Acid-base titrations and curves: TCE Chemistry (Tasmania)

Syllabus dot point

“Interpret titration curves for the four acid-base combinations and select a suitable indicator.”

9 min readUpdated 2 Jun 2026

Quick answer

A titration curve plots pH against volume of titrant added. The steep, near-vertical section marks the equivalence point. The four acid-base combinations give differently shaped curves with different equivalence pH values, set by the conjugate of any weak partner. You choose an indicator whose colour-change range falls inside that steep section, so the end point coincides with the equivalence point.

What this dot point is asking

TASC expects you to read titration curves, distinguish the equivalence point from the end point, recognise the buffer region, and select an appropriate indicator with justification.

Equivalence point and end point

Equivalence point

The point in a titration at which the moles of added titrant exactly react with the moles of the analyte, in the ratio given by the balanced equation.

The end point is where the indicator changes colour. A well chosen indicator gives an end point that coincides with the equivalence point. These are separate ideas: equivalence is a chemical fact about moles, while the end point is what the indicator shows.

The four combinations

Strong acid with strong base: the curve starts low, rises gently, then jumps steeply over a wide pH range (roughly $3$ to $11$ ), with the equivalence point at pH $7$ . The salt formed is neutral.
Strong acid with weak base: the equivalence point is below $7$ (acidic), because the salt contains the conjugate acid of the weak base.
Weak acid with strong base: the equivalence point is above $7$ (basic), because the salt contains the conjugate base of the weak acid. The curve also has a flatter buffer region before the jump.
Weak acid with weak base: the jump is small and gradual, so there is no sharp equivalence point and indicators are unreliable; use a pH meter.

Equivalence pH depends on the salt

Only strong-strong gives a neutral (pH $7$ ) equivalence point. A weak acid titrated with strong base gives a basic equivalence point; a weak base titrated with strong acid gives an acidic one. Predict it from the conjugate of the weak partner.

The buffer region and half-equivalence

In a weak acid against strong base titration, the early flat region is a buffer of the weak acid and its conjugate base. At the half-equivalence point exactly half the acid is neutralised, so $[\text{acid}] = [\text{base}]$ and therefore $\text{pH} = pK_a$ . This is a quick way to read $pK_a$ straight off a curve.

Choosing an indicator

The indicator must change colour within the steep vertical part of the curve, so that one drop of titrant carries the pH through the colour change. Match the indicator's $pK_a$ (its working range, roughly $pK_a \pm 1$ ) to the equivalence pH.

Phenolphthalein (range about $8.3$ to $10$ ) suits weak acid with strong base.
Methyl orange (range about $3.1$ to $4.4$ ) suits strong acid with weak base.
Either suits strong acid with strong base, because the jump spans both ranges.

Picking an indicator

Identify the weak partner

Ethanoic acid (weak) is titrated with sodium hydroxide (strong). The weak partner is the acid.

Predict the equivalence pH

A weak acid with a strong base gives a basic equivalence point, around pH $8.7$ , because the ethanoate ion is a conjugate base that hydrolyses water.

Match the indicator to the steep region

The steep section lies in the basic region, roughly pH $7$ to $11$ . Phenolphthalein changes colour between pH $8.3$ and $10$ , inside this section, so it is suitable. Methyl orange would change far too early (pH $3.1$ to $4.4$ ) and is unsuitable.

Reading features off a curve

A titration curve carries more information than just the equivalence point. The initial pH reflects the strength and concentration of the analyte: a strong acid starts very low, a weak acid starts noticeably higher for the same concentration. The half-equivalence point lies at half the equivalence volume and, for a weak acid, gives $\text{pH} = pK_a$ directly. The equivalence volume itself lets you calculate the unknown concentration through the mole ratio, exactly as in volumetric analysis. A polyprotic acid (such as carbonic or phosphoric acid) shows two or more separate steep regions, one per proton removed, each with its own equivalence point.

Why the jump is steep

Near equivalence almost all the analyte has reacted, so a tiny further addition of titrant produces a large relative change in the amount of excess acid or base, and therefore a large change in pH. This near-vertical region is what makes an indicator end point sharp. For weak-weak combinations the jump is shallow because neither partner is fully ionised, so no indicator gives a clean colour change and a pH meter must be used instead.

Confusing end point with equivalence point

Students often treat the indicator colour change as the definition of the equivalence point. The equivalence point is fixed by reaction stoichiometry; the end point only matches it if the right indicator is chosen. Using the wrong indicator places the end point away from equivalence and gives a titration error. A second slip is assuming every equivalence point is pH $7$ , which is true only for strong acid against strong base.

In the exam, identify which partner is weak, predict whether the equivalence point is acidic, neutral or basic, then select an indicator whose range sits inside the steep part of the curve and justify the choice.

Exam-style questions

Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

Practice4 marks

25.00\ \text{mL}

0.100\ \text{mol L}^{-1}

ethanoic acid (weak) is titrated with

0.100\ \text{mol L}^{-1}

sodium hydroxide (strong). (a) State, with a reason, whether the pH at the equivalence point is below, equal to or above

7

. (b) Select a suitable indicator from phenolphthalein (range

8.3

10.0

) and methyl orange (range

3.1

4.4

), justifying your choice.

Show worked answer →

(a) The equivalence point is above $7$ (basic). At equivalence all the ethanoic acid has been converted to sodium ethanoate; the ethanoate ion $\text{CH}_3\text{COO}^-$ is the conjugate base of a weak acid and hydrolyses water, $\text{CH}_3\text{COO}^- + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{COOH} + \text{OH}^-$ , making the solution basic. (2 marks)

(b) Phenolphthalein is suitable. Its range ( $8.3$ to $10.0$ ) lies inside the steep section of the curve, which is in the basic region for this combination, so its colour change coincides with the equivalence point. Methyl orange ( $3.1$ to $4.4$ ) would change far too early, in the acidic region, giving a large titration error. (2 marks)

Practice3 marks

Explain the difference between the equivalence point and the end point of a titration, and explain why choosing the correct indicator matters for an accurate result.

Show worked answer →

The equivalence point is the point at which the moles of titrant added exactly react with the moles of analyte in the ratio given by the balanced equation; it is a chemical fact determined by stoichiometry. (1 mark)

The end point is the volume at which the indicator changes colour, which is what the experimenter actually observes. (1 mark)

The two coincide only if the indicator's colour-change range falls within the steep (near-vertical) part of the titration curve. A poorly chosen indicator changes colour away from the equivalence point, so the recorded titre differs from the true equivalence volume, introducing a systematic titration error. (1 mark)

Practise this

Sources & how we know this

Chemistry Level 4 (CHM415115) course document (2015 onwards) — TASC