Page 32 - Pharmaceutical analytical chemistry |

3. pH at the equivalence point: on adding 50 mL of base, the
titration is at the equivalence point and neither acid nor base is
present in excess. The system is therefore an aqueous solution of
NaCl and the pH = 7.0.

4. pH beyond the equivalence point: on adding base beyond the
equivalence point, excess base will remain and since it is
essentially completely ionized, pH is calculated from the [OH-]. On
adding 60 mL of base there will be 10 mL of base in excess.

The magnitude of the break (pH change in the region of equivalence
point) will depend on both the concentration of the acid and the
concentration of the base. Moreover, the reverse titration, i.e. titration of
strong bases using strong acids, gives the mirror image of these curves.

The point at which the reaction is observed to be complete is called the
end point. A measurement is chosen such that the end point coincides
with or is very close to the equivalence point. The most obvious way of
determining the end point is to measure the pH at different points of the
titration and make a plot of this versus the milliliters of titrant added. This
is done with a pH meter. It is usually more convenient to add an indicator
to the solution and visually detect a color change. Any acid base indicator

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