Chemistry • Year 12 • Module 6 • Lesson 15
Indicators: Mechanism & Selecting the Right One
Lock in the vocabulary of indicator equilibria, the three key indicator pH ranges, and the rule for matching an indicator to a titration type.
1. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: indicator, HIn, In–, endpoint, equivalence point, methyl orange, bromothymol blue, phenolphthalein, Ka(In), hydrolysis. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | A weak acid whose acid and conjugate base forms have different colours, used to signal when a titration is complete. | |
| 1.2 | The acid (protonated) form of an indicator — carries one colour. | |
| 1.3 | The conjugate base (deprotonated) form of an indicator — carries a different colour. | |
| 1.4 | The acid dissociation constant of the indicator; its value determines the pH at which the colour transition occurs. | |
| 1.5 | The point in a titration where stoichiometrically equivalent amounts of acid and base have reacted; its pH depends on the salt formed. | |
| 1.6 | The point at which the indicator changes colour, which should coincide with the equivalence point for an accurate result. | |
| 1.7 | An indicator with a transition range of pH 3.1–4.4; suitable for strong acid / weak base titrations (EP pH < 7). | |
| 1.8 | An indicator with a transition range of pH 6.0–7.6; suitable for strong acid / strong base titrations. | |
| 1.9 | An indicator with a transition range of pH 8.3–10.0; suitable for weak acid / strong base titrations (EP pH > 7). | |
| 1.10 | The reaction of a conjugate base (e.g. CH₃COO–) with water to produce OH–, explaining why the equivalence point pH > 7 in a weak acid / strong base titration. |
2. Complete the indicator reference table
Fill in the blanks (marked with ___) in the table below. Use your lesson notes. 10 marks — 1 per blank
| Indicator | pH transition range | Colour in acid | Colour in base | Titration type(s) suitable for |
|---|---|---|---|---|
| Methyl orange | ___ | Red | ___ | Strong acid / weak base; strong acid / strong base |
| Bromothymol blue | 6.0–7.6 | ___ | Blue | ___ |
| Phenolphthalein | 8.3–10.0 | ___ | Pink / magenta | ___; strong acid / strong base |
2.4 A fourth indicator, alizarin yellow, has a transition range of pH 10.1–12.0. Would it be suitable for any of the four standard titration types? Explain your reasoning in one sentence.
3. True or false — with correction
Circle T or F. If the statement is false, write the corrected version on the line below. 10 marks — 1 T/F, 1 correction where false
3.1 The equivalence point of any acid-base titration is always at pH 7. T / F
3.2 An indicator changes colour at a single, precise pH rather than over a range. T / F
3.3 Phenolphthalein is colourless in acidic solution and pink in basic solution. T / F
3.4 For a weak acid / strong base titration, the equivalence point pH is below 7 because the weak acid is not fully ionised. T / F
3.5 Using an indicator whose transition range does not include the equivalence point pH introduces a systematic error into the titration result. T / F
4. Fill in the blanks — indicator equilibrium
Complete the paragraph below using words from the word bank. Each word is used once. 8 marks
An indicator is a (1) _______________ whose acid form, written (2) _______________, has a different (3) _______________ from its conjugate base form, (4) _______________. The pH at which the colour transition occurs is governed by (5) _______________, the acid dissociation constant of the indicator. For an accurate titration result, the indicator must be chosen so that its transition range (6) _______________ the (7) _______________ point pH. When the indicator changes colour, the student records the (8) _______________ volume.
5. Function recall
Answer each in 1–2 sentences using precise lesson vocabulary. 8 marks — 2 each
5.1 Why must an indicator be a weak acid (not a strong acid) to function as a colour-change signal?
5.2 Explain why the equivalence point pH of a strong acid / weak base titration is below 7.
5.3 State one reason why wine acid titrations in Australian wine laboratories (e.g. AWRI protocols) use phenolphthalein as the standard indicator.
5.4 Explain what would happen to the recorded titre volume if a student used methyl orange (range 3.1–4.4) for a weak acid / strong base titration where the equivalence point pH is approximately 8.7.
Q1 — Term–definition match
1.1 indicator • 1.2 HIn • 1.3 In– • 1.4 Ka(In) • 1.5 equivalence point • 1.6 endpoint • 1.7 methyl orange • 1.8 bromothymol blue • 1.9 phenolphthalein • 1.10 hydrolysis
Q2 — Indicator table blanks
Methyl orange: range 3.1–4.4; base colour yellow.
Bromothymol blue: acid colour yellow; suitable for strong acid / strong base titrations (EP near neutral).
Phenolphthalein: acid colour colourless; also suitable for weak acid / strong base titrations.
2.4 No — alizarin yellow's range (pH 10.1–12.0) is above the highest typical equivalence point pH for any of the four standard titration types, so the indicator would change colour only in a region of excess strong base, not at equivalence.
Q3 — True / False
3.1 False. Correction: the equivalence point pH depends on the salt formed — strong/strong → pH 7; weak acid/strong base → pH > 7; strong acid/weak base → pH < 7.
3.2 False. Correction: an indicator changes colour gradually over a pH range (approximately pKa(In) ± 1), not at a single precise pH.
3.3 True.
3.4 False. Correction: for a weak acid / strong base titration the equivalence point pH is above 7, because the conjugate base of the weak acid hydrolyses to produce OH–. The EP is below 7 only for a strong acid / weak base titration (conjugate acid hydrolyses to give H¹).
3.5 True.
Q4 — Cloze answers (in order)
(1) weak acid • (2) HIn • (3) colour • (4) In– • (5) Ka(In) • (6) brackets • (7) equivalence • (8) endpoint
Q5.1 — Why a weak acid indicator?
A strong acid indicator would be completely dissociated at all pH values above ~0, so it would exist only in its conjugate base (In–) form throughout the titration and never show the HIn colour. Only a weak acid indicator maintains a significant concentration of both HIn and In– near the equivalence point, allowing the equilibrium to shift and produce a visible colour change.
Q5.2 — Why EP pH < 7 for strong acid / weak base?
At the equivalence point, the conjugate acid of the weak base (e.g. NH₄¹) is present in solution. This conjugate acid is a weak acid itself and partially ionises: NH₄¹ ⇌ H¹ + NH₃. The H¹ produced lowers the pH below 7.
Q5.3 — Wine lab indicator choice
Wine contains tartaric acid and other weak acids titrated against NaOH (strong base) — a weak acid / strong base titration. The equivalence point pH is approximately 8.2–8.5, which falls within phenolphthalein's transition range (8.3–10.0). AWRI protocols specify NaOH with phenolphthalein as indicator precisely for this reason.
Q5.4 — Wrong indicator → effect on titre
Methyl orange changes colour at pH 3.1–4.4, which is in the buffer region of the weak acid titration curve — far below the equivalence point. The student would stop adding NaOH long before the true equivalence point is reached, recording a titre that is much too small. The calculated concentration of the weak acid would be significantly underestimated.