Topic 3 of 6
Titration curves and indicators
Identify what controls pH before, at and after equivalence.
A-Level 9476 (2026-2027)
Different regions of a titration are controlled by different species
Equivalence means stoichiometric reaction, not necessarily pH 7.
Strong and weak acids titrated with strong base
Both curves reach equivalence at 25 cubic centimetres of sodium hydroxide. Strong acid begins near pH 1 and has equivalence pH 7. Weak acid begins near pH 2.87, has a buffer region and has an alkaline equivalence near pH 8.72. Beyond equivalence the curves approach one another because excess hydroxide controls pH.
| Region | Main chemistry | What controls pH |
|---|---|---|
| Before any base | Weak acid partly ionises. | Ka and initial acid concentration. |
| Before equivalence | Added OH- converts HA into A-. Both HA and A- remain. | The acid/conjugate-base buffer ratio. At half-equivalence, pH = pKa. |
| At equivalence | All initial HA has been converted stoichiometrically into A-. | Hydrolysis of the weak conjugate base: A- + H2O ⇌ HA + OH-. pH is above 7 at 298 K. |
| After equivalence | Strong base is in excess. | Excess OH- divided by total solution volume. |
| Titration pair | Equivalence at 298 K | Curve feature |
|---|---|---|
| Strong acid + strong base | Approximately pH 7. | Large, steep pH change around equivalence. |
| Weak acid + strong base | Above pH 7, from conjugate-base hydrolysis. | A buffer region before equivalence; lower-pH part of the jump is shortened. |
| Strong acid + weak base | Below pH 7, from conjugate-acid dissociation. | When acid is added to weak base, pH falls through a base/conjugate-acid buffer region. |
| Weak acid + weak base | Depends on relative Ka and Kb, not automatically 7. | Usually no large sharp jump, making a visual indicator endpoint unreliable. |
Worked example
After equivalence, count the excess
30.0 cm3 of 0.100 mol dm-3 NaOH is added to 25.0 cm3 of 0.100 mol dm-3 monoprotic acid. Find pH at 298 K when strong-base excess controls the result.
- Initial acid = 0.00250 mol; added OH- = 0.00300 mol.
- Excess OH- = 0.000500 mol in 0.0550 dm3.
- [OH-] = 0.00909 mol dm-3; pOH = 2.04.
pH = 11.96. Using only the base volume in the denominator would overestimate [OH-].
Choose an indicator whose transition sits inside the steep change
An endpoint should closely match equivalence for the actual titration curve.
An acid-base indicator is usually a weak acid or base whose two forms have different colours. Its visible colour change occurs over a pH interval, commonly around pKa ± 1. The endpoint is the observed colour change; the equivalence point is the stoichiometric point. They should be close, but they are not definitions of the same event.
| Situation | Suitable reasoning |
|---|---|
| Strong acid/strong base with a broad steep jump | Several indicators may work if their entire useful transition interval lies in the steep region. |
| Weak acid/strong base | An alkaline-range indicator such as phenolphthalein, roughly pH 8.2-10.0, commonly matches the steep region; methyl orange, roughly pH 3.1-4.4, changes in the buffer region too early. |
| Strong acid/weak base | Choose an indicator in the acidic steep region using the actual curve; an alkaline-range indicator usually changes too early when acid is added. |
| Weak acid/weak base | A broad gradual transition makes visual endpoint selection poor; a pH meter and suitable analysis may be preferable. |
A transition interval does not have to be centred at pH 7. The useful question is how much titrant volume changes during the indicator transition. In a steep region this volume is small, so the endpoint error is small. If given unfamiliar indicators, compare their ranges directly with the curve rather than choosing a familiar name.