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Acid Base Indicators

Learn and apply Acid Base Indicators in the published Chemistry course sequence.

  • GCE A-Level H1 Chemistry 8873-2027
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Choosing Acid–Base Indicators: Orientation

Indicator choice is an evidence-matching problem: the transition range must lie within the rapid pH change near the end-point.

H1 8873 scope
  • Select indicators from acid/base strengths and given indicator data; full titration-curve treatment is excluded.

Definitions (Must Know)

  • An acid–base indicator is a weak acid/base system whose conjugate forms have different colours.
  • Its transition range is the pH interval over which the observed colour changes.

Detailed Explanations

Indicator choice is made by matching supplied transition-range data to the rapid pH change expected from the strengths of the reacting acid and base.

A weak acid–weak base titration normally has no sufficiently sharp pH change for a visual indicator to locate the equivalence point accurately.

An indicator does not cause the end-point; its acid and base forms have different colours and respond to the changing pH.

The equivalence-point pH alone is insufficient: compare the whole steep region with the indicator range.

Worked Examples

Modelled example 1

Choose an indicator from acid and base strengths

Core

Problem

Choose between methyl orange (pH 3.1–4.4) and phenolphthalein (pH 8.2–10.0) for hydrochloric acid titrated with aqueous ammonia.
Study the worked solution

  1. Classify the reacting pair

    Method

    Identify hydrochloric acid as strong and ammonia as weak.

    Reason

    The acid/base strengths locate the rapid pH change in the acidic region.

    Working

    This is a strong acid–weak base titration.

  2. Match the indicator

    Method

    Choose the acidic-range indicator.

    Reason

    Its transition is reached during the rapid pH change, whereas phenolphthalein changes too far into the alkaline region.

    Working

    Methyl orange, pH 3.1–4.4, is suitable.

Guided practice 2

Select from supplied transition data

About 5 min

Problem

A titration changes rapidly from pH 3.0 to 10.5 near equivalence. Would indicators with ranges 3.1–4.4 and 8.2–10.0 both be suitable?

Try this before viewing the solution

Hints

Hint 1: check each interval
Compare both ends of each indicator range with pH 3.0–10.5.
Hint 2: apply the endpoint rule
A suitable range should be crossed during the rapid pH change.
View solution step by step

  1. Check the lower range

    Method

    Compare pH 3.1–4.4 with the supplied rapid-change interval.

    Reason

    Most of this transition occurs at the lower edge of the rapid change.

    Working

    The whole range lies within pH 3.0–10.5, so this indicator is suitable.

  2. Check the upper range

    Method

    Compare pH 8.2–10.0 with pH 3.0–10.5.

    Reason

    The whole upper range lies inside the rapid pH change.

    Working

    This indicator is also suitable; both give an end-point close to equivalence.

Common misconception 3

Choose by transition range, not familiar colour

Find and correct the mistake

Learner claim

A learner always chooses phenolphthalein because its colour change is easy to see, then checks only whether pH 8.2 is close to the equivalence pH. Identify and correct the method.

Choose the controlling test

The indicator decision should use

View solution step by step

  1. Reject the colour shortcut

    Method

    Separate visibility from chemical suitability.

    Reason

    A clear colour is unhelpful if it changes well before or after the rapid pH change.

    Working

    Familiarity with phenolphthalein does not make it universally suitable.

  2. Use the whole transition interval

    Method

    Compare both ends of the indicator range with the rapid pH interval.

    Reason

    An indicator changes colour over a range, not at one exact pH.

    Working

    Accept it only when the transition is completed within the rapid pH change.

Examiner practice 4

Justify an indicator from supplied data

4 marks

Problem

A weak acid is titrated with a strong base. The pH changes rapidly from 7.2 to 10.4 near equivalence. Methyl orange changes at pH 3.1–4.4 and phenolphthalein at pH 8.2–10.0. Select the suitable indicator and justify why the other is unsuitable. [4 marks]

Try this before viewing the solution

View solution step by step

  1. Select phenolphthalein

    1 mark

    Method

    Name phenolphthalein.

    Reason

    Its transition occurs in the alkaline rapid-change interval.

    Working

    Suitable indicator: phenolphthalein.

  2. Use both ends of its range

    1 mark

    Method

    Compare pH 8.2–10.0 with pH 7.2–10.4.

    Reason

    The full transition lies within the supplied rapid pH change.

    Working

    8.2 > 7.2 and 10.0 < 10.4.

  3. Link range to volume accuracy

    1 mark

    Method

    Explain why rapid pH change matters.

    Reason

    Only a small titrant-volume change is needed to cross the indicator range.

    Working

    The observed end-point is close to the equivalence volume.

  4. Reject methyl orange

    1 mark

    Method

    Compare its acidic range with the supplied interval.

    Reason

    pH 3.1–4.4 is outside pH 7.2–10.4.

    Working

    Methyl orange changes too early and is unsuitable.

Challenge 5

Recognise when a visual indicator is unreliable

Minimal support

Problem

Near equivalence, a titration changes gradually from pH 5.4 to 6.2 over a substantial titrant volume. Assess a visual indicator with transition range pH 5.6–6.0 and recommend a better way to detect the end-point.

Try this before viewing the solution

Hints

Hint 1: consider volume
A pH range can be numerically enclosed yet still require a large volume change when the pH rise is shallow.
Hint 2: choose a measurement
Consider a method that follows pH directly rather than relying on one colour interval.
View solution step by step

  1. Test the transition

    Method

    Notice that pH 5.6–6.0 is crossed slowly.

    Reason

    The supplied pH change occurs over a substantial titrant volume.

    Working

    The colour transition would persist across an appreciable volume interval.

  2. Assess the endpoint

    Method

    Reject the visual indicator as unreliable for a precise end-point.

    Reason

    Judging one colour within a long transition can shift the recorded volume appreciably.

    Working

    The indicator end-point may differ substantially from equivalence.

  3. Recommend direct measurement

    Method

    Use a calibrated pH meter or probe.

    Reason

    It records the pH response without depending on a sharp visual colour change.

    Working

    Monitor pH against added titrant volume.

Mind Stretchers

Attempt the independent prompts before opening a hint or solution.

  • Choose a suitable indicator range for ethanoic acid titrated with sodium hydroxide and justify the choice.
  • Explain why an indicator is unreliable for a weak acid–weak base titration.

Mind stretcher 1: Rejecting a poor indicatorExtension

Question. An indicator changes from pH 5.0 to 6.5, but a titration curve is shallow throughout this interval. Explain the likely experimental error.

Show Hint

Relate a broad volume change to the colour interval.

Show Answer

A substantial titrant volume would be needed to cross the indicator range, so the observed end-point could differ appreciably from the equivalence volume.

Mind stretcher 2: Data without a named titrationExtension

Question. Two indicators have ranges 2.8–4.0 and 6.0–7.6. The steep region is pH 2.5–5.0. Select and justify.

Show Hint

Choose by overlap, not by proximity to pH 7.

Show Answer

The 2.8–4.0 indicator is suitable because its full transition lies in the steep region. The 6.0–7.6 range lies outside it.