Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 14.16 Indicator selection-Study Notes - New Syllabus

Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 14.16 Indicator selection- Study Notes- New syllabus

Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 14.16 Indicator selection- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.

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Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics

14.16 Selecting a Suitable Indicator for a Titration

An indicator is used to identify the end point of a titration. To be suitable, its colour change must occur at (or very close to) the equivalence point, where the reaction is complete.

Key Principle

A suitable indicator changes colour within the steep vertical region of the titration curve.

How to Select an Indicator

  1. Identify the equivalence point pH from the titration curve.
  2. Locate the steep vertical section.
  3. Choose an indicator whose pH transition range lies within this region.

Common Indicators

  • Methyl orange: pH 3.1 – 4.4
  • Phenolphthalein: pH 8.2 – 10.0

Indicator Selection for Different Titrations

1. Strong Acid vs Strong Base

  • Equivalence point ≈ pH 7.
  • Very steep vertical region.
  • Both methyl orange and phenolphthalein are suitable.

2. Weak Acid vs Strong Base

  • Equivalence point > 7 (basic).
  • Vertical region lies in alkaline range.
  • Phenolphthalein is suitable.
  • Methyl orange is unsuitable (changes too early).

3. Weak Base vs Strong Acid

  • Equivalence point < 7 (acidic).
  • Vertical region lies in acidic range.
  • Methyl orange is suitable.
  • Phenolphthalein is unsuitable (changes too late).

4. Weak Acid vs Weak Base

  • No sharp vertical region.
  • pH changes gradually.
  • No suitable indicator.

Important Insight

  • The indicator’s colour change must occur within the steep region, not just at the equivalence point.
  • This ensures minimal error between end point and equivalence point.

Key Features

  • Match indicator range to vertical section of curve.
  • Strong systems → flexible choice.
  • Weak systems → careful selection required.
  • Weak–weak titrations → no suitable indicator.

Example 1:

Which indicator is suitable for titrating ethanoic acid with sodium hydroxide?

▶️ Answer/Explanation

This is a weak acid–strong base titration.

Equivalence point is above pH 7.

Therefore, phenolphthalein is suitable.

Example 2:

Explain why methyl orange is unsuitable for a weak acid–strong base titration.

▶️ Answer/Explanation

Methyl orange changes colour in the acidic range (pH 3–4).

The equivalence point for a weak acid–strong base is above pH 7.

Therefore, it changes colour before the equivalence point is reached.

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