8.8 Properties of Buffers

What makes a buffer resist pH change

A buffer contains comparable amounts of a weak acid and its conjugate base (or weak base and conjugate acid). Added H⁺ reacts with the conjugate base; added OH⁻ reacts with the weak acid. Because both species are present in significant concentrations, small additions of acid or base are consumed without large shifts in pH.

Buffer capacity

Buffer capacity is the amount of acid or base a buffer can neutralize before pH changes significantly. It depends on the concentrations of the weak acid/base pair — higher concentrations mean greater capacity. Capacity is maximized when the ratio of [A⁻]/[HA] is close to 1, i.e., when pH ≈ pKa.

Calculating buffer pH

Buffer pH is found using the Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]). When [A⁻] = [HA], pH = pKa, giving the buffer its maximum resistance to change. Adding small amounts of strong acid/base shifts the ratio slightly, changing pH only a little compared to an unbuffered solution.

Key terms

Buffer

A solution with a weak acid and its conjugate base (or weak base/conjugate acid) that resists pH changes.

Buffer capacity

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

Henderson-Hasselbalch equation

pH = pKa + log([A⁻]/[HA]); used to calculate the pH of a buffer solution.

Maximum buffer effectiveness

Occurs when [A⁻] ≈ [HA], i.e., pH ≈ pKa.

How a buffer neutralizes added acid

The conjugate base reacts with added H⁺, converting it into the weak acid and limiting pH change.

Try an AP-style question

What determines the pH of a buffer?

1. Only the total volume of the solution
2. The pKa and the ratio of [A⁻] to [HA]
3. Only the temperature of the solution
4. The amount of water used to dissolve it