In 2015, the Australian Therapeutic Goods Administration (TGA) investigated a pharmaceutical quality failure in which a batch of ammonium-based antacid tablets was approved at 112% stated purity — because the quality control technician used methyl orange (pKa 3.46) for a titration whose equivalence point was at pH 9.2. The wrong indicator caused the endpoint to be recorded 8 pH units too early, before the reaction was anywhere near complete. Understanding indicator selection prevents exactly this error.
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
A quality control technician runs a strong acid/weak base titration using methyl orange. The equivalence point pH is 9, but methyl orange changes colour at pH 3.1–4.4. The technician records a false endpoint and approves a batch at 112% purity.
Why did the wrong indicator lead to a false result? What property of the indicator determines whether it is suitable for a given titration? Write your best explanation before reading on.
Core Content
An indicator is not a passive observer — it is a weak acid that participates in its own equilibrium, and understanding this equilibrium explains why the colour change occurs over a range of pH values rather than at a single sharp point.
An indicator is a weak acid, represented as HIn, where the acid form (HIn) and its conjugate base (In⁻) have different colours. The equilibrium is:
HIn(aq) ⇌ H⁺(aq) + In⁻(aq)
[Colour A] ⇌ [Colour A] + [Colour B]
In acidic solution (high [H⁺]): Le Chatelier's principle shifts the equilibrium left → [HIn] > [In⁻] → acid colour (HIn) dominates. In basic solution (low [H⁺]): equilibrium shifts right → [In⁻] > [HIn] → base colour (In⁻) dominates. The colour change is gradual across a pH range of approximately pKIn ± 1.
Why the colour change is gradual: The human eye perceives a colour change when one form is approximately 10× more concentrated than the other. This 10:1 ratio spans about one pH unit either side of the pKIn — giving the indicator its transition range of approximately 2 pH units.
For the indicator to work correctly: the indicator's colour change range must encompass the equivalence point pH of the titration. If the range falls outside the sharp pH jump region, the endpoint recorded will be at the wrong pH — producing a systematic error in the titre and consequently in all calculated concentrations.
| Indicator | Acid form (HIn) colour | Base form (In⁻) colour | Transition range | pKIn |
|---|---|---|---|---|
| Methyl orange (MO) | Red | Yellow | 3.1–4.4 | ~3.5 |
| Bromothymol blue (BTB) | Yellow | Blue | 6.0–7.6 | ~7.1 |
| Phenolphthalein (Ph) | Colourless | Pink | 8.3–10.0 | ~9.1 |
Indicator = weak acid: HIn ⇌ H⁺ + In⁻ (acid and conjugate base have different colours). High [H⁺]: HIn form dominates (acid colour). Low [H⁺]: In⁻ form dominates (base colour). MO: red(H)/yellow(In⁻), 3.1–4.4. BTB: yellow(H)/blue(In⁻), 6.0–7.6. Phenolphthalein: colourless(H)/pink(In⁻), 8.3–10.0. Indicator range must encompass the equivalence point pH.
Pause — copy the highlighted definition into your book before moving on.
In an acidic solution (pH 2), what form of phenolphthalein (HIn ⇌ H⁺ + In⁻) is dominant, and what colour is observed?
We just saw the HIn equilibrium — the indicator range must encompass the equivalence point pH. That raises a question: What is the equivalence point pH for each of the four titration types, and how does salt hydrolysis determine it? This card answers it → SA+SB: EP = 7; WA+SB: EP > 7; SA+WB: EP < 7; WA+WB: no reliable endpoint.
The equivalence point pH is not always 7 — it depends entirely on whether the salt produced at equivalence is itself acidic, basic, or neutral, and understanding this salt hydrolysis argument is the key to justifying any indicator choice in the HSC.
Case 1 — Strong acid + strong base (e.g. HCl + NaOH): The salt formed is NaCl. Neither Na⁺ nor Cl⁻ hydrolyses in water — they are spectator ions. The solution at equivalence is neutral: EP pH = 7. Any indicator whose range encompasses pH 7 is valid. The large, sharp pH jump (~pH 4–10) means all three common indicators (MO, BTB, Ph) give a valid endpoint.
Case 2 — Weak acid + strong base (e.g. CH₃COOH + NaOH): The salt formed is CH₃COONa. The conjugate base CH₃COO⁻ hydrolyses: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻. OH⁻ is produced → solution is basic at equivalence: EP pH > 7 (typically 8.5–9.5). Indicator: phenolphthalein (range 8.3–10.0). Methyl orange (3.1–4.4) is completely unsuitable — it changes colour in the buffer region, far before equivalence.
