Titration of a weak acid

Titration of a weak acid (acetic acid) with a strong base (NaOH) is shown in figure 3.1. Addition of NaOH neutralizes the H⁺ ions present in the solution; thereby increasing its pH. Consumption of H⁺ ions drives the dissociation of the CH₃COOH into CH₃COO⁻ and H⁺ (Le Chatlier's principle). As more and more of NaOH is added, the pH of the solution increases. This increase in pH is small when the solution reaches the pH close to the pK_a of acetic acid. A weak acid, therefore, provides a resistance to change in pH near its pK_a value. This resistance to change in pH by an acid near its pK_a is the concept of a buffer.

 

 

Figure 3.1: Titration curve of acetic acid. Acetic acid is titrated with NaOH and pH is measured after every addition. The pH is plotted against the amount of NaOH expressed as a fraction of the total amount of NaOH required to ionize all the acetic acid. Addition of NaOH drives the dissociation of CH₃COOH into CH₃COO⁻ and H⁺. Addition of NaOH causes increase in pH; notice the buffering region (blue) wherein addition of NaOH causes little increase in pH.