MadSci Network: Chemistry

Re: Ozone Detection

Area: Chemistry
Posted By: John Christie, Faculty, School of Physical Chemistry, La Trobe University, Bundoora, Victoria, Australia
Date: Wed Oct 29 18:51:32 1997
Area of science: Chemistry
ID: 877746670.Ch

(1) If I had a small, covered container with normal (from the room) air in it under a UV light source, would Ozone be produced?

Ozone can be produced by UV irradiation of air, but to produce it directly would require very short wavelength UV light, of wavelength < 220 nm. In practice, this means that you would have to have a quartz window rather than a glass one both on your UV light source and on your gas vessel, and also not too much air gap between lamp and vessel.

This wavelength is needed to break down molecular oxygen into atomic oxygen

O2 --> O + O.    (UV light wavelength < 220 nm )
This is the way atomic oxygen is produced above 50 km altitude in the upper atmosphere.

Heavily polluted air contains nitrogen dioxide, a brown coloured gas, and any UV light is sufficient to produce atomic oxygen.

NO2 --> NO + O.   (UV light wavelength < 400 nm )
This is the way atomic oxygen is produced in episodes of photochemical smog in the atmosphere at ground level.

In either case, once you have atomic oxygen, ozone is produced by

O + O2 + another molecule --> O3 + another molecule.
A three body collision is required, but it does not much matter what the third molecule is. In air, the oxygen or nitrogen already present will serve that purpose.

(2) Also would ozone be produced if the normal air had a higher-than-normal oxygen percentage?

You would get a bit more ozone, but not enough to be worth the extra trouble of enriching the air.

(3) If Ozone would be produced, is there a fairly simple way to detect it?

Ozone has fairly similar reactions to normal oxygen, but is much more reactive, and is a much stronger oxidizing agent. One good simple way to detect it is by its reaction with potassium iodide. Make up some potassium iodide solution (about 1-2 g/L) in some alkaline borate buffer (pH in the 9-10 range). Add a little soluble starch indicator or 'Iotect'(reg. Trade Mark) indicator. Either shake up 20 mL of this solution with your gas sample in the gas vessel, or bubble the gas from your gas vessel through the solution. A dark blue colour forming in the solution indicates the presence of ozone ***BUT*** other strong oxidizing agents, most notably NO2, will also produce this colour. O2 gas should not.

N.B. You may well get this blue colour when you first make up the solution. If you do, it means that the potassium iodide has 'gone off', i.e. has already started to oxidise. Make up some sodium thiosulfate solution, and add just enough of it to remove the blue colour before starting your test.


If the main aim of your exercise is simply to produce ozone, there is a much simpler and more effective procedure than UV iradiation, involving a silent electrical discharge in a foil-lined tube through which air is flowing. The details can be found in many advanced chemistry textbooks.

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