Case 3 — Strong acid + weak base (e.g. HCl + NH₃): The salt formed is NH₄Cl. The conjugate acid NH₄⁺ hydrolyses: NH₄⁺ ⇌ H⁺ + NH₃. H⁺ is produced → solution is acidic at equivalence: EP pH < 7 (typically 4.5–5.5). Indicator: methyl orange (range 3.1–4.4). Phenolphthalein (8.3–10.0) is unsuitable — its range is entirely above the equivalence point and the sharp pH jump.
Case 4 — Weak acid + weak base: The sharp pH jump at equivalence essentially disappears — the pH changes gradually through a wide range. No common indicator gives a reliable, sharp endpoint for weak acid + weak base titrations. These titrations are avoided in practice, or a pH meter is used instead of an indicator.
| Titration type | Salt formed | Hydrolysis | EP pH | Correct indicator |
|---|---|---|---|---|
| Strong acid + strong base | Neutral salt (e.g. NaCl) | None | = 7 | MO, BTB, or Ph (all valid) |
| Weak acid + strong base | Salt of weak acid (e.g. CH₃COO⁻) | A⁻ + H₂O ⇌ HA + OH⁻ | > 7 (~8.5–9.5) | Phenolphthalein |
| Strong acid + weak base | Salt of weak base (e.g. NH₄⁺) | BH⁺ ⇌ H⁺ + B | < 7 (~4.5–5.5) | Methyl orange |
| Weak acid + weak base | Mixed salt | Both — gradual pH change | Variable | No reliable indicator |
SA + SB: EP pH = 7 (neutral salt, no hydrolysis) → any common indicator valid. WA + SB: EP pH > 7 (A⁻ hydrolyses → OH⁻) → phenolphthalein. SA + WB: EP pH < 7 (BH⁺ hydrolyses → H⁺) → methyl orange. WA + WB: no reliable indicator — no sharp pH jump. Justification template: state titration type, EP pH and why, indicator range encompasses EP pH.
Add the highlighted point to your notes before the check below.
A student titrates NH₃ (weak base) with HCl (strong acid). What is the correct indicator and why?
We just saw that the EP pH depends on salt hydrolysis — SA+SB = 7, WA+SB > 7, SA+WB < 7. That raises a question: What is the systematic two-step decision process for selecting an indicator, and what goes wrong quantitatively if the wrong indicator is used? This card answers it → two steps: determine EP pH from salt type → select indicator whose range encompasses EP pH; wrong indicator causes titre errors that propagate through all four calculation steps.
Rather than memorising a table, understanding the two-step logic of indicator selection lets you work out the answer for any titration you encounter in the HSC — including unfamiliar examples with pKa values given in the question.
Step 1: Determine the equivalence point pH.
Step 2: Select the indicator whose range encompasses the EP pH.
When pKa is given: A more precise EP pH can be calculated. For a weak acid (HA) with pKa = 4.75 + NaOH titration: at equivalence, [A⁻] = c(initial)/2 (at equal volumes); Kb(A⁻) = Kw/Ka; [OH⁻] = √(Kb × [A⁻]); pOH = −log[OH⁻]; EP pH = 14 − pOH. This value confirms phenolphthalein is correct.
The wrong indicator error: If methyl orange (range 3.1–4.4) is used for a weak acid + strong base titration (EP pH ≈ 8.7), the colour change occurs when pH ≈ 4 — in the buffer region. This is only partway through the titration. The recorded titre is far too small → n(base) calculated is too small → c(acid) is drastically underestimated.
| If wrong indicator used | Effect on titre | Effect on calculated c | Direction of error |
|---|---|---|---|
| MO for WA + SB (EP pH > 7) | Too small — endpoint at pH 4, before equivalence | Underestimated | n(base) too small → c(acid) too low |
| Ph for SA + WB (EP pH < 7) | Too large — endpoint only after excess base added (pH > 8.3) | Overestimated | n(base) too large → c(acid) too high |
| Ph for SA + SB | Correct — EP jump encompasses Ph's range | Correct | No error |
Indicator selection two-step: (1) determine EP pH from salt hydrolysis; (2) select indicator whose range encompasses EP pH. Wrong indicator for WA+SB: titre too small → c(acid) underestimated. Wrong indicator for SA+WB: titre too large → c(acid) overestimated. Half-equivalence point pH = pKa ≠ equivalence point pH — never confuse them.
Pause — write the highlighted definition into your book before moving on.
A student uses methyl orange for a CH₃COOH + NaOH titration. What happens to the calculated concentration of acetic acid?
