Direct Answer
Choose an indicator whose pH transition range lies within the steep vertical section of the titration curve and includes, or is very close to, the equivalence point. The indicator must change colour sharply when chemically equivalent amounts of acid and base have reacted.
The Mechanism
An acid-base indicator is usually a weak acid whose conjugate acid-base pair has different colours:
The observed colour depends on the relative concentrations of and . The colour transition normally occurs over approximately , so different indicators change colour over different pH ranges.
The end point is the observed indicator colour change. It should be as close as possible to the equivalence point, which is the theoretical stoichiometric point. If the transition range falls outside the steep section, the end point occurs too early or too late, producing a systematic error in the titre.
| Titration | Equivalence-point region | Suitable indicator |
|---|---|---|
| Strong acid and strong base | Near pH 7 with a large, steep pH change | Methyl orange or phenolphthalein |
| Weak acid and strong base | Above pH 7 | Phenolphthalein |
| Strong acid and weak base | Below pH 7 | Methyl orange |
| Weak acid and weak base | No sufficiently steep pH change | No suitable visual indicator; use a pH probe |
A common misconception is that the indicator must change colour at exactly pH 7. This is incorrect: the equivalence-point pH depends on the strengths of the acid and base, and only a strong acid-strong base titration has an equivalence point near pH 7 at 298 K.
Exam Technique
This is Additional Higher Level content in Reactivity 3.1.14-3.1.15. When asked to justify an indicator, identify the equivalence-point region from the titration curve, state the indicator's transition range, and explain that this range lies within the curve's steep vertical section.
