CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.7.13 Indicators used in Titration

Indicators

  • 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
  • Both indicators change colour over a specific pH range

Equilibria pH Range Indicator, downloadable AS & A Level Chemistry revision notes

The diagram shows the change in colour from red to yellow of methyl orange over a pH range of 3.1-4.4 and from colourless to pink of phenolphthalein over a pH range of 8.3-10

Indicator & pH range table

Equilibria Table 1_Indicators, downloadable AS & A Level Chemistry revision notes

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, both indicators can be used

 

Equilibria Strong Acid Strong Alkali Indicator, downloadable AS & A Level Chemistry revision notes

The diagram shows that 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

 

Equilibria Strong Acid Weak Alkali Indicator, downloadable AS & A Level Chemistry revision notes

The diagram shows that 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

 

Equilibria Weak Acid Strong Alkali Indicator, downloadable AS & A Level Chemistry revision notes

The diagram shows that 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

 

Equilibria Weak Acid Weak Alkali Indicator, downloadable AS & A Level Chemistry revision notes

The diagram shows that nether indicators can be used to determine the endpoint of the titration of a weak acid and weak alkali

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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