Page 34 - Pharmaceutical analytical chemistry |

3. pH at the equivalence point: at the equivalence, the acetic acid is
quantitatively converted to sodium acetate.
pH = 14 - ½ (pKb + pCs) = 8.73

4. pH beyond the equivalence point: the excess of NaOH present
represses the hydrolysis of the acetate ion to such an extent that
its contribution becomes negligibly small. So on adding 60 mL of
base, the pH is determined by the concentration of excess OH-.
and

The slowly rising region before the equivalence point is called the buffer
region. It is flattest at the midpoint, and so the buffering capacity is
greatest at a pH corresponding to pKa. The buffering capacity also
depends on the concentrations of HAc and Ac- and the total buffering
capacity increases as the concentration increases.
Notice that the corresponding region for a strong acid-strong base
titration is much flatter than for a weak acid-strong base titration. In this
respect, a solution of a strong acid or a strong base is much more resistant
to pH change upon addition of H+ or OH- than the buffer system we have
discussed.
The transition range of the indicator for this titration of a weak acid must
fall within a pH range of about 7 to 10. Phenolphthalein fits this nicely. If
an indicator such as methyl red was used it would begin changing color
shortly after the titration began and would gradually change to the
alkaline color up to pH 6, before the equivalence point has been reached.

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