8.3.4Acids and Bases
Acid-Base Indicators — AP CHEMISTRY Revision Notes & Topical Questions
5 min read 5 parts Lesson 21 of 21 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5How an Indicator Works
Only a tiny amount of indicator is used so that it does not change the pH or consume the titrant.
Weak acid system
- Weak acid system An indicator is a weak acid HIn whose conjugate base In⁻ has a different colour, giving the equilibrium HIn ⇌ H⁺ + In⁻.
Acidic solution
- Acidic solution Extra H⁺ shifts the equilibrium left by Le Chatelier's principle, so the HIn colour dominates.
Basic solution
- Basic solution Added OH⁻ removes H⁺, shifting the equilibrium right, so the In⁻ colour dominates.
Gradual change
- Gradual change The colour changes over a range of pH rather than at a single value, because the eye sees mixtures of the two colours.
Section 2 of 5Link to pKa
1
The indicator equilibrium has Ka = [H⁺][In⁻]/[HIn], which rearranges to pH = pKa + log([In⁻]/[HIn]).
2
When [In⁻] = [HIn] the ratio is 1, the logarithm is zero and the pH equals the indicator's pKa; the colour is a mixture of both.
3
The eye needs roughly ten times more of one form to see its colour clearly, so the colour change spans about pKa ± 1.
4
Below pKa - 1 the acid colour is seen, and above pKa + 1 the base colour is seen.
Section 3 of 5Choosing an Indicator
Choosing an Indicator
Match to the steep part
- Match to the steep part The indicator's colour-change range must fall inside the near-vertical section of the titration curve around equivalence.
Strong acid + strong base
- Strong acid + strong base The vertical section is very long, so almost any common indicator works, including methyl orange and phenolphthalein.
Weak acid + strong base
- Weak acid + strong base Equivalence is above 7, so phenolphthalein (pKa about 9) is suitable but methyl orange is not.
Strong acid + weak base
- Strong acid + weak base Equivalence is below 7, so methyl red or methyl orange is suitable but phenolphthalein is not.
Weak + weak
- Weak + weak There is no sharp vertical section, so no ordinary indicator gives a clear end point.
| Indicator | Acid colour | Base colour | Approximate range |
|---|---|---|---|
| Methyl orange | Red | Yellow | 3.1 - 4.4 |
| Methyl red | Red | Yellow | 4.4 - 6.2 |
| Bromothymol blue | Yellow | Blue | 6.0 - 7.6 |
| Phenolphthalein | Colourless | Pink | 8.3 - 10.0 |
Section 4 of 5Worked Example
Question
- Methyl red has pKa = 5.1.
- Find the ratio [In⁻]/[HIn] at pH 4.1 and at pH 6.1, and decide whether it suits a titration of 0.100 mol/L ethanoic acid with NaOH (pH at equivalence 8.72).
- Use pH = pKa + log([In⁻]/[HIn]), so [In⁻]/[HIn] = 10^(pH - pKa).
- At pH 4.1: ratio = 104.1 - 5.1) = 10⁻¹ = 0.10, so the acid colour dominates.
- At pH 6.1: ratio = 106.1 - 5.1) = 10 so the base colour dominates; the range is 4.1 to 6.1.
- The vertical section of the weak acid-strong base curve lies near pH 7 to 10, which is outside this range.
- Methyl red would change colour too early, during the buffer region, so it is unsuitable.
Answer
- Ratios are 0.10 and 10;
- methyl red is unsuitable because pH 8.72 is outside its range.
Section 5 of 5Exam Checklist & Tips
8.3.4 – Key Points for the Exam
- Indicator A weak acid or base whose conjugate forms have different colours.
- End point The point at which the indicator visibly changes colour.
- Transition range The pH interval, about pKa ± 1, over which the colour change is seen.
- Equivalence point The stoichiometric point of the titration, which the end point should match closely.
Common Mistakes
- Believing the indicator changes colour at exactly one pH; it changes across a range of about two pH units.
- Choosing an indicator by its colour rather than by whether its range lies on the steep part of the curve.
- Using too much indicator, which is itself an acid or base and can alter the result.
- Treating end point and equivalence point as identical; a well-chosen indicator only makes them very close.
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