The Titration Experiment
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 1.1: 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 1.2: 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.
However, if a strong base is used to titrate a weak acid, the pH at the
equivalence point will not be 7. There is a lag in reaching the equivalence
point, as some of the weak acid is converted to its conjugate base. You should
recognize the pair of a weak acid and its conjugate base as a buffer. In
, we see the resultant lag that precedes the equivalence point,
called the buffering region. In the buffering
region, it takes a large
amount of NaOH to produce a small change in the pH of the receiving solution.
Figure 1.3: Titration curve of a strong base titrating a weak acid
Because the conjugate base is basic, the pH will
be greater than 7 at the
equivalence point.
You will need to calculate the pH using the Henderson-Hasselbalch
equation, and inputting the
pKb and concentration of the
conjugate base of the weak acid.
The titration of a base with an acid produces a flipped-over version of the
titration curve of an acid with a base. pH is decreased upon addition of
the acid.
Note that the pH of a solution at the equivalence point has nothing
to do with the volume of
titrant necessary to reach the equivalence point; it is a property inherent to
the composition of the solution. The pH at the
equivalence point is calculated in
the same manner used to calculate the pH of weak base solutions in
Calculating pH's.
When polyprotic acids are titrated with strong bases, there
are multiple equivalence points. The titration curve of a polyprotic acid shows
an equivalence point for the each protonation:
Figure 1.4: Titration curve of a strong base titrating a polyprotic acid
The titration curve shown above is for a diprotic acid such as
H2SO4 and is not unlike two stacked . For a diprotic acid, there are two
buffering regions and two
equivalence points. This proves the earlier assertion that
polyprotic acids lose their
protons in a stepwise manner.
