and will give falsely high control results.
6. Collect the test strips after a 2-3 hour exposure. Note the collection time on the
index card, and seal the test strips in a zip-lock bag. If the results are not being
recorded that day, the strips should be double bagged to protect from exposure
to the air. Place strips out of sunlight.
7. When ready to photograph and record the results of the test strips, remove
from the ziplockTM bags. Dip each strip about ¾ of the way into a beaker of tap
water. Lay moistened strip on a paper towel and photograph with your digital
camera. The control strip should also be dipped in water and photographed along
side the test strip for accurate comparison. Carefully observe the color change.
Photographing your Test Strips:
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For best results use natural or fluorescent lighting conditions.
Hold camera a fixed distance from the strips. A distance of
approximately 18 inches works well.
Stabilize your camera with a tripod if you have one or by placing it
on top of something steady like a ring stand, or a stack of 3-4 text
books.
Turn flash mode to OFF.
Place a gray scale at the top of the image near the strips for use as
a relative comparison to RGB values. This also helps to check the
exposure.