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whose transition range falls within the vertical part of the titration curve
can be used with a sharp end point and a negligible titration error. The
selection of the indicator becomes more critical as the solution becomes
more dilute.

(2) Titration curves of weak acids versus strong bases
An example of weak acids is acetic acid which is only a few percent
ionized. It is neutralized to water and to an equivalent amount of the salt
sodium acetate. Before the titration is started, we have 0.1 M HAc, and
the pH is calculated as described for weak acids. As soon as the titration
is started, some of the HAc is converted to NaAc, and a buffer system is
set up. As the titration proceeds, the pH slowly increases as the ratio
[Ac-] / [HAc] changes. At the midpoint of the titration, [Ac-] = [HAc],
and the pH is equal to pKa. At the equivalence point, we have a solution
of NaAc which is a Brönsted base that hydrolyses and the pH at the
equivalence point will be alkaline. The pH will depend on the
concentration of NaAc so that the greater its concentration the higher will
be the pH. As excess NaOH is added beyond the equivalence point, the
ionization of the base Ac- is suppressed to a negligible amount, and the
pH is determined only by the concentration of excess OH-. Therefore, the
titration curve beyond the equivalence point follows that for the titration
of a strong acid.
Derive the titration curve of 50 mL of 0.1 M acetic with 0.1 M NaOH
(pKa = 4.76).

1. pH before adding titrant: the pH of HAc is calculated from:
pH = ½ (pKa + pCa) = 2.88

2. pH during titration: the addition of base produces a buffer of
acetic acid and sodium acetate. Therefore, on adding 10 mL base

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