✏️ Worked Examples
For each titration, identify the correct indicator and justify your choice. (a) Titration of ethanoic acid (CH₃COOH) with NaOH. (b) Titration of HCl with ammonia (NH₃). (c) Titration of HCl with NaOH.
CH₃COOH + NaOH — weak acid + strong base:
Salt formed: CH₃COONa. CH₃COO⁻ hydrolyses: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻ → OH⁻ produced → EP pH > 7 (≈ 8.7).
Indicator: Phenolphthalein (range 8.3–10.0). The EP pH of 8.7 falls within this range → valid endpoint.
HCl + NH₃ — strong acid + weak base:
Salt formed: NH₄Cl. NH₄⁺ hydrolyses: NH₄⁺ ⇌ H⁺ + NH₃ → H⁺ produced → EP pH < 7 (≈ 5.3).
Indicator: Methyl orange (range 3.1–4.4). The EP pH of 5.3 and the sharp pH jump occur in the acidic region; MO's range encompasses this. Phenolphthalein (8.3–10.0) would be entirely above the jump → unusable or wildly incorrect.
HCl + NaOH — strong acid + strong base:
Salt formed: NaCl — spectator ions; no hydrolysis. EP pH = 7.
The sharp pH jump spans approximately pH 4–10 → all three indicators (MO, BTB, Ph) have their ranges within this jump → any of the three is valid. BTB (6.0–7.6) is closest to pH 7 and gives the most precise endpoint, but MO or Ph are equally acceptable.
ANSWERS: (a) Phenolphthalein — WA+SB, EP pH ≈ 8.7 > 7, within Ph range. (b) Methyl orange — SA+WB, EP pH ≈ 5.3 < 7, within MO range. (c) MO, BTB, or Ph all valid — SA+SB, EP pH = 7, large pH jump encompasses all three indicator ranges.
A student titrates 25.00 mL of 0.0800 mol/L ethanoic acid (CH₃COOH, Ka = 1.8 × 10⁻⁵) with 0.0800 mol/L NaOH. (a) Write the equation for the hydrolysis of the ion formed at the equivalence point and explain why EP pH > 7. (b) Calculate the equivalence point pH. (c) Select the appropriate indicator and justify in two sentences.
Hydrolysis equation:
CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻
OH⁻ is produced → solution is basic at equivalence → EP pH > 7.
EP pH calculation:
At equivalence, equal volumes mixed → [CH₃COO⁻] = 0.0800/2 = 0.0400 mol/L
Kb(CH₃COO⁻) = Kw/Ka = 1.0 × 10⁻¹⁴ / 1.8 × 10⁻⁵ = 5.56 × 10⁻¹⁰
[OH⁻] = √(Kb × c) = √(5.56 × 10⁻¹⁰ × 0.0400) = √(2.22 × 10⁻¹¹) = 4.71 × 10⁻⁶ mol/L
pOH = −log(4.71 × 10⁻⁶) = 5.33 → EP pH = 14.00 − 5.33 = 8.67
Indicator selection:
Correct indicator: Phenolphthalein (range 8.3–10.0). The equivalence point pH (8.67) falls within phenolphthalein's transition range; the indicator changes from colourless to pink as the solution passes through the sharp pH jump at equivalence. Methyl orange (3.1–4.4) is unsuitable — its range corresponds to the buffer region of the weak acid titration, not the equivalence point.
ANSWERS: (a) CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻; OH⁻ produced → EP pH > 7. (b) [CH₃COO⁻] = 0.0400 mol/L; Kb = 5.56 × 10⁻¹⁰; [OH⁻] = 4.71 × 10⁻⁶; EP pH = 8.67. (c) Phenolphthalein — EP pH 8.67 within range 8.3–10.0.
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An indicator works by being a ____ where the acid and base forms have different colours. For a strong acid–strong base titration, the equivalence point pH is ____, so either phenolphthalein or methyl orange is suitable. For a weak acid–strong base titration the equivalence point pH is ____, so ____ must be used.
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Check Your Understanding
1. A student titrates NH₃ solution with HCl. The equivalence point pH is approximately 5.3. The student selects phenolphthalein (transition range 8.3–10.0) as the indicator. Which statement correctly predicts the outcome?
2. Which of the following correctly matches a titration type with a suitable indicator and a valid justification?
3. During a titration of 0.100 mol/L HCl with 0.100 mol/L NaOH, Student 1 uses BTB (range 6.0–7.6) and records an endpoint at 25.15 mL. Student 2 uses phenolphthalein (range 8.3–10.0) for the same titration. At approximately what volume would Student 2 record their endpoint, and is phenolphthalein valid?
