FIGURE 2-28 The titration curve of phosphoric acid (H₃PO₄), a common buffer in biological systems. This biologically ubiquitous molecule has three hydrogen atoms that dissociate at different pH values; thus phosphoric acid has three pK_a values, as noted on the graph. The shaded areas denote the pH ranges—within one pH unit of the three pK_a values—where the buffering capacity of phosphoric acid is high. In these regions, the addition of an acid (or base) will cause relatively small changes in the pH.