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Titration is a general class of experiment where a known property of
one solution is used to
infer an unknown property of another solution. In acid-base
chemistry, we often use titration to determine the pH of a certain solution.
A setup for the titration of an acid with a base is shown in :
Figure %: A titration setup
We use this instrumentation to calculate the amount of unknown
acid in the receiving
flask by measuring the amount of base, or titrant, it takes to neutralize
the acid.
There are two major ways to
know when the solution has been neutralized. The first uses a pH meter in
the receiving flask adding
base slowly until the pH reads exactly 7. The second method uses an
indicator. An indicator is an
acid or base whose conjugate acid or conjugate base has a color
different from that of the original
compound. The color changes when the solution contains a 1:1 mixture of
the differently colored
forms of the indicator. As you know from the Henderson-Hasselbalch
equation, the pH equals
the pKa of the indicator at the endpoint of the
indicator. Since we know the pH of the
solution and
the volume of
titrant added, we can then deduce how much base was needed to neutralize
the unknown sample.
Titration Curves
A titration curve is drawn by plotting data attained during a titration,
titrant volume on the x-axis and pH on the y-axis. The titration
curve serves to profile the unknown solution. In the shape of the curve lies
much chemistry and an
interesting summary of
what we have learned so far about acids and bases.
The titration of a strong acid with a strong base produces the
following titration curve:
Figure %: Titration curve of a strong base titrating a strong acid
Note the sharp transition region near the equivalence point on the
.
Also remember that the equivalence point for a strong acid-strong base
titration curve is exactly 7
because the salt produced does not undergo any hydrolysis
reactions.
