Choose indicators from titration data
Key idea: An indicator's acid and conjugate base have different colours. Choose one whose entire transition range lies within the steep pH change near equivalence, so its end-point is close to equivalence.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Explain how the indicator equilibrium changes its colour.
- Fit the whole colour-change range inside the steep section.
- Compare both ends of each range with the steep section.
- Choose an indicator for the ethanoic-acid titration.
- Explain your choice using pH ranges and added volume.
- Choose indicators for the strong-acid titration.
- Explain why more indicators suit the strong-acid titration.
- Identify the acid and decide whether thymol blue is suitable.
- Explain your acid classification, salt hydrolysis and indicator choice.
- Compare ranges when adding acid makes the pH fall.
- Choose an indicator for the ammonia titration.
- Explain how salt hydrolysis changes the suitable indicator range.
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ContinueAn indicator shows the end of a titration by changing colour. It only works if its colour change happens in the steep part of the titration curve, where a single drop of titrant moves the pH a long way. This lesson explains how an indicator works and how to choose one from a titration curve or from pH data.
How an indicator works
An indicator is a weak acid, HIn, whose undissociated form and conjugate base have different colours:
In acid, the high [H⁺] pushes the equilibrium to the left, so the colour of HIn shows. In alkali, hydroxide ions remove H⁺, the equilibrium moves to the right, and the colour of In⁻ shows. The colour changes gradually over a range of about two pH units, centred near the indicator’s pKₐ.
| Indicator | Colour in acid | Colour in alkali | pH range of colour change |
|---|---|---|---|
| methyl orange | red | yellow | 3.1–4.4 |
| phenolphthalein | colourless | pink | 8.3–10.0 |
The end-point is where the indicator changes colour. A good indicator gives an end-point close to the equivalence point.
Choosing an indicator
Near equivalence, the pH changes by several units for a tiny volume of titrant. Choose an indicator whose whole colour-change range lies inside that steep section. Then the colour change happens over a small volume interval close to equivalence. If the range lies outside the steep section, the colour changes gradually over several cm³, too early or too late.
Worked example 1
Compare both ends of each range
Problem
Show full solution
Compare each range with the steep section
Method
Check both the lower and upper limits of each range.Reason
The whole colour change must happen in the steep section.Working
Answer
X: 6.0 < 8.0 and 9.0 < 10.5, so all of X’s range is inside. Y: 4.0 is below 6.0, so most of Y’s range is outside.Decide
Method
Choose X.Reason
X changes colour completely in the steep section near equivalence. Y starts changing well before equivalence, in the gently sloping part of the curve.Working
Answer
X is suitable; Y is not.
Quick check
Check your understanding 1
The steep interval is pH 7.14–10.30. Choose phenolphthalein (8.3–10.0) or methyl orange (3.1–4.4), then explain the data-to-volume link in the worked problem.
Show answer
Phenolphthalein lies entirely within 7.14–10.30. Methyl orange lies outside, giving an earlier end-point. Use the next response to explain the small volume over which the suitable colour change happens.
Guided practice 2
Ethanoic acid and sodium hydroxide
Problem
When 0.100 mol dm⁻³ sodium hydroxide is added to 25.0 cm³ of 0.100 mol dm⁻³ ethanoic acid, the pH rises from 7.14 at 24.90 cm³ to 10.30 at 25.10 cm³. Is phenolphthalein (8.3–10.0) or methyl orange (3.1–4.4) suitable? Explain using both ranges.
Try this before viewing the solution
Hints
Hint 1
steep section
Between 24.90 and 25.10 cm³ the pH jumps from 7.14 to 10.30.
Hint 2
compare
Check whether each indicator’s whole range lies between 7.14 and 10.30.
Show solution step by step
Find the steep section
Method
The pH rises from 7.14 to 10.30 over only 0.20 cm³.Reason
The equivalence point is alkaline, because ethanoate ions are a weak base.
Working
Answer
Steep section: pH 7.14 to 10.30.Choose the indicator
Method
Phenolphthalein is suitable; methyl orange is not.Reason
Phenolphthalein’s whole range, 8.3–10.0, lies inside the steep section. Methyl orange’s range, 3.1–4.4, is reached long before equivalence, in the buffer region.
Working
Answer
7.14 < 8.3 and 10.0 < 10.30
Quick check
Methyl orange would change colour gradually over many cm³ near the start of the titration, giving an end-point far too early.
Check your understanding 2
For 25.0 cm³ of 0.100 mol dm⁻³ hydrochloric acid titrated with 0.100 mol dm⁻³ sodium hydroxide, the pH rises from 3.09 at 24.60 cm³ to 10.90 at 25.40 cm³. Which of methyl orange and phenolphthalein can be used? Why is the choice wider here than for ethanoic acid?
Show answer
Both ranges, 3.1–4.4 and 8.3–10.0, lie inside the steep section from pH 3.09 to 10.90, so either indicator changes colour completely between 24.6 and 25.4 cm³, around equivalence. A strong acid–strong base curve has a much longer steep section, from about pH 3 to 11, so more indicators fit. With ethanoic acid the steep section starts at about pH 7, which rules out methyl orange.
