8.4 Acid-Base Reactions and Buffers

Buffers resist pH change

A buffer is a solution containing a weak acid and its conjugate base (or weak base and conjugate acid) in comparable amounts. It resists changes in pH when small amounts of acid or base are added, because the weak acid neutralizes added base and the conjugate base neutralizes added acid, keeping [H+] relatively stable.

Henderson-Hasselbalch equation

For a buffer, pH = pKa + log([A⁻]/[HA]). When [A⁻] = [HA], pH = pKa — this is the point of maximum buffer capacity. Buffers work best within about ±1 pH unit of the pKa of the weak acid used.

Buffer capacity and reactions with strong acid/base

Buffer capacity is the amount of acid or base a buffer can absorb before pH changes significantly; it depends on the concentrations of HA and A⁻. Adding strong acid converts some A⁻ to HA; adding strong base converts some HA to A⁻. Use stoichiometry to find new moles, then Henderson-Hasselbalch for pH.

Key terms

Buffer

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

Henderson-Hasselbalch equation

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

Buffer capacity

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

Maximum buffer capacity condition

Occurs when [HA] = [A⁻], so pH = pKa.

Effect of adding strong acid to a buffer

H+ reacts with the conjugate base (A⁻), converting it to HA; pH decreases only slightly.

Try an AP-style question

Which mixture forms a buffer?

1. CH₃COOH and CH₃COONa
2. NaOH and NaCl
3. HCl and NaCl
4. HNO₃ and KNO₃