6.1.15Additional lessons
Choosing an Acid-Base Indicator (HL) — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 129 of 172
Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
Part 1 of 4The Matching Rule
A suitable indicator must change colour where the pH curve is steepest.
Steep section
- The pH changes by several units over a tiny volume near equivalence, so the indicator must change colour within this range.
Working range
- The indicator's colour change range, about pKin ± 1, should lie entirely in the vertical section of the curve.
Sharp end point
- A single drop should change the colour so the end point and equivalence point differ by a negligible volume.
Data booklet
- The pKin values and colours of common indicators are tabulated and are used to make the choice.
Part 2 of 4Choosing for Each Type
Choosing for Each Type
Strong acid + strong base
- The vertical section covers about pH 3 to 11, so methyl orange, bromothymol blue and phenolphthalein all work.
Weak acid + strong base
- The vertical section lies in the alkaline region, so phenolphthalein is suitable and methyl orange is not.
Strong acid + weak base
- The vertical section lies in the acidic region, so methyl orange is suitable and phenolphthalein is not.
Weak acid + weak base
- No indicator is suitable, because the curve has no steep section; a pH meter is used instead.
| Titration | Suitable indicator |
|---|---|
| Strong acid + strong base | Any of the three |
| Weak acid + strong base | Phenolphthalein |
| Strong acid + weak base | Methyl orange |
| Weak acid + weak base | None; use a pH meter |
Part 3 of 4Decision Steps
1
Identify the strengths of the acid and the base, and sketch the corresponding pH curve.
2
Mark the pH range of the vertical section on the curve.
3
Check the working range of each indicator available against that range.
4
Select the indicator whose full colour change lies inside the steep section.
5
State the colour change at the end point, such as colourless to pink for phenolphthalein with an alkali.
Part 4 of 4Exam Checklist & Tips
6.1.15 – Key Points for the Exam
- Steep section The nearly vertical region of the pH curve close to the equivalence point.
- Suitable indicator One whose colour change range lies within the steep section.
- pH meter An electronic device that measures pH directly and can be used when no indicator is suitable.
- Equivalence point The point at which acid and base have reacted in exactly stoichiometric amounts.
Common Mistakes
- Choosing an indicator because its colour is clear, without checking its range.
- Using phenolphthalein for strong acid with weak base titrations.
- Assuming that the indicator must change colour at pH 7 in every titration.
- Forgetting that the choice of indicator depends on the strengths of both reactants.
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