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

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