This animation illustrates the titration of acetic acid (CH3COOH) with aqueous base (OH-).
When a weak acid, such as acetic acid, is titrated with a strong base (OH-), the pKa value of the weak acid can be determined by plotting a titration curve. Because acetic acid is a monoprotic acid, only one equivalent of base is needed to ionize acetic acid to its conjugate base (CH3COO-). Notice that at the midpoint of the titration, there is an inflection point in the curve. At this point the concentration of weak acid equals the concentration of conjugate base and, therefore, according the Henderson-Hasselbalch equation, the pH = PKa. At the endpoint of the titration, all of the acetic acid has been ionized to its conjugate base.