weirdfacts
Year 12 / Buffers and indicators

Resist the change. Reveal the change.

A buffer contains appreciable amounts of a weak acid and its conjugate base, or a weak base and its conjugate acid. Added H+H^{+} is consumed by the basic component; added OH⁻ is consumed by the acidic component. The pH changes only a little until the buffer’s capacity is exceeded.

An indicator is itself an acid–base equilibrium with differently coloured forms. Around pH = pKa both forms are appreciable; the usual transition interval is approximately pKa ± 1.

Choose an indicator whose transition interval lies within the steep part of the titration curve. This makes the observed end point close to the stoichiometric equivalence point. Buffer calculations are not required by this syllabus; focus on the chemical explanation.

INTERACTIVE MODEL

Strong is not concentrated

H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺HA: undissociated · H⁺ and A⁻: dissociatedpH 2.00014
c = 0.010 M · pH 2.00 · ionised ≈ 100.0%

At 25 °C. Compare HCl with a model weak acid (Ka = 1.8 × 10⁻⁵). The pH calculation includes water; particle counts are schematic.

Explore: Add acid or base and follow how a conjugate pair resists a large pH change.

CH₃COO⁻ + H⁺ → CH₃COOH CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O
WORKED EXAMPLE

Explain how an ethanoic acid/ethanoate buffer responds to a little added HCl.

  1. CH₃COO⁻ + H+H^{+} → CH₃COOH.
  2. The removal of most added H+H^{+} limits the pH decrease.

Answer: Ethanoate consumes added H+H^{+} to form ethanoic acid, so the increase in free [H+H^{+}] is limited.

Assumed knowledge

Conjugate pairs differ by H⁺; neutralisation consumes acid or base.

Learning checkpoints & sourceYOUR SYLLABUS CHECKPOINT
  • Explain how buffers respond to added acid or base.
  • Choose indicators using pKa and titration range.
QCAA Chemistry 2025 v1.3 · p. 34 ↗
READY TO TRY IT?

Turn the idea into an answer.

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