8.9 Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation

For a buffer made of a weak acid and its conjugate base, pH = pKa + log([A⁻]/[HA]). This links pH to the ratio of base to acid concentrations, letting you calculate buffer pH without doing a full equilibrium calculation, as long as the approximation that initial concentrations ≈ equilibrium concentrations holds.

When pH = pKa

When [A⁻] = [HA], the log term equals zero, so pH = pKa. This is the buffer's most resistant point — maximum buffering capacity — and occurs at the half-equivalence point of a titration, since exactly half the weak acid has been converted to its conjugate base.

Using the equation practically

The equation shows buffer pH depends on the ratio of conjugate base to acid, not the absolute amounts. Diluting a buffer doesn't change its pH (ratio stays constant) until concentrations get very low. To raise pH, increase [A⁻] relative to [HA]; to lower pH, increase [HA] relative to [A⁻].

Key terms

Henderson-Hasselbalch equation

pH = pKa + log([A⁻]/[HA]); relates buffer pH to pKa and the conjugate base/acid ratio.

When does pH = pKa?

When [A⁻] = [HA], i.e., at the half-equivalence point of a titration.

Buffering capacity

A buffer's ability to resist pH change; maximized when [A⁻] ≈ [HA] (pH near pKa).

Effect of dilution on buffer pH

Little to no change, since dilution doesn't alter the ratio [A⁻]/[HA].

Half-equivalence point

The point in a titration where half the weak acid has been converted to its conjugate base; pH = pKa here.

Try an AP-style question

A buffer has pKa = 4.74 and [A⁻] = [HA]. What is its pH?

1. 5.74
2. 9.26
3. 4.74
4. 7.00