Check your understanding 3
Explain why the strong acid–strong base data above allow both indicators, while the ethanoic acid data exclude methyl orange. Use the full ranges and the pH change over a small added volume.
Show answer
Both 3.1–4.4 and 8.3–10.0 fit within 3.09–10.90, traversed over 0.80 cm³ around equivalence. The ethanoic-acid steep interval starts at 7.14, above the whole methyl-orange range; methyl orange therefore changes before equivalence. The range comparison connects colour change with volume error, rather than just an acidic or alkaline label.
Check your understanding 4
At 25 °C, a 0.100 mol dm⁻³ monobasic acid titrated with NaOH starts at pH 3.0 and has equivalence pH 8.8. Its steep interval is 7.0–10.5. Classify the acid and decide whether thymol blue (8.0–9.6) is suitable.
Show answer
This is a weak acid–strong base titration. Thymol blue is suitable. In the next problem, connect the starting concentration/pH, salt hydrolysis and complete range to your conclusion.
Exam-style question 1
Judge an indicator from a curve
Problem
An unknown 0.100 mol dm⁻³ monobasic acid is titrated with sodium hydroxide at 25 °C. The curve starts at pH 3.0, rises steeply from pH 7.0 to 10.5 near equivalence, and the equivalence point is at pH 8.8. Identify the type of titration, and decide whether thymol blue, which changes colour over pH 8.0–9.6, is a suitable indicator. [4 marks]
Try this before viewing the solution
Show solution step by step
Classify the acid
1 markMethod
The acid is weak.Reason
A strong acid at 0.100 mol dm⁻³ would start at pH 1.0, not 3.0.
Working
Answer
Starting pH 3.0 means only about 1% is dissociated.Classify the base
1 markReason
Sodium hydroxide is fully dissociated.Working
Answer
Weak acid–strong base titration.Use the equivalence pH
1 markMethod
An equivalence point at pH 8.8 supports this.Reason
The conjugate base of a weak acid makes the solution alkaline.Working
Answer
Equivalence above 7 fits a weak acid and a strong base.Judge the indicator
1 markMethod
Thymol blue is suitable.Reason
Its whole range, 8.0–9.6, lies inside the steep section from 7.0 to 10.5.Working
Answer
Its colour-change range lies completely inside the steep section near equivalence.
Self-mark with the mark scheme
Tick each mark point that your answer includes.
Credit the weak acid, the strong base, the alkaline-equivalence evidence and the range comparison.
When the pH falls
When acid is added to a base, the curve falls instead of rising, but the rule is the same: the whole colour-change range must lie inside the steep section.
Try it yourself 3
Ammonia and hydrochloric acid
Problem
25.0 cm³ of 0.100 mol dm⁻³ ammonia is titrated with 0.100 mol dm⁻³ hydrochloric acid at 25 °C:
| Acid added / cm³ | pH |
|---|---|
| 24.90 | 6.85 |
| 25.00 | 5.28 |
| 25.10 | 3.70 |
Indicator R changes colour over pH 4.0–6.0; indicator S over pH 8.0–10.0. Choose an indicator. In the next response, explain why the alkaline-range indicator that suits ethanoic acid with sodium hydroxide is unsuitable here.
Try this before viewing the solution
Hints
Hint 1
direction
The pH falls as acid is added. Find the pH values on either side of 25.00 cm³.Show solution step by step
Find the steep section
Method
The pH falls from 6.85 to 3.70 between 24.90 and 25.10 cm³.Reason
That is a fall of over 3 pH units for 0.20 cm³ of acid.Working
Answer
Steep section: pH 6.85 to 3.70.Choose
Method
Choose R.Reason
Its whole range, 4.0–6.0, lies inside the steep section. S changes between pH 10 and 8, well before equivalence.Working
Answer
3.70 < 4.0 and 6.0 < 6.85Explain the difference
Method
This equivalence point is acidic; the ethanoic acid one was alkaline.Reason
Ammonium ions are a weak acid, while ethanoate ions are a weak base.Working
Answer
The steep section moves to lower pH, so a different indicator fits.
Quick check
Check your understanding 5
Explain the R/S decision from the descending data above, then compare the salt formed here with the salt in the ethanoic acid–NaOH titration.
Show answer
The steep interval spans pH 3.70–6.85 in a 0.20 cm³ added-volume interval. R’s whole 4.0–6.0 range fits; S’s 8.0–10.0 range is reached before equivalence. Here NH₄⁺ is a weak acid and gives acidic equivalence. Ethanoate is a weak base and gives alkaline equivalence, placing its steep interval higher. Curve direction changes the order of readings, rather than the whole-range criterion.
Common mistakes
- Choosing an indicator because the equivalence point is “alkaline” or “acidic” without comparing ranges with the steep section.
- Accepting an indicator when only part of its range overlaps the steep section.
- Treating the colour change as happening at one exact pH. It happens over a range.
- Confusing the end-point, where the colour changes, with the equivalence point, where the amounts match.
- Expecting any indicator to work for a weak acid–weak base titration, which has no steep section.
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Syllabus and review details
- GCE A-Level H2 Chemistry 2027 · 9476-2027
9476 (2027), complete syllabus
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