H2o

Now invert —log [HA]/[A ], which involves changing its sign, to obtain the Henderson-Hasselbalch equation:

Stated more generally, pH = pKa + log

[proton acceptor] [proton donor]

This equation fits the titration curve of all weak acids and enables us to deduce a number of important quantitative relationships. For example, it shows why the p^a of a weak acid is equal to the pH of the solution at the midpoint of its titration. At that point, [HA] equals [A—], and pH = pKa + log 1 = pKa + 0 = pKa

As shown in Box 2-3, the Henderson-Hasselbalch equation also allows us to (1) calculate pKa, given pH and the molar ratio of proton donor and acceptor; (2) calculate pH, given pKa and the molar ratio of proton donor and acceptor; and (3) calculate the molar ratio of proton donor and acceptor, given pH and pKa.

Weak Acids or Bases Buffer Cells and Tissues against pH Changes

The intracellular and extracellular fluids of multicellu-lar organisms have a characteristic and nearly constant

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