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.
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 hydrolisis reaction
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 %: 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 pK b 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 calculation Ph
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 %: 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.
