8.10 Buffer Capacity

What is buffer capacity?

Buffer capacity is the amount of acid or base a buffer can absorb before its pH changes significantly. It depends on the concentrations of the weak acid/base and its conjugate — not on the ratio itself (which sets the pH). Higher concentrations of both components mean a larger buffer capacity.

Ratio effects on resistance

A buffer resists pH change best when the ratio of [HA] to [A⁻] is close to 1:1, near the midpoint of a titration curve. Here, the buffer can neutralize added acid or base almost equally well in either direction, minimizing pH shifts.

Buffer capacity on titration curves

On a titration curve, the flattest region (buffer region) surrounding the half-equivalence point shows where buffer capacity is greatest and pH changes slowly. Near the equivalence point, small additions cause large pH swings because the buffering components are nearly used up.

Key terms

Buffer capacity

The amount of acid or base a buffer can neutralize before its pH changes significantly.

What determines buffer capacity?

The concentrations of the weak acid and conjugate base present in the buffer.

Best buffering ratio

A 1:1 ratio of weak acid to conjugate base (at the half-equivalence point) maximizes resistance to pH change.

Buffer region on titration curve

The flat region around the half-equivalence point, where pH changes slowly with added titrant.

Why pH jumps near equivalence point

The buffering components are nearly exhausted, so there's little left to resist pH change.

Try an AP-style question

Buffers A (1.0 M HA / 1.0 M A⁻) and B (0.10 M HA / 0.10 M A⁻) have the same pH. Which has the greater capacity?

1. B
2. Neither is a buffer
3. They're equal
4. A