Chemistry • Year 12 • Module 6 • Lesson 15
Indicators: Mechanism & Selecting the Right One
Apply indicator selection rules to real titration data, interpret a titration curve with overlaid indicator ranges, and reason about systematic errors from wrong indicator choices.
1. Interpret titration results from a NATA-accredited laboratory
A NATA-accredited analytical chemistry laboratory ran eight replicate titrations of the same analyte (0.100 mol/L NaOH added to 25.00 mL portions of an unknown acid solution) using three different indicators. The table records the volume of NaOH at the colour change. 8 marks
| Run | Indicator used | NaOH titre (mL) | Notes |
|---|---|---|---|
| 1 | Methyl orange (3.1–4.4) | 14.82 | Colour change: red → orange |
| 2 | Methyl orange (3.1–4.4) | 14.78 | Colour change: red → orange |
| 3 | Bromothymol blue (6.0–7.6) | 22.14 | Colour change: yellow → blue |
| 4 | Bromothymol blue (6.0–7.6) | 22.09 | Colour change: yellow → blue |
| 5 | Phenolphthalein (8.3–10.0) | 22.18 | Colour change: colourless → pale pink |
| 6 | Phenolphthalein (8.3–10.0) | 22.11 | Colour change: colourless → pale pink |
| 7 | Phenolphthalein (8.3–10.0) | 22.16 | Colour change: colourless → pale pink |
| 8 | Bromothymol blue (6.0–7.6) | 22.12 | Colour change: yellow → blue |
1.1 Calculate the mean titre for runs using bromothymol blue and the mean titre for runs using phenolphthalein. Show working.
1.2 The methyl orange titres (Runs 1–2) are approximately 7.3 mL less than the BTB / phenolphthalein titres. Using your knowledge of indicator selection, explain why methyl orange gives a lower titre and what type of systematic error this represents.
1.3 Identify the most likely titration type (strong/strong; weak acid/strong base; strong acid/weak base) consistent with the data. Justify with reference to the approximate equivalence point pH and which indicators agree.
1.4 A NATA auditor reviewing this data recommends discarding the methyl orange results and reporting only the mean of Runs 3–8 as the official titre. Evaluate this recommendation in one sentence.
2. Interpret a titration curve with indicator range overlays
The graph below shows a titration curve for 25.00 mL of 0.100 mol/L ethanoic acid (CH₃COOH) titrated with 0.100 mol/L NaOH. Three coloured bands indicate the transition pH ranges of the three standard indicators. Use the graph to answer questions 2.1–2.4. 9 marks
2.1 Read the equivalence point pH from the graph. State which of the three indicators (MO, BTB, phenolphthalein) has its transition band overlapping the sharp pH jump at equivalence and explain why overlapping the jump — rather than the equivalence point pH itself — is the important criterion.
2.2 The methyl orange band (pH 3.1–4.4) overlaps the buffer region of this curve. At approximately what volume of NaOH does the pH reach 3.75 (the middle of MO's range)? Predict what a student using MO would record as their endpoint volume, and explain the consequence for a calculated acid concentration.
2.3 Use the half-equivalence point marker on the graph to read off the pKa of ethanoic acid and explain why pH = pKa at exactly half the equivalence volume.
2.4 Bromothymol blue (BTB, 6.0–7.6) does not overlap the equivalence point pH of 8.72. Using the graph, describe where in the curve the BTB band falls, and predict whether using BTB would overestimate or underestimate the titre, and by roughly how much (read from the graph).
3. Cause-and-effect chain — wrong indicator in a pharmaceutical QC lab
A quality control technician at a pharmaceutical manufacturer is titrating NH₃ solution (weak base) with HCl (strong acid) to determine purity. The equivalence point pH is 5.3. The technician mistakenly uses phenolphthalein (range 8.3–10.0). Trace the cause-and-effect chain below. 6 marks
4. Compare indicators across four titration types
Complete the table. For each titration type, state the approximate equivalence point pH, the correct indicator, and one reason why each of the other two indicators is unsuitable. 12 marks — 1 per cell
| Titration type | Approx. EP pH | Correct indicator | Why MO fails (if wrong) | Why BTB fails (if wrong) | Why Ph fails (if wrong) |
|---|---|---|---|---|---|
| Strong acid / strong base | 7 | Any of the three is valid — explain why below | |||
| Weak acid / strong base | N/A (this is the correct choice) | ||||
| Strong acid / weak base | N/A (this is the correct choice) | ||||
| Weak acid / weak base | ~7 (varies) | None suitable |
4.1 For the strong acid / strong base row, explain in one sentence why all three indicators are valid, even though their ranges differ significantly.
