8.11 Chemistry of Buffers

What makes a buffer work

A buffer resists changes in pH when small amounts of acid or base are added. It contains both a weak acid and its conjugate base (or a weak base and its conjugate acid) in significant, comparable concentrations. The weak acid neutralizes added base, while the conjugate base neutralizes added acid, keeping pH relatively stable.

Calculating buffer pH

Buffer pH is found using the Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]). When [A⁻] = [HA], pH = pKa — this is the point of maximum buffering capacity. The ratio of conjugate base to acid determines whether pH is above or below pKa.

Buffer capacity

Buffer capacity is the amount of acid or base a buffer can absorb before pH changes significantly. It depends on the concentrations of the weak acid and conjugate base — more concentrated buffers resist bigger changes. Capacity is greatest when [HA] and [A⁻] are large and nearly equal to each other.

Key terms

Buffer

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

Henderson-Hasselbalch equation

pH = pKa + log([A⁻]/[HA]); used to calculate buffer pH from the ratio of conjugate base to weak acid.

Maximum buffering capacity

Occurs when [A⁻] = [HA], so pH = pKa.

Buffer capacity

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

How a buffer neutralizes added acid

The conjugate base (A⁻) reacts with added H+ to form more weak acid (HA), minimizing pH change.

Try an AP-style question

What happens when HCl is added to an acetate buffer?

1. CH₃COO⁻ + H⁺ → CH₃COOH
2. CH₃COOH + H⁺ → CH₃COOH₂⁺
3. H⁺ stays free in solution
4. Cl⁻ + CH₃COOH → HCl