Indicators used in Titration (Cambridge International A-Level Chemistry)

Indicators

Spec point — 7.2.10

• Indicators are substances that change colour when they are added to acidic or alkaline solutions
• When choosing the appropriate indicator, the pH of the equivalence point is very important
• The two most common indicators that are used in titrations are methyl orange and phenolphthalein

Indicator & pH range examples

• Both indicators change colour over a specific pH range

  • Methyl orange 3.1 - 4.4
  • Phenolphthalein 8.3 - 10.0

Diagram showing the colour changes for methyl orange and phenolphthalein

Methyl orange changes from red to yellow over a pH range of 3.1 - 4.4, while phenolphthalein changes from colourless to pink over a pH range of 8.3 - 10.0

Choosing indicators for titrations

Strong acid and strong alkali

• The colour change for both indicators takes place at a pH range that falls within the vertical region of the curve
• Therefore, either indicator can be used

Methyl orange and phenolphthalein in a strong acid + strong alkali titration

Both indicators can be used to determine the endpoint of the titration of a strong acid and strong alkali

• Strong acid and weak alkali

  • Only methyl orange will change colour at a pH close to the equivalence point and within the vertical region of the curve

Methyl orange and phenolphthalein in a strong acid + weak alkali titration

Only methyl orange can be used to determine the endpoint of the titration of a strong acid and weak alkali

• Weak acid and strong alkali

  • Now, only phenolphthalein will change colour at a pH close to the equivalence point and within the vertical region of the curve
  • The pH range at which methyl orange changes colour falls below the curve

Methyl orange and phenolphthalein in a weak acid + strong alkali titration

Only phenolphthalein can be used to determine the endpoint of the titration of a weak acid and strong alkali

• Weak acid and weak alkali

  • Neither indicator is useful, and a different method should be considered

Methyl orange and phenolphthalein in a weak acid + weak alkali titration

Neither indicator can be used to determine the endpoint of the titration of a weak acid and weak alkali