The applet below draws the graph of pH against volume of titrant added for the titration of weak acid by a strong base. Specifically:

a solution of 0.1 mol/L NaOH (aq) is the titrant added from a burette 25 cm3 of a solution of 0.1 mol/L acid (aq) is the analyte in a flask below the burette. You can select from a range of acids. Ethanoic acid solution is an approproate example to start with. titrant is added beyond the equivalence point when 25ml of titrant has been added. turn on the data table feature to see the pH value of the equivalence point.

Select a weak acid (note: default is strong acid/strong base) and then press the run button to draw the pH curve for the weak acid/strong base titration.

Note that for the titration of a weak acid with a strong base:

the equivalence point is greater than pH 7.0

The shape of the graph is explained as follows:

Before the equivalence point:

Added NaOH(aq) titratant is neutralised by the weak acid in the analyte solution to produce a salt and water. In the case of ethanoic acid:

NaOH(aq) + CH₃CO₂H(aq) --> CH₃CO₂Na(aq) + H₂O(l)

Overall the added base is neutralised, acid is consumed and a solution of the salt, sodium ethanoate is produced.

However, the conjugate base of a weak acid is generally a strong base and so the conjugate base of the ethanoic acid, the ethanoate anion, can react further. The ethanoate anion combines with hydronium ions in solution, H₃O⁺(aq), reducing their concentration and raising the pH according to:

pH = -log₁₀[H⁺]

Where [H⁺] is equivalent to [H₃O⁺].

The overall result is that the pH rises faster before the equivalance point than in the strong acid/strong base titration.

At the equivalence point:

For a solution of ethanoic acid as the analyte, at the equivalence point the acid has just been neutralised by the base to give a solution of sodium ethanoate in water. However, the ethanoate anion is a base, combines with hydronium ions in solution, H₃O⁺(aq), reducing their concentration and raising the pH. The result is that the pH value of the solution at the equivalence point is greater than pH 7.0.

After the equivalence point:

After the equivalence point is reached the dominant factor is the continued addition of hydroxide ions, OH^-(aq), from the burette and so the curve follows a similar pattern to the strong acid/strong base and for the same reasons.