4. A student titrates formic acid (HCOOH, Ka = 1.77 × 10⁻⁴, pKa = 3.75) with NaOH. At the half-equivalence point, the pH reads 3.75. Which indicator is most appropriate for this titration?
5. A student uses phenolphthalein for a titration and observes the colour change right at pH 7.0 — coinciding exactly with the equivalence point. Which of the following correctly identifies what is happening?
UnderstandBand 3–4(4 marks) Q6. Explain why phenolphthalein is suitable for a titration of ethanoic acid (CH₃COOH) with sodium hydroxide (NaOH), but not for a titration of hydrochloric acid (HCl) with ammonia (NH₃). Your response must include the equivalence point pH for each titration and the reasoning behind the EP pH direction.
ApplyBand 4–5(5 marks) Q7. A student titrates 25.00 mL of 0.0800 mol/L ethanoic acid (CH₃COOH, Ka = 1.8 × 10⁻⁵) with 0.0800 mol/L NaOH. (a) Write the equation for the hydrolysis of the ion formed at the equivalence point and explain why the EP pH > 7. (b) Calculate the equivalence point pH. (c) Select the appropriate indicator from MO (3.1–4.4), BTB (6.0–7.6), and Ph (8.3–10.0), and justify your choice in two sentences.
EvaluateBand 6(7 marks) Q8. A student is given an unknown solution and titrates 20.00 mL of it with 0.1000 mol/L NaOH. The titration curve shows: (i) starting pH ≈ 2.9; (ii) a buffer plateau around pH 4.7 spanning 5–20 mL; (iii) equivalence point at 25.00 mL; (iv) EP pH ≈ 8.7.
(a) Identify whether the unknown is HCl or CH₃COOH. Justify using three pieces of evidence from the curve. (b) Calculate the initial concentration of the unknown acid. (c) From the curve, determine the pKa of the acid at the half-equivalence point and explain why pH = pKa at this specific volume. (d) Select the appropriate indicator and explain why methyl orange would fail for this titration.
For strong acid + weak base (HCl + NH₃), the EP pH ≈ 5.3 and the sharp pH jump occurs in the acidic region (~pH 3.5–7.5). Phenolphthalein's range (8.3–10.0) is entirely above this jump. If HCl is added to the NH₃ flask, the solution pH falls through the jump without phenolphthalein changing colour. If NH₃ is in the burette, phenolphthalein changes colour only after a large excess of NH₃ is added (pH > 8.3) — giving a titre much larger than the stoichiometric amount. Option A incorrectly states phenolphthalein detects pH 5.3 (its range starts at 8.3). Option D has the colours backwards — phenolphthalein is colourless in acidic solution.
For weak acid + strong base, the conjugate base (e.g. CH₃COO⁻) hydrolyses to produce OH⁻ → EP pH > 7 (typically 8.5–9.5). Phenolphthalein (8.3–10.0) encompasses this EP pH → suitable. Option A: MO (3.1–4.4) falls in the buffer region of a weak acid titration, far below the EP. Option B: phenolphthalein is unsuitable for strong acid + weak base (EP pH ≈ 5.3, below Ph's range). Option D: not all EPs are near pH 7; strong acid + weak base gives EP pH < 7.
For strong acid + strong base, the sharp pH jump spans approximately pH 4–10. Both BTB (6.0–7.6) and phenolphthalein (8.3–10.0) have their ranges within this jump — both change colour within a fraction of a drop of the same equivalence point. Student 2 records approximately the same volume (within ±0.10 mL). Phenolphthalein is completely valid for strong/strong.
The half-equivalence point pH = pKa = 3.75 tells us this is a weak acid (HCOOH). HCOOH + NaOH is a weak acid + strong base titration → EP pH > 7 (HCOO⁻ hydrolyses: HCOO⁻ + H₂O ⇌ HCOOH + OH⁻). Phenolphthalein (8.3–10.0) is appropriate. Option A is a critical error — the half-equivalence point pH is NOT the equivalence point pH. The half-EP is in the buffer region; the EP is well above 7.
A titration with an equivalence point at pH 7.0 is a strong acid + strong base titration. The pH jump spans ~pH 4–10, encompassing phenolphthalein's range (8.3–10.0). The colour change occurs within the sharp jump near equivalence — a valid endpoint. Options B and C: weak acid + strong base gives EP pH ≈ 8.7 (not 7.0); strong acid + weak base gives EP pH < 7. Option D: weak acid + weak base has no sharp jump.
