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Indicator questions test whether you really understand what’s happening in a titration, not just whether you can calculate. This set covers colours, choosing the right indicator, reading titration curves, the chemistry of the indicator equilibrium, and one classic advanced application: the double-indicator titration.
You’ll find these reference ranges useful:
| Indicator | Colour in acid | Colour in alkali | pH 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 | 8.2–10.0 |
| Litmus | red | blue | about 4.5–8.3 |
If you need a refresher first, see acid–base indicators and how to choose an indicator.
Section A: colours
1. What colour is methyl orange at pH 2? At pH 6?
2. What colour is phenolphthalein at pH 7? At pH 11?
3. Bromothymol blue is added to a solution of pH 6.8. What colour would you expect?
4. A solution turns phenolphthalein colourless and methyl orange yellow. What range of pH could it have?
5. Red cabbage indicator turns green-yellow in a solution. Is the solution acidic, neutral or alkaline?
Section B: choosing indicators
6. Which indicator would you use to titrate nitric acid with ammonia solution? Explain.
7. Which indicator would you use to titrate methanoic acid with potassium hydroxide? Explain.
8. A student uses universal indicator in a titration. Explain why this is a poor choice.
9. For a titration of hydrochloric acid with sodium hydroxide, a student says only phenolphthalein can be used. Are they right?
Section C: titration curves
10. A titration curve starts at pH 2.9, has a flat region around pH 4.8, and a steep section from about pH 7 to 11. Identify the type of titration and a suitable indicator.
11. On the same curve, the equivalence point is at 22.0 cm³. What is the pKa of the acid?
12. A curve starts at pH 11.1 and falls, with an equivalence point at about pH 5.3. What type of titration is this?
Section D: indicator chemistry
13. An indicator HIn has Ka = 1.0 × 10⁻⁵. At pH 4.0, what is the ratio [In⁻] : [HIn], and which colour will you see?
14. Explain, using an equilibrium, why adding acid turns phenolphthalein from pink to colourless.
Section E: double-indicator titration
15. 25.0 cm³ of a solution containing sodium carbonate and sodium hydrogencarbonate is titrated with 0.100 mol/dm³ HCl. With phenolphthalein, the first end point is at 10.00 cm³. Methyl orange is then added and titration continues; the second end point is reached after a further 15.00 cm³. Calculate the concentrations of Na₂CO₃ and NaHCO₃.
Answers
1. pH 2: red (below 3.1). pH 6: yellow (above 4.4).
2. pH 7: colourless (below 8.2). pH 11: pink.
3. pH 6.8 is in the middle of bromothymol blue’s range, where both forms are present, so the colour is green (a mix of yellow and blue).
4. Phenolphthalein colourless means pH below 8.2; methyl orange yellow means pH above 4.4. So the pH is between about 4.4 and 8.2.
5. Green-yellow indicates alkaline (roughly pH 11–12). See red cabbage indicator.
6. Methyl orange (or methyl red). This is a strong acid–weak base titration, so the equivalence point is below 7 (around pH 5). The steep section runs from about 7 to 3, and methyl orange’s range (3.1–4.4) lies inside it. Phenolphthalein would change far too early.
7. Phenolphthalein. This is a weak acid–strong base titration; the equivalence point is above 7 (about 8–9), where phenolphthalein changes. Methyl orange would change during the buffer region, long before equivalence.
8. Universal indicator is a mixture that changes colour gradually across the whole pH range, so there’s no single sharp colour change to mark the end point.
9. No. For a strong acid–strong base titration, the pH jumps from about 3.5 to 10.5 in a drop or so near equivalence. Methyl orange, methyl red, bromothymol blue and phenolphthalein all change within that jump, so any of them works.
10. Starting at 2.9 and showing a buffer region means a weak acid in the flask; an equivalence section from 7 to 11 means a strong base was added. Use phenolphthalein. See titration curves.
11. At half-equivalence (11.0 cm³), pH = pKa. The flat buffer region is centred around pH 4.8, so pKa ≈ 4.8 (read the exact value at 11.0 cm³ from the curve).
12. Starting at 11.1 (a weak base) and ending with an equivalence point below 7: a weak base titrated with a strong acid, such as ammonia with hydrochloric acid.
13. [In⁻] ÷ [HIn] = Ka ÷ [H⁺] = 1.0 × 10⁻⁵ ÷ 1.0 × 10⁻⁴ = 0.10, so the ratio is 1 : 10. HIn outnumbers In⁻ ten to one, so you see the acid colour.
14. Phenolphthalein is a weak acid: HIn (colourless) ⇌ H⁺ + In⁻ (pink). Adding acid increases [H⁺], so by Le Chatelier’s principle the equilibrium shifts to the left. The pink In⁻ is converted into colourless HIn.
15.
- First end point (phenolphthalein): only carbonate reacts, turning into hydrogencarbonate: CO₃²⁻ + H⁺ → HCO₃⁻. n(HCl) = 0.100 × 0.01000 = 1.00 × 10⁻³ mol = n(CO₃²⁻) [Na₂CO₃] = 1.00 × 10⁻³ ÷ 0.0250 = 0.0400 mol/dm³
- Second stage (methyl orange): all hydrogencarbonate now present reacts: HCO₃⁻ + H⁺ → H₂O + CO₂. n(HCl) = 0.100 × 0.01500 = 1.50 × 10⁻³ mol = total HCO₃⁻ Of this, 1.00 × 10⁻³ mol came from the carbonate in stage 1. Original HCO₃⁻ = 1.50 × 10⁻³ − 1.00 × 10⁻³ = 5.0 × 10⁻⁴ mol [NaHCO₃] = 5.0 × 10⁻⁴ ÷ 0.0250 = 0.0200 mol/dm³
Why indicators change over about two pH units
The colour of an indicator solution depends on the ratio of its two forms, In⁻ and HIn, which is set by the pH: [In⁻] ÷ [HIn] = Ka ÷ [H⁺]. When the pH equals the indicator’s pKa, the two forms are present in equal amounts and you see a blend of both colours. Human eyes can usually pick out one colour clearly once it outnumbers the other by roughly ten to one, which happens one pH unit either side of the pKa. That’s where the rule of thumb “an indicator changes over pKin ± 1” comes from, and it’s why question 13 has a clear answer: at a ratio of 1 : 10, the acid colour dominates.
Reviewing your mistakes
- Colour errors usually come from mixing up which colour belongs to acid and which to alkali. Remember phenolphthalein is colourless in acid and neutral solutions.
- Indicator choice errors usually come from forgetting that weak acid/strong base equivalence points are above 7, and strong acid/weak base below 7.
- Curve reading errors usually come from reading pKa at the equivalence point instead of half-equivalence.
Key takeaways
- Each indicator changes colour over roughly pKin ± 1.
- Choose an indicator whose range lies inside the steep section of the titration curve.
- Strong/strong: most indicators work; weak acid/strong base: phenolphthalein; strong acid/weak base: methyl orange.
- The colour you see depends on the ratio [In⁻] : [HIn], which depends on pH.
- Double-indicator titrations use two end points to analyse carbonate mixtures.
- For the practical side, see acid–base titration.
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