Chemistry • Year 12 • Module 6 • Lesson 15
Indicators: Mechanism & Selecting the Right One
Synthesise quantitative equivalence-point data, evaluate multi-indicator scenarios, and critique scientific claims about indicator selection.
1. Evaluating indicator selection in a water-treatment quality audit
The Australian Water Quality Guidelines (NHMRC, 2018) require that drinking water alkalinity be determined by titration. A municipal water treatment plant performs monthly acid-base titrations to determine total alkalinity (primarily HCO₃– and CO₃²–). A senior analyst selects the indicator for three different alkalinity tests run in the same batch. 8 marks
Scenario context:
Test A — 0.100 mol/L HCl (aq) titrated against 25.00 mL NaOH (aq, unknown concentration). Indicator chosen: bromothymol blue.
Test B — 0.050 mol/L H₂SO₄ (aq, strong acid) titrated against 25.00 mL Na₂CO₃ (aq, 0.050 mol/L). Na₂CO₃ is a salt whose CO₃²– ion is the conjugate base of the weak acid HCO₃–. Indicator chosen: methyl orange.
Test C — 0.100 mol/L NaOH (aq) titrated against 25.00 mL propionic acid (CH₃CH₂COOH, Ka = 1.34 × 10–⁵). Indicator chosen: bromothymol blue.
| Test | Titration type | Approx. EP pH (calculate or state) | Indicator chosen | Valid? (Y/N) | Consequence if wrong |
|---|---|---|---|---|---|
| A | Bromothymol blue | ||||
| B | Methyl orange | ||||
| C | Bromothymol blue |
Using the data, scenario context, and your completed table, write an extended response that:
- Identifies which tests use a valid indicator and which do not.
- Explains, for each invalid choice, which indicator should have been selected and why — with reference to the equivalence point pH.
- For Test B, calculates the approximate EP pH using the Kb of CO₃²– (Kw/Ka2, where Ka2 of H₂CO₃ = 4.7 × 10–¹¹) to justify the indicator choice. Show full working.
- Evaluates the overall risk to the water-treatment plant if the wrong indicators are used systematically — with reference to Australian drinking water safety requirements.
2. Source critique — evaluating a laboratory protocol claim
"Bromothymol blue is the most versatile acid-base indicator and can be used for any titration. Because it changes colour right at pH 7, it detects the neutral equivalence point that all titrations share. It is therefore the indicator of choice in any university or TAFE analytical chemistry laboratory."
— Excerpt from a fictitious student laboratory manual circulating on a study-sharing platform, 2024
This claim contains multiple scientific errors. 7 marks
2.1 Identify three distinct scientific errors or misleading claims in the excerpt. For each error, state the correct scientific explanation.
2.2 Propose one experimental investigation a NATA-accredited laboratory could perform to demonstrate that BTB is not universally valid — describe the independent variable, the dependent variable, and what result would falsify the claim. (included in 7 marks)
2.3 The Australian Wine Research Institute (AWRI) specifies phenolphthalein (not BTB) for routine titratable acidity measurements in wine. Using your knowledge of wine acidity and titration chemistry, explain why this choice is scientifically justified and why the manual's BTB recommendation would give inaccurate results in a wine analysis context.
Q1 — Table + extended response
Test A (HCl / NaOH, strong/strong): EP pH = 7. BTB range (6.0–7.6) overlaps the large pH jump (~4–10). Valid. No error consequence.
Test B (H₂SO₄ / Na₂CO₃, strong acid / conjugate base of weak acid): CO₃²– is the conjugate base of HCO₃– (itself the conjugate base of H₂CO₃); for the EP pH calculation, use the first equivalence point (CO₃²– → HCO₃–):
Kb(CO₃²–) = Kw / Ka2(H₂CO₃) = 1.0 × 10–¹´ / 4.7 × 10–¹¹ = 2.13 × 10–´
At EP, [CO₃²–] = 0.025 mol/L (diluted 1:1). [OH–] = √(Kb × c) = √(2.13 × 10–´ × 0.025) = √(5.33 × 10–⁶) = 2.31 × 10–³ mol/L
pOH = –log(2.31 × 10–³) = 2.64; EP pH = 14.00 – 2.64 = 11.36.