CH₃COOH + NaOH: Salt formed is CH₃COONa. CH₃COO⁻ hydrolyses: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻ → OH⁻ produced → EP pH > 7 (≈ 8.7). Phenolphthalein (range 8.3–10.0) encompasses this EP pH → appropriate. ✓ (2 marks)
HCl + NH₃: Salt formed is NH₄Cl. NH₄⁺ hydrolyses: NH₄⁺ ⇌ H⁺ + NH₃ → H⁺ produced → EP pH < 7 (≈ 5.3). Phenolphthalein (range 8.3–10.0) is entirely above the EP pH and the sharp pH jump — phenolphthalein does not change colour at the equivalence point → completely unsuitable. ✓ (2 marks)
(a) Hydrolysis equation: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻. OH⁻ is produced → solution is basic → EP pH > 7. ✓ (1 mark for equation + 1 mark for reasoning)
(b) At EP, equal volumes mixed → [CH₃COO⁻] = 0.0400 mol/L.
Kb(CH₃COO⁻) = Kw/Ka = 1.0 × 10⁻¹⁴ / 1.8 × 10⁻⁵ = 5.56 × 10⁻¹⁰
[OH⁻] = √(5.56 × 10⁻¹⁰ × 0.0400) = √(2.22 × 10⁻¹¹) = 4.71 × 10⁻⁶ mol/L
pOH = 5.33 → EP pH = 14.00 − 5.33 = 8.67 ✓ (2 marks)
(c) Phenolphthalein (8.3–10.0). The EP pH (8.67) falls within phenolphthalein's transition range; the indicator changes from colourless to pink as the solution passes through the sharp pH jump at equivalence. Methyl orange (3.1–4.4) is unsuitable — its range corresponds to the buffer region, not the equivalence point. ✓ (1 mark)
(a) Unknown is CH₃COOH — three pieces of evidence: ① Starting pH ≈ 2.9 (not pH ~1 as expected for a strong acid at ~0.1 mol/L) — consistent with partial dissociation of a weak acid; ② A buffer plateau exists at pH ≈ 4.7 (strong acids have no buffer region); ③ EP pH ≈ 8.7 > 7 — strong acid gives EP pH = 7; basic EP indicates conjugate base hydrolysis, confirming a weak acid. ✓ (3 marks)
(b) n(NaOH) at EP = 0.1000 × 0.02500 = 2.500 × 10⁻³ mol = n(CH₃COOH). c(CH₃COOH) = 2.500 × 10⁻³ / 0.02000 = 0.1250 mol/L ✓ (1 mark)
(c) Half-equivalence volume = 25.00 / 2 = 12.50 mL. At this point, n(CH₃COO⁻) = n(CH₃COOH) → Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]) = pKa + log(1) = pKa. Reading from curve at 12.50 mL → pH ≈ 4.7 → pKa ≈ 4.7. ✓ (2 marks)
(d) Correct indicator: phenolphthalein (range 8.3–10.0 encompasses EP pH 8.7). Methyl orange (3.1–4.4) fails because its range falls within the buffer region of the weak acid titration (centred on pKa ≈ 4.7 ± 1 = pH 3.7–5.7). The colour change would occur when only a fraction of CH₃COOH has been neutralised — far before the equivalence point — giving a titre that is drastically too small and a calculated concentration that is severely underestimated. ✓ (1 mark)
Go back and check your predictions. Recall the 2015 TGA investigation: methyl orange (pKa 3.46) used for a weak acid/strong base titration with EP pH 9.2 — the endpoint was recorded 8 pH units too early. Phenolphthalein used for strong acid + weak base ✗ — phenolphthalein is completely unsuitable for EP pH ≈ 5.3; no usable endpoint. Phenolphthalein for weak acid + strong base ✓ — correct choice; EP pH ≈ 8.7 falls within its range. Any indicator for weak acid + weak base ✗ — no indicator is suitable; the gradual pH change means no sharp endpoint can be located.
You can now explain indicator equilibria, determine EP pH for all four titration types, select the correct indicator with full justification, and predict the direction of error when the wrong indicator is used.
Review
A pharmaceutical company uses titration to check the purity of a weak base drug (pKb = 4.5, M = 181 g/mol). A 0.500 g tablet is dissolved and titrated with 0.0500 mol/L HCl. Average titre = 47.20 mL. (a) Write the equation for the reaction and identify the conjugate acid formed. (b) Explain why the equivalence point pH is below 7 for this titration and estimate whether it is closer to pH 4, 5, or 6. (c) Select the appropriate indicator and justify using the EP pH and indicator range. (d) If the analyst used phenolphthalein instead, explain the direction of error in the reported purity. (e) Calculate the % purity of the tablet. (8 marks)
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