FIGURE 2-27 The titration curve of the buffer acetic acid (CH₃COOH). The pK_a for the dissociation of acetic acid to hydrogen and acetate ions is 4.75. At this pH, half the acid molecules are dissociated. Because pH is measured on a logarithmic scale, the solution changes from 91 percent CH₃COOH at pH 3.75 to 9 percent CH₃COOH at pH 5.75. The acid has maximum buffering capacity in this pH range.