← Equilibrium principles in aqueous systems

10 Titration curves

Reading the curve

• The equivalence point is where moles acid = moles base (steep vertical jump).
• Strong acid + strong base: equivalence at pH 7.
• Weak acid + strong base: equivalence is basic (> 7, the salt is a conjugate base).
• Strong acid + weak base: equivalence is acidic (< 7).
• At the half-equivalence point for a weak acid, [HA] = [A⁻], so pH = pK_a, a buffer region.

Indicator choice: pick an indicator whose colour-change range lies within the steep vertical part of the curve. Phenolphthalein (8.3–10) suits weak acid–strong base; methyl orange (3.1–4.4) suits strong acid–weak base. ✓

Half-equivalence trick

To find K_a of a weak acid from a titration curve: read the pH at exactly half the volume needed to reach equivalence, that pH equals pK_a.

Practise just this