Methyl orange (3.1–4.4) is far below this EP. Invalid. Methyl orange would detect a false endpoint in the acidic region, giving a titre far smaller than required. For the first EP, phenolphthalein (8.3–10.0) is also below 11.36, making this a case where a special indicator for pH >10 (e.g. alizarin yellow) may be needed, or a pH meter is required.
Test C (NaOH / propionic acid, weak acid / strong base): pKa = –log(1.34 × 10–⁵) = 4.87. At EP: [propanoate] ≈ 0.050 mol/L. Kb(propanoate) = 1.0 × 10–¹´ / 1.34 × 10–⁵ = 7.46 × 10–¹°. [OH–] = √(7.46 × 10–¹° × 0.050) = √(3.73 × 10–¹¹) = 6.11 × 10–⁶ mol/L. pOH = 5.21. EP pH = 8.79. BTB (6.0–7.6) is below this EP. Invalid. Phenolphthalein (8.3–10.0) is the correct choice.
Risk evaluation: If wrong indicators are used systematically, titratable alkalinity measurements are consistently biased. For Test B, underestimating alkalinity could lead to under-dosing of neutralisation treatment, leaving drinking water at dangerously low or high pH, violating NHMRC guidelines (pH 6.5–8.5). The systematic nature of the error means it cannot be detected by replicate precision alone — a calibration standard check or pH meter comparison would be needed.
Q2.1 — Three errors in the claim
Error 1: "BTB can be used for any titration." Incorrect — BTB (range 6.0–7.6) is only valid where the equivalence point pH and the sharp pH jump include this range (primarily strong/strong). For weak acid / strong base (EP ≈ 8.7) or strong acid / weak base (EP ≈ 5.3), BTB falls outside the jump and produces a systematically incorrect endpoint.
Error 2: "BTB changes colour right at pH 7." Incorrect — BTB transitions over the range pH 6.0–7.6, not at a single precise pH. It appears yellow at pH < 6.0, green in the transition zone, and blue at pH > 7.6. There is no single "right at pH 7" change.
Error 3: "All titrations share a neutral equivalence point." Incorrect — the equivalence point pH depends on the nature of the acid and base and the salt formed. Strong acid / strong base gives pH = 7; weak acid / strong base gives pH > 7; strong acid / weak base gives pH < 7. Only one of the four standard titration types has an EP at pH 7.
Q2.2 — Experimental design to falsify the claim
Independent variable: titration type (strong acid/strong base vs weak acid/strong base vs strong acid/weak base).
Dependent variable: titre volume recorded using BTB compared with the titre volume recorded using a pH meter (or a validated correct indicator) for the same analyte.
Falsifying result: If BTB gives a statistically different (discordant) titre for the weak acid / strong base and strong acid / weak base conditions compared with the pH meter endpoint, this falsifies the "universally valid" claim. The strong/strong condition should agree (controls the experiment).
Q2.3 — AWRI wine analysis and phenolphthalein
Wine contains tartaric, malic, and citric acids — all weak organic acids. Titrating wine against NaOH is therefore a weak acid / strong base titration. The equivalence point pH is above 7 (typically ~8.2–8.5 for the primary acids at wine concentrations). Phenolphthalein's range (8.3–10.0) encompasses this EP pH; the colour change from colourless to pale pink occurs within the sharp pH jump at equivalence, giving an accurate titre. BTB's range (6.0–7.6) is below the equivalence point for a weak acid / strong base titration and falls in the gradual buffer region of the curve. Using BTB for wine analysis would produce a premature colour change, recording an endpoint before the true equivalence point, underestimating titratable acidity — a serious error in a commercial quality control context where acidity determines regulatory compliance and commercial value.