IB DP Chemistry - R3.1.15 Appropriate indicators for titration - Study Notes - New Syllabus - 2026, 2027 & 2028
IB DP Chemistry – R3.1.15 Appropriate indicators for titration – Study Notes – New Syllabus
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Reactivity 3.1.15 — Appropriate Indicator for a Titration
Reactivity 3.1.15 — Appropriate Indicator for a Titration
Equivalence Point
The point in a titration where stoichiometrically equivalent amounts of acid and base have reacted.
- Determined by reaction stoichiometry.
- pH at equivalence varies depending on acid–base strength.
- Sharpest change in pH occurs at this point in strong acid–base titrations.
End Point
The point where the indicator changes colour during a titration.
- Determined by the indicator’s pKa.
- Should match the equivalence point as closely as possible.
- Visually observed and may vary slightly from the exact equivalence point.
Choosing an Appropriate Indicator for a Titration
Indicators must be selected so that their colour change occurs close to the equivalence point of the titration. This depends on the relative strengths of the acid and base involved, and the nature of the salt formed.
- Strong acid + strong base: pH ≈ 7 → indicators like bromothymol blue
- Strong acid + weak base: pH < 7 → indicators like methyl orange
- Weak acid + strong base: pH > 7 → indicators like phenolphthalein
- Weak acid + weak base: no sharp jump → no suitable indicator (use a pH meter)
The identity of the salt formed reveals whether the solution at the equivalence point is acidic, basic, or neutral – guiding indicator choice.
Example
In a titration between hydrochloric acid (HCl) and sodium hydroxide (NaOH), which indicator is appropriate and why?
▶️Answer/Explanation
HCl and NaOH are strong acid and base, respectively. The equivalence point is at pH 7. An ideal indicator would change colour near pH 7 — bromothymol blue (pH 6.0–7.6) is well-suited for this titration.
Example
What indicator is appropriate for titrating nitric acid with ammonia?
▶️Answer/Explanation
Nitric acid is a strong acid and ammonia is a weak base. The salt formed is ammonium nitrate, and the solution at equivalence is acidic. An indicator like methyl orange (pH 3.1–4.4) is suitable, as it changes colour in the acidic range near the equivalence point.
Example
Ethanoic acid (CH₃COOH) is titrated with sodium hydroxide. Identify a suitable indicator and justify.
▶️Answer/Explanation
This is a weak acid (CH₃COOH) with a strong base (NaOH). The salt formed is sodium ethanoate, and the solution at equivalence is slightly basic due to hydrolysis of CH₃COO⁻. Phenolphthalein (pH range 8.3–10.0) is suitable as it changes colour in the basic range, matching the pH at equivalence.
Example
In a titration involving ethanoic acid and methylamine (CH₃NH₂), the student used phenolphthalein as an indicator. Comment on the suitability of this choice.
▶️Answer/Explanation
Both acid and base are weak, meaning the titration curve has no sharp pH jump. The equivalence point is vague and does not align well with any indicator’s range. Phenolphthalein is not reliable here – in fact, no indicator is appropriate for this case. A pH meter should be used instead to monitor pH throughout the titration.