Select an indicator from titration data
Choose an acid–alkali indicator whose transition range lies within the steep pH change around equivalence.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Equilibrium constants apply at a fixed temperature; include only gaseous or aqueous species in the relevant expressions. For acid–base work, distinguish strength from concentration. Written practice is self-checked
Before you begin
- Balance equations, interpret reversible arrows and use concentration in mol dm⁻³.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where should an indicator's transition range lie?
What you will learn
- Select an indicator from supplied transition data
- Match indicator range to the vertical region of a titration curve
- Recognise when a weak–weak titration lacks a sharp indicator endpoint
The transition range must match the pH jump
An indicator changes colour over a transition range. Select one whose range lies within the steep vertical pH change near the equivalence point, so a small volume change gives a visible endpoint close to equivalence.
Read the supplied indicator data directly. The syllabus does not require using pKa values for indicator selection.
A suitable indicator depends on both reactants
Strong acid–strong base curves have a wide steep region around pH 7, so several transition ranges may fit. Weak-acid/strong-base equivalence is above 7; strong-acid/weak-base equivalence is below 7.
A weak acid–weak base titration may have no sharp pH jump, so a colour indicator may not give a reliable endpoint. Use the curve and the transition ranges supplied rather than memorising one universal indicator choice.
Pause and explain
Which data should you compare when selecting an indicator?
Worked example
The steep pH region for a titration is pH 6.5–10.0. Indicators A, range 3–5; B, range 7–9; C, range 10–12. Which is most suitable?
Show the worked solution
- Compare each transition range with the steep pH region.
- B lies within 6.5–10.0.
- A is below and C is mostly above the sharp change.
Answer Indicator B is most suitable because its transition range lies within the steep pH region.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A titration has a steep pH region 4–10. Candidate ranges are X: 2–4, Y: 5–8, Z: 10–12. Choose one.
Give me a hint
Pick the interval that lies inside the vertical region.
Compare my reasoning
- X is at or below its lower boundary.
- Y is within 4–10.
- Z is at or above the upper boundary.
Y is the most suitable indicator because its transition range 5–8 lies within the steep 4–10 region.
Look for these in your work
- Ranges compared
- Suitable range selected
- Reason stated
A weak acid is titrated with a strong base and the equivalence point is above pH 7. Which of two indicators, ranges 3–5 and 8–10, is more likely suitable if the steep region is pH 7.5–10.5?
Give me a hint
Match the transition range to the supplied steep section.
Compare my reasoning
- The equivalence region is above 7.
- The 8–10 range lies within the steep section.
- The 3–5 range is too acidic.
The 8–10 indicator is more suitable because its range falls within the steep pH interval 7.5–10.5.
Look for these in your work
- Curve chemistry interpreted
- Range matched to steep region
- Unsuitable interval rejected
A weak acid–weak base titration curve changes gradually through equivalence. Explain the limitation on selecting an indicator.
Give me a hint
An indicator endpoint is most accurate when its transition occurs during a sharp pH jump.
Compare my reasoning
- There is no clear steep region.
- A small volume change may not produce a sharp colour endpoint near equivalence.
- A visual indicator may be unreliable; use an appropriate measurement method if available.
Because the curve has no sharp pH jump, an indicator may change gradually over a broad volume range and give an uncertain endpoint.
Look for these in your work
- Curve feature recognised
- Endpoint accuracy limitation explained
- Practical implication stated
Common mistakes
- Choosing an indicator because its colour is familiar
- Selecting a transition range far from equivalence
- Using pKa where the question supplies transition ranges
- Assuming weak–weak titrations always have a sharp endpoint
What can you explain now?
Where should an indicator's transition range lie?
Compare with the explanation
Within the steep pH change near equivalence
Then the colour change occurs close to the endpoint.
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Make it stick
How do you choose an indicator using a titration curve and supplied transition ranges?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.