FIGURE 2-26 The relationship between pH, pK_a, and the dissociation of an acid. As the pH of a solution of carbonic acid rises from 0 to 8.5, the percentage of the compound in the undissociated, or un-ionized, form (H₂CO₃) decreases from 100 percent and that of the ionized form increases from 0 percent. When the pH (6.4) is equal to the acid’s pK_a, half of the carbonic acid has ionized. When the pH rises to above 8, virtually all of the acid has ionized to the bicarbonate form (HCO₃^–).