Q1.1 — Mean titres
BTB (Runs 3, 4, 8): mean = (22.14 + 22.09 + 22.12) / 3 = 22.12 mL.
Phenolphthalein (Runs 5, 6, 7): mean = (22.18 + 22.11 + 22.16) / 3 = 22.15 mL.
Q1.2 — Why MO gives a lower titre
Methyl orange (range 3.1–4.4) transitions in the buffer region of the weak acid titration curve, well before the equivalence point (~pH 8.7). The student sees the colour change at approximately pH 3.75–4.4, records the endpoint prematurely, and adds far less NaOH than needed to reach equivalence. This is a systematic error (always in the same direction — titre too small).
Q1.3 — Identify titration type
Weak acid / strong base titration. Evidence: BTB and phenolphthalein agree closely (~22.1–22.2 mL), and both their ranges (6.0–7.6 and 8.3–10.0) overlap the strong pH jump in this region, which suggests the equivalence point pH is in the range 7–10 — consistent with a weak acid / strong base EP (typically pH 8–9.5). MO gives a drastically different (smaller) titre, confirming it is unsuitable.
Q1.4 — Auditor recommendation
The recommendation is correct: the methyl orange results introduce a large systematic underestimate of the titre; discarding them and averaging the six BTB and phenolphthalein results (which agree within normal experimental variation) produces an accurate, traceable result consistent with NATA accreditation requirements for uncertainty minimisation.
Q2.1 — Which indicator overlaps the jump?
EP pH ≈ 8.72 from the graph. Phenolphthalein (8.3–10.0) is the only indicator whose band overlaps the sharp pH jump region (~pH 7.5–11 on this curve). The criterion is the jump, not just the EP pH exactly, because the jump is so steep that even a band slightly offset from the EP pH will still produce a colour change within a fraction of a mL of the equivalence volume — an acceptably small error.
Q2.2 — MO false endpoint
From the graph, the curve crosses pH 3.75 at approximately 2–5 mL NaOH (early in the buffer region). A student using MO would record an endpoint at this volume — roughly 20 mL short of the true equivalence volume (25.00 mL). The calculated acid concentration would be severely underestimated (by ~80%).
Q2.3 — Half-equivalence pKa
The half-equivalence marker sits at 12.50 mL NaOH; the graph reads pH ≈ 4.74 at this point. At the half-equivalence volume, exactly half the CH₃COOH has been converted to CH₃COO–, so [CH₃COOH] = [CH₃COO–]. Substituting into Henderson–Hasselbalch: pH = pKa + log([A–]/[HA]) = pKa + log(1) = pKa. Therefore pH = pKa ≈ 4.74.
Q2.4 — BTB error
From the graph, the BTB band (6.0–7.6) lies in the gradual rising section of the curve before the sharp jump at equivalence. The curve crosses this band at approximately 22–23 mL (below the 25.00 mL EP). Using BTB would underestimate the titre by approximately 2–3 mL for this weak acid / strong base titration, producing a calculated acid concentration lower than the true value. (Note: for a strong/strong titration, BTB would be valid because the jump is large enough to include BTB's range.)
Q3 — Cause-and-effect chain
Effect 1: entirely above the equivalence point pH and the sharp pH jump region.
Effect 2: a large excess of NH₃ has been added (pH is pushed above 8.3 by unreacted base), or the endpoint is not detectable at all within the expected titration range.
Effect 3: larger (the student must add excess NaOH/acid to push pH above 8.3, or the endpoint is entirely missed).
Overall outcome: overestimated; systematic error.
Q4 — Comparison table
Strong acid / strong base (EP pH = 7): All three indicators valid. MO, BTB, and Ph all have ranges within the very large pH jump (~pH 4–10) — 4.1 below.
Weak acid / strong base (EP pH > 7, typically ~8.5–9.5): Phenolphthalein correct. MO fails — range (3.1–4.4) is in the buffer region; endpoint is premature and titre is far too small. BTB fails — range (6.0–7.6) is below the EP pH and the sharp jump; slight underestimate.
Strong acid / weak base (EP pH < 7, typically ~4.5–5.5): Methyl orange correct. BTB fails — range (6.0–7.6) is above the EP; titre is overestimated. Ph fails — range (8.3–10.0) is far above EP; no endpoint detected.
Weak acid / weak base: None suitable — no sharp pH jump exists at equivalence. All three indicators change colour gradually and at unpredictable volumes.
4.1: For strong acid / strong base, the pH jump spans approximately pH 4–10 — a jump of ~6 pH units — so all three indicator ranges (MO 3.1–4.4, BTB 6.0–7.6, Ph 8.3–10.0) lie within this steep jump, meaning all three produce a sharp colour change within a fraction of a drop of the equivalence volume.