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9476 / 2027
Chemistry of Aqueous Solutions

Lesson 3 of 6

Titration curves and indicators

Identify what controls pH before, at and after equivalence.

A-Level 9476 (2026-2027)

  • I can explain acid-base titration pH changes.Syllabus

    Syllabus 9476, 10.1(d). Explain acid-base titration pH changes.

  • I can choose indicators from supplied titration data.Syllabus

    Syllabus 9476, 10.1(e). Choose indicators from supplied titration data.

Different regions of a titration are controlled by different species

Equivalence means stoichiometric reaction, not necessarily pH 7.

Make a guess. You are not marked.

Ethanoic acid is titrated with sodium hydroxide. What is the pH at the equivalence point?

  1. Above 7, as ethanoate is a weak base
  2. Exactly 7, as the acid is neutralised
  3. Below 7, as ethanoic acid is left over
Show the answer

Above 7, as ethanoate is a weak base

At equivalence the flask holds sodium ethanoate. Ethanoate is a weak base and reacts with water to make some OH-, so the solution is alkaline.

Strong and weak acids titrated with strong base

Both curves reach equivalence at 25 cubic centimetres of sodium hydroxide. Strong acid begins near pH 1 and has equivalence pH 7. Weak acid begins near pH 2.87, has a buffer region and has an alkaline equivalence near pH 8.72. Beyond equivalence the curves approach one another because excess hydroxide controls pH.

Calculated ideal dilute-solution curves at 298 K: 25.0 cm3 of 0.100 mol dm-3 monoprotic acid, titrated with 0.100 mol dm-3 NaOH. Weak-acid Ka = 1.80 × 10-5 mol dm-3. Curves use charge balance; students do not need to solve the numerical model to interpret them.
Follow a weak acid with strong base through four regions
RegionMain chemistryWhat controls pH
Before any baseWeak acid partly ionises.Ka and initial acid concentration.
Before equivalenceAdded OH- converts HA into A-. Both HA and A- remain.The acid/conjugate-base buffer ratio. At half-equivalence, pH = pKa.
At equivalenceAll initial HA has been converted stoichiometrically into A-.Hydrolysis of the weak conjugate base: A- + H2O ⇌ HA + OH-. pH is above 7 at 298 K.
After equivalenceStrong base is in excess.Excess OH- divided by total solution volume.
The four acid-base strength combinations
Titration pairEquivalence at 298 KCurve feature
Strong acid + strong baseApproximately pH 7.Large, steep pH change around equivalence.
Weak acid + strong baseAbove pH 7, from conjugate-base hydrolysis.A buffer region before equivalence; lower-pH part of the jump is shortened.
Strong acid + weak baseBelow pH 7, from conjugate-acid dissociation.When acid is added to weak base, pH falls through a base/conjugate-acid buffer region.
Weak acid + weak baseDepends on relative Ka and Kb, not automatically 7.Usually no large sharp jump, making a visual indicator endpoint unreliable.

Worked exampleAfter equivalence, count the excess

30.0 cm3 of 0.100 mol dm-3 NaOH is added to 25.0 cm3 of 0.100 mol dm-3 monoprotic acid. Find pH at 298 K when strong-base excess controls the result.

Try it first. Then show the worked stepsHide the worked steps
  1. Initial acid = 0.00250 mol; added OH- = 0.00300 mol.
  2. Excess OH- = 0.000500 mol in 0.0550 dm3.
  3. [OH-] = 0.00909 mol dm-3; pOH = 2.04.
Answer

pH = 11.96. Using only the base volume in the denominator would overestimate [OH-].

Choose an indicator whose transition sits inside the steep change

An endpoint should closely match equivalence for the actual titration curve.

An acid-base indicator is usually a weak acid or base whose two forms have different colours. Its colour changes over a range of pH, often around pKa ± 1. The endpoint is where you see the colour change. The equivalence point is where the amounts have reacted exactly. The two should be close, but they are not the same thing.

Use the supplied transition interval and curve
SituationSuitable reasoning
Strong acid/strong base with a broad steep jumpSeveral indicators may work if their entire useful transition interval lies in the steep region.
Weak acid/strong baseAn alkaline-range indicator such as phenolphthalein, roughly pH 8.2-10.0, commonly matches the steep region; methyl orange, roughly pH 3.1-4.4, changes in the buffer region too early.
Strong acid/weak baseChoose an indicator in the acidic steep region using the actual curve; an alkaline-range indicator usually changes too early when acid is added.
Weak acid/weak baseA broad gradual transition makes visual endpoint selection poor; a pH meter and suitable analysis may be preferable.

An indicator does not need to change colour at pH 7. What matters is how much titrant you add while the colour changes. On a steep part of the curve, this volume is tiny, so the endpoint error is small. If you are given unfamiliar indicators, compare their ranges with the steep part of the curve. Do not just pick a name you know.

Check your understanding

25.0 cm3 of 0.100 mol dm-3 aqueous ammonia is titrated with 0.100 mol dm-3 HCl. At 25.0 cm3 of acid, the pH falls steeply from about 6.5 to about 3.5. Which indicator is most suitable?

  1. Bromothymol blue, which changes colour over pH 6.0-7.6
  2. Methyl red, which changes colour over pH 4.4-6.2
  3. Phenolphthalein, which changes colour over pH 8.2-10.0
  4. Thymol blue, which changes colour over pH 1.2-2.8
Show the answer

Methyl red, which changes colour over pH 4.4-6.2

Its whole range lies inside the steep drop, so a fraction of a drop changes the colour at equivalence. Equivalence is below pH 7 because NH4+ is slightly acidic.