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
Indicator & pH range table
| Indicator | pH range |
| Methyl orange | 3.1 - 4.4 |
| Phenolphthalein | 8.3 - 10.0 |
- Both indicators change colour over a specific pH range
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
