A pharmaceutical quality control technician selects phenolphthalein instead of methyl orange for a strong acid/weak base drug purity test. The endpoint occurs at pH 9 — long after the equivalence point at pH 5. The batch passes QC with a reported purity of 112%. Impossible. Undetected until patient harm occurs. Indicator selection is not a minor procedural detail.
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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.
Wrong: The endpoint of a titration is always at pH 7, so any indicator that changes near pH 7 will work.
Right: The equivalence point pH depends on the salt formed. Strong acid + strong base → pH 7; weak acid + strong base → pH > 7; strong acid + weak base → pH < 7. The indicator must have a colour change range that brackets the equivalence point pH — otherwise the endpoint occurs before or after the true equivalence point, producing a systematic error.
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
Question 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?
Question 2. Which of the following correctly matches a titration type with a suitable indicator and a valid justification?
Question 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?
Question 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?
Question 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?
Question 6. 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.
Question 7. 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.
Question 8. 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. Either way, the endpoint is wrong or undetectable. Option A incorrectly states phenolphthalein detects pH 5.3 (its range starts at 8.3). Option C is a dangerous generalisation. 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 — wrong. Option B: phenolphthalein is unsuitable for strong acid + weak base (EP pH ≈ 5.3, below Ph's range) — wrong. Option D: not all EPs are near pH 7; strong acid + weak base gives EP pH < 7, and BTB is only borderline there — wrong.
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. Option B is wrong — the jump is so sharp that no extra base is needed; both indicators transition within the same tiny volume range near EP.
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 conceptual 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. MO at the half-EP would give a false endpoint far before equivalence.
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 at all — an indicator colour change would be gradual and unreliable.
CH₃COOH + NaOH: At the equivalence point, CH₃COONa is formed. CH₃COO⁻ is the conjugate base of the weak acid CH₃COOH and hydrolyses: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻. OH⁻ is produced → EP pH > 7 (≈ 8.7). Phenolphthalein (range 8.3–10.0) encompasses this EP pH → phenolphthalein is appropriate. ✓ (2 marks)
HCl + NH₃: At the equivalence point, NH₄Cl is formed. NH₄⁺ is the conjugate acid of the weak base NH₃ and hydrolyses: NH₄⁺ ⇌ H⁺ + NH₃. H⁺ is 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 → phenolphthalein is 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 are 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 = −log(4.71 × 10⁻⁶) = 5.33 → EP pH = 14.00 − 5.33 = 8.67 ✓ (2 marks)
(c) Phenolphthalein (8.3–10.0). Justification: 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, 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, exactly half the CH₃COOH has been converted to CH₃COO⁻ → n(CH₃COO⁻) = n(CH₃COOH) → by Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]) = pKa + log(1) = pKa. Reading from the curve at 12.50 mL → pH ≈ 4.7 → pKa ≈ 4.7 (consistent with Ka ≈ 2 × 10⁻⁵). ✓ (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. Student 1 (BTB for strong/strong) ✓ — BTB is valid; the large pH jump encompasses all three indicators. Student 2 (Ph for strong acid + weak base) ✗ — phenolphthalein is completely unsuitable for EP pH ≈ 5.3; no usable endpoint. Student 3 (Ph for weak acid + strong base) ✓ — phenolphthalein is the correct choice; EP pH ≈ 8.7 falls within its range. Student 4 (BTB for weak/weak) ✗ — no indicator is suitable for weak acid + weak base; the gradual pH change means no sharp endpoint can be located.
Advanced Titration Calculations