Choosing an Acid-Base Indicator (HL)
Choosing a suitable indicator
- Around the equivalence point of a titration, the pH changes very rapidly
- Indicators change colour over a narrow pH range, approximately centred around the pKa of the indicator
- An indicator will be appropriate for a titration if the pH range of the indicator falls within the rapid pH change for that titration
- Section 18 of the data booklet contains information about acid-base indicators
Common Indicators and their colours table
| Indicator | Colour in acid | Colour in alkali | pKa | pH range of colour change |
| Methyl orange | red | yellow | 3.7 | 3.1 – 4.4 |
| Bromophenol blue | yellow | blue | 4.2 | 3.0 – 4.6 |
| Methyl red | red | yellow | 5.1 | 4.4 – 6.2 |
| Phenolphthalein | colourless | pink | 9.6 | 8.3 – 10.0 |
Strong acid - strong base
- In strong acid – strong base titrations, the pH changes from 4 to 10 at the end-point so a suitable indicator must change colour within this range
- Methyl red and phenolphthalein are suitable indicators for these titrations
- Methyl orange is not ideal but it shows a significant enough colour change at the endpoint so is widely used
Weak acid - strong base
- In weak acid – strong base titrations, the pH changes from 7 to 10 at the endpoint so a suitable indicator must change colour within this range
- Phenolphthalein is the only suitable indicator for weak acid – strong base titrations that is widely available
Strong acid - weak base
- In strong acid – weak base titrations, the pH changes from 4 to 7 at the end-point so a suitable indicator must change colour within this range
- Methyl red is the most suitable indicator for these titrations
- However, methyl orange is often used since it shows a significant enough colour change at the endpoint and is more widely available than methyl red
Weak acid - weak base
- In weak acid - weak alkali titrations, there is no sudden pH change at the end-point and thus there are no suitable indicators for these titrations
- The endpoints of these titrations cannot be easily determined
Weak acid - weak base titration curve including indicators
The overlay on the graph shows that both phenolphthalein and methyl orange would change colour outside the point of inflection in a weak acid-weak base titration so they would not be able to show the equivalence point of the titration
