- Weigh 5 g of KI.
- Dissolve KI in the liquid starch.
- Cut filter paper into strips about 2-3 cm wide.
- Using a small paintbrush, apply the starch/KI solution to the filter paper strips.
- Allow the filter paper strips to dry. You may do this in a drying oven at low temperature,
or under a fume hood.
- When the filter paper strips are dry, store them in a sealable plastic bag until you are
ready to use them.
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Corn starch Potassium iodide 250 ml beaker Filter paper |
Small paintbrush Sealable plastic bags Wet bulb psychometer |
The Ozone Detector
You may not know this, but ozone can't hide from a clever chemist. Ozone is a reactive
substance. One of the things it reacts easily with is potassium iodide (KI). Here's the
reaction that takes place between KI, ozone, and water:
This leaves us with potassium hydroxide (KOH), oxygen (O2),
and iodine (I2). The iodine is what we're really concerned
with, because iodine turns corn starch purple. That's right, iodine reacts with corn starch,
and the reaction products are usually purple or bluish in color.
This will help us find that ozone that is lurking around here in the troposphere where it doesn't
belong. All we have to do is make a mixture of KI and corn starch, and we're ready to go ozone
hunting. The KI will react to any ozone around to produce iodine. The iodine will then react
with the corn starch, turning it purple. Therefore, if the mixture turns purple, we know ozone
is in the air.
Using this knowledge we can make a nifty ozone detector by mixing corn starch and KI in water,
and painting the mixture onto sheets of paper. Below are instructions on how to make and use
detector.
Making the Detector
Place the detector in a safe secure place outdoors for at least eight hours. If ozone is present
the detector will turn a purple color. The more ozone is present, the darker the color will be.
Strong light can interfere
with the reaction of ozone with KI, so make sure you put your detector in the shade. You will
need to know the humidity at the time and place where you carry out the experiment. You can
measure the humidity yourself with a wet bulb psychometer. If you don't have a wet bulb
psychometer, you
an get that information from a weather report. For example, The Weather Channel has easily accessible current local weather conditions
for any location in the United States. After eight hours, compare the color of your detector to
the scale below:
Schšnbein number scale.
Look at the color of your detector and find the same shade on the sale to determine the
Schšnbein number. The Schšnbein number is just a measure of how dark your detector has become.
You will use this number to estimate the ozone level at the location where you placed your
detector. You can estimate the ozone level using the graph below.
Schšnbein number vs. ozone level at various
humidities.
The Big Picture
Carry out your ozone measurement somewhere near your home, in your backyard, or on a window
ledge if you live in an apartment. Then together with the other students in your class, plot
your results on a map of your town. Do some parts of town show higher ozone levels than others?
If so, can you find a cause for the higher levels in some places?
Using the Detector
For more information, at other Web sites...
AIRNOW — contains information on
ground-level ozone, including ozone level maps for many states, from the U.S. Environmental
Protection Agency, Office of Air Quality Planning & Standards.