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Fraction of dissociated CH3COOH Added OH-->

Fraction of dissociated CH3COOH Added OH-->

▲ FIGURE 2-22 The titration curve of acetic acid (CH3COOH). The pKa 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.

Added OH-

▲ FIGURE 2-23 The titration curve of phosphoric acid (H3PO4). This biologically ubiquitous molecule has three hydrogen atoms that dissociate at different pH values; thus, phosphoric acid has three pKa values, as noted on the graph. The shaded areas denote the pH ranges—within one pH unit of the three pKa values—where the buffering capacity of phosphoric acid is high. In these regions the addition of acid (or base) will cause relatively small changes in the pH.

in the A~ form. At pH values more than one unit above or below the pKa, the buffering capacity of weak acids and bases declines rapidly. In other words, the addition of the same number of moles of acid to a solution containing a mixture of HA and A~ that is at a pH near the pKa will cause less of a pH change than it would if the HA and A~ were not present or if the pH were far from the pKa value.

All biological systems contain one or more buffers. Phosphate ions, the ionized forms of phosphoric acid, are present in considerable quantities in cells and are an important factor in maintaining, or buffering, the pH of the cytoplasm. Phosphoric acid (H3PO4) has three protons that are capable of dissociating, but they do not dissociate simultaneously. Loss of each proton can be described by a discrete dissociation reaction and pKa as shown in Figure 2-23. The titration curve for phosphoric acid shows that the pKa for the dissociation of the second proton is 7.2. Thus at pH 7.2, about 50 percent of cellular phosphate is H2PO4~ and about 50 percent is HPO42~ according to the Henderson-Hasselbalch equation. For this reason, phosphate is an excellent buffer at pH values around 7.2, the approximate pH of the cytoplasm of cells, and at pH 7.4, the pH of human blood.

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