Page 35 - Pharmaceutical analytical chemistry |

The equivalence point for the titration of any weak acid with a strong
base will be alkaline. The weaker the acid (the smaller the Ka), the larger
the Kb of the salt and the more alkaline is the equivalence point. The
following figure shows the titration curves for 0.1 M solution of weak
acids of different Ka values titrated with 0.1 M NaOH. The sharpness of
the end point will decrease as Ka decreases.
The titration of weak bases with strong acids is completely analogous to
the above case, but the titration curves are the reverse of those for weak
acids versus strong bases. The titration curve for 0.1 M ammonia titrated
with 0.1 M HCl is shown in the following figure. Because Kb for
ammonia is equal to Ka for acetic acid, the titration curve for ammonia
versus a strong acid is just the mirror image of the titration curve for
acetic acid versus a strong base.
The indicator for such a titration must have a transition range within
about 4 to 7. Methyl red meets this requirement. If phenolphthalein has
been used as the indicator, it would have gradually lost its color between
pH 8 and 10, i.e. before the equivalence point was reached.

(3) Titration curves of polyprotic acids
A polyprotic acid has more than one ionizable proton per molecule. Such
acids ionize in steps, each step is characterized by its own ionization
constant, for example a diprotic acid H2A ionizes as follows:

H2A ↔ H+ + HA-
HA- ↔ H+ + A2-
>> as the removal of a proton becomes increasingly difficult with
successive steps, because the protons must be removed from residues
which bear negative charges.
In order to obtain good end point breaks for the titration of the first
proton, should be at least 104 × . If is in the required range of

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