Chemistry • Year 12 • Module 6 • Lesson 16

Titration Curves: Interpreting & Analysing All Four Types

Apply curve-reading skills and the five-region method to real quantitative data, Australian industrial contexts, and indicator selection problems.

Apply · Band 4–5

1. Interpret a titration curve — wine acidity analysis (AWRI context)

The Australian Wine Research Institute (AWRI) routinely determines the concentration of tartaric acid (H₂Tart, a diprotic weak acid; first K_a = 1.0 × 10^–3, pK_a1 = 3.00) in wine samples by titrating a 20.00 mL aliquot with standardised 0.1000 mol/L NaOH. The curve below is adapted from a representative AWRI quality-control run. 10 marks

1.1 Read the first equivalence point volume (V_EP1) and EP pH from the curve. Show how you identified the midpoint of the jump. 2 marks

1.2 Use the half-equivalence point labelled on the curve to determine pK_a1 of tartaric acid. State the calculation method. 2 marks

1.3 The first EP pH is 8.8 (above 7). Explain why this confirms that tartaric acid is a weak acid, not a strong acid. 2 marks

1.4 Select an appropriate indicator for detecting the first equivalence point of this titration. Name the indicator, give its pH range, and justify the selection. 2 marks

1.5 Using V_EP1 = 25.00 mL and c(NaOH) = 0.1000 mol/L for the 20.00 mL wine aliquot, calculate the concentration of tartaric acid in the wine sample. 2 marks

Stuck? Use the formula n(acid) = n(NaOH at EP) and c = n/V.

2. Cause-and-effect chain — why the strong/strong jump is largest

Complete the cause-and-effect chain below. Each cause box is filled; write the consequence in the effect box. Finish with the overall outcome. 5 marks

Cause 1: In a strong acid + strong base titration, no weak acid HA is present.

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Effect 1: There is no _____________ region before the equivalence point.

Cause 2: Without a buffer, nothing resists pH change near the equivalence point.

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Effect 2: A tiny excess of acid or base (one drop) causes a pH change of ________ units.

Cause 3: Near the equivalence point, [H⁺] and [OH^–] both approach 10^–7 mol/L simultaneously.

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Effect 3: The logarithmic pH scale amplifies this __________, producing the steepest possible jump.

Overall outcome (so…): The strong acid + strong base titration produces a pH jump of ______ pH units, larger than any other combination, because _______________________________.

Stuck? Revisit Content Card 3 in the lesson (buffer capacity determines jump size).

3. Interpret data table — HNO₃ concentration in the Orica manufacturing process

Orica, Australia’s largest explosives manufacturer, monitors the concentration of nitric acid (HNO₃) in process streams at its Kooragang Island plant (NSW). Quality-control staff titrate 10.00 mL samples of process HNO₃ with standardised 0.5000 mol/L NaOH. The data below are from five consecutive daily samples. 7 marks

Sample	V_EP (mL NaOH)	EP pH	Buffer region before EP?	Starting pH
Mon	20.00	7.01	No	1.0
Tue	20.20	7.00	No	1.0
Wed	19.85	7.02	No	1.0
Thu	20.00	6.99	No	1.0
Fri (anomaly)	20.00	8.72	Yes	2.8

3.1 Identify the curve type for samples Mon–Thu using all four diagnostic features from the data table. 2 marks

3.2 Calculate the average concentration of HNO₃ for Mon–Thu (use Mon–Thu V_EP values only). Show your working. 2 marks

3.3 On Friday, the EP pH was 8.72 and a buffer region was present. Suggest what contamination may have occurred in the Friday sample. Justify your suggestion using the diagnostic features. 2 marks

3.4 State one quality-control advantage of using a titration curve rather than a single indicator colour change for monitoring HNO₃ concentration. 1 mark

Stuck? Apply the four diagnostic checks: starting pH / buffer before EP / EP pH / jump size.

4. Predict and justify — changing the titrant

4 marks

Scenario. A student sets up a titration of 25.00 mL of 0.100 mol/L acetic acid (CH₃COOH, K_a = 1.8 × 10^–5) with 0.100 mol/L NaOH and records a curve with an equivalence point at 25.00 mL and EP pH = 8.72. The student then repeats the experiment but mistakenly uses 0.100 mol/L NH₃ (a weak base, K_b = 1.8 × 10^–5) as the titrant instead of NaOH.

4.1 Predict two specific ways the new curve (acetic acid + NH₃) will differ from the original curve (acetic acid + NaOH). 2 marks

4.2 Justify whether any standard indicator could reliably detect the equivalence point of the acetic acid + NH₃ titration. 2 marks

Stuck? Think about what happens when both acid and base are weak (Content Card 1 — Curve 4).

Answers — Do not peek before attempting

Q1.1 — Reading V_EP1 and EP pH

V_EP1 = 25.00 mL: identified as the midpoint of the steepest section of the curve (the inflection point of the first pH jump, between approximately 23–27 mL). EP pH = 8.8, read from the y-axis at V = 25 mL. (1 mark for correct V_EP; 1 mark for correct EP pH reading method.)

Q1.2 — Determining pK_a1 from the half-EP

Half-EP volume = V_EP1/2 = 25.00/2 = 12.50 mL. At 12.50 mL, pH = 3.0 (read from the curve at the labelled half-EP point). Therefore pK_a1 = 3.0. (1 mark for identifying half-EP at 12.50 mL; 1 mark for reading pH = 3.0 and stating pK_a1 = 3.0.) Note: the literature value for tartaric acid pK_a1 = 2.98 — consistent with the graphical reading.

