Demonstrating The Effect Of Co2 On The Ph Of Water


Demonstrating the effect of CO
on the pH of water – The bad breath indicator
Charles J. Marzzacco
Department of Chemistry
Florida Institute of Technology
Melbourne FL 32901
The colorful demonstration that I describe is one I have
3. Add 20 mL of a buffer solution that is 1.0 M with respect to
performed successfully in front of elementary, middle, high
and 1.0 M with respect to NH
Cl to the first indicator
school and college students. It makes an ideal addition to a
solution. This should result in a blue-violet color.
chemistry demonstration show. It never fails to get a laugh, and
4. Add 0.10 M NH
to the second solution until the color of the
it nicely demonstrates the acidity of CO
solution is identical to that of the first solution. It should take
about 6 mL.
The demonstration involves adding a quantity of universal
indicator to each of two samples of distilled water. Both
5. Use a 20 inch piece of plastic tubing for each student to
solutions are adjusted to a pH slightly above 9. In the first
breathe into. Tell the students to avoid having their face
solution, a 1:1 NH
Cl buffer solution is used. This buffered
directly over the bottle.
solution should exhibit a blue-violet color characteristic of a pH
slightly above 9. In the second solution, the adjustment is made
by adding dilute ammonia a little at a time until the color is the
same as that of the first solution.
The pH of the unbuffered solution drops because of the CO
exhaled breath. The CO
dissolves in water where it acts as a
The two solutions are prepared in advance. The students
Lewis acid and forms carbonic acid, a weak acid.
observe two identical looking blue-violet solutions. They are led
to believe that the two solutions are identical and are told that
(aq) + H
they will be used to test the breath quality of two students to be
- (aq) + H
O + (aq)
selected by the instructor. Each student is assigned one of the
(aq) + H
solutions. The student assigned to the buffered solution is
asked to gently and repeatedly blow through a tube into the
Once the small amount of NH
in the unbuffered solution is
solution. Little change in the color of the solution will be
neutralized, the pH drops into the acidic range. The NH
observed because the pH doesn’t change. The class is told that
concentration in the buffered solution is much larger than that in
the lack of color change indicates that this student has good
the unbuffered solution. The passage of many breaths into the
breath. The student assigned to the unbuffered solution is then
buffered solution does not supply enough CO
to significantly
asked to gently and repeatedly blow into his or her solution.
deplete the large amount of NH
that is present in the solution.
After about five breaths, the solution will change from blue-violet
There is only a slight change in the pH of the solution.
to green. After a total of about 10 breaths it will be yellow-green,
Therefore, the color of this solution does not change
and by 15 breaths it will be yellow. You can expect to hear
considerable laughter from the class. There is no need to tell
them that this student has very bad breath. The observation
Standby uses a lot of electricity
and laughter says it all.
A report to the British House of Commons reveals that the 74
While the second student is breathing into the solution, the pH
million TV sets in the United Kingdom use about 7 TWh
slowly drops from 9 to about 5.5. The change is due to the
(1 terawatt = 10
watts) of electricity each year merely by being
acidity of the CO
in the breath. The nature of the class
left in standby mode instead of being turned off. (Read about it
discussion following the demonstration will depend on the level
at ) This
of chemical knowledge of the students.
amount of electricity costs about $1.5 billion and is equivalent to
the output of two electricity generating stations. Some British
TV sets use as much as two-thirds the amount of power in
standby mode as when they are in actual use. A survey shows
1. Add one liter of distilled or tap water to each of two used
many laypeople think that standby mode uses almost no
2.5-L acid bottles.
electricity while a few think that standby mode uses less power
2. Add 5 mL of Fisher Universal Indicator solution to each.
than when the TV is turned off! Some members of Parliament
If distilled water is used, the solutions will likely exhibit a
are contemplating legislation prohibiting the manufacture of TV
yellow appearance indicating a pH of about 5.5. With tap
sets with a standby button. Have your students calculate how
water the color could be yellow, green or blue, depending on
much electricity is used by the average television set in standby
the acidity of the water.
mode, assuming it is in that condition, say, 20 hours per day.
4 CHEM 13 NEWS/March 2006


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