Q1.3 — EP pH 8.8 confirms weak acid

At the equivalence point, all tartaric acid has been converted to its conjugate base (hydrogen tartrate / tartrate ions). These conjugate bases undergo base hydrolysis, producing OH– and raising the pH above 7. A strong acid would produce a salt with no hydrolysis tendency (Cl– from HCl, for example) and EP pH = 7.00; the EP above 7 is diagnostic of a weak acid whose conjugate base hydrolyses. (1 mark for stating conjugate base hydrolysis produces OH–; 1 mark for contrasting with strong acid EP at pH 7.)

Q1.4 — Indicator selection

Phenolphthalein (range 8.3–10.0) is appropriate. The first EP pH = 8.8, which falls within phenolphthalein’s transition range. As the curve passes through pH 8.3–10.0 during the sharp jump at 25 mL, phenolphthalein changes from colourless to faint pink, signalling the EP. Methyl orange (pH 3.1–4.4) would change colour in the buffer region far from the EP and is unsuitable. (1 mark for naming phenolphthalein with correct range; 1 mark for justification using EP pH.)

Q1.5 — Concentration of tartaric acid

n(NaOH) = c × V = 0.1000 × 0.02500 = 2.500 × 10^–3 mol. From the balanced equation H₂Tart + NaOH → NaHTart + H₂O (first ionisation, 1:1 ratio): n(H₂Tart) = 2.500 × 10^–3 mol. c(H₂Tart) = 2.500 × 10^–3 / 0.02000 = 0.1250 mol/L. (1 mark for correct mole calculation; 1 mark for correct concentration.)

Q2 — Cause-and-effect chain

Effect 1: There is no buffer region before the equivalence point.

Effect 2: A tiny excess causes a pH change of approximately 6–8 pH units (accept 3–8; the key point is a very large change).

Effect 3: The logarithmic pH scale amplifies this concentration change, producing the steepest possible jump.

Overall outcome: The strong acid + strong base titration produces a pH jump of ~6–8 pH units, larger than any other combination, because there is no buffer capacity (no HA or A– species) to resist pH change near the equivalence point.

(1 mark each for Effects 1, 2, 3 and the overall outcome — 4 marks; accept reasonable variation in wording. Note: only 5 marks total for Section 2.)

Q3.1 — Curve type for Mon–Thu

Strong acid + strong base. Diagnostic evidence: (1) starting pH = 1.0 — low, consistent with a strong acid at ~1 mol/L fully ionised; (2) no buffer region before EP; (3) EP pH = 7.00 ± 0.02; (4) by implication a large sharp jump (no buffer to moderate it). (1 mark for correct curve type; 1 mark for at least two diagnostic features.)

Q3.2 — Average c(HNO₃)

Mon–Thu V_EP average = (20.00 + 20.20 + 19.85 + 20.00)/4 = 80.05/4 = 20.01 mL. n(NaOH) = 0.5000 × 0.02001 = 0.01001 mol = n(HNO₃). c(HNO₃) = 0.01001/0.01000 = 1.001 mol/L (accept 1.00 mol/L). (1 mark for correct average V_EP; 1 mark for correct concentration.)

Q3.3 — Friday anomaly

The Friday sample shows EP pH = 8.72, starting pH = 2.8, and a buffer region — all diagnostic of a weak acid + strong base titration. The process stream appears to have been contaminated with a weak acid (for example, acetic acid or another organic acid) instead of, or mixed with, HNO₃. The higher starting pH (2.8 vs 1.0) confirms partial ionisation (weak acid character), and the EP above 7 and buffer region before the EP confirm the weak acid identification. (1 mark for identifying contamination with a weak acid; 1 mark for using at least two diagnostic features to justify.)

Q3.4 — Advantage of using a titration curve

A full titration curve reveals the EP pH and buffer region, allowing the nature of the acid (strong vs weak) to be confirmed simultaneously with the concentration determination. A single indicator colour-change only indicates when the EP is reached, providing no information about acid identity or possible contamination.

Q4.1 — Two differences when NH₃ replaces NaOH

(1) The equivalence point pH will be near (but not exactly) pH 7 rather than ~8.72, because both species are weak (acetic acid pK_a = 4.74; NH₄⁺ pK_a = 9.26; EP pH ≈ (4.74 + 9.26)/2 = 7.00 theoretically for equal K values, but in practice close to 7 with no sharp jump). (2) There will be no sharp pH jump at the equivalence point — dual buffer regions from both HA/A– and NH₄⁺/NH₃ overlap, giving a gradual rise with no detectable inflection. (1 mark per correct difference.)

Q4.2 — Indicator viability for weak/weak

No standard indicator could reliably detect the equivalence point. Because there is no sharp pH jump, there is no narrow pH range over which the colour change would occur simultaneously with the EP. Both methyl orange (pH 3.1–4.4) and phenolphthalein (pH 8.3–10.0) would change colour well before or after the actual EP, giving incorrect titration volumes. A potentiometric (pH meter) endpoint is required. (1 mark for stating no standard indicator works; 1 mark for explaining absence of sharp jump as the reason.)