More indicators
The colour change of an indicator can be used to detect the equivalence point in a titration if an indicator can be found that changes colour near the equivalence point pH, that is in the range shown by the blue box.
For a titration of a weak acid with a strong base (as in the diagram) or a strong acid with a weak base, the change in pH is usually 4-5 units.
For a titration of a strong acid with a strong base, the pH change is 8-10 units.
The volume at which the indicator changes colour is known as the end point in the titration.
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- the pH at the equivalence point (this is half way up the steep rise)This determines the pH that must be detected by the colour change.
- the pKa of the indicator.This determines the pH at which the indicator changes colour.
The colour of the indicator will be due to the acid form at pH one unit less than its pKa.
The colour of the indicator will be due to the base form at pH one unit more than its pKa.
- the steepness and extent of the pH change at the equivalence pointThis determines the volume of titrant over which the pH changes occurs.
For a titration of a weak acid with a strong base (as in the diagram) or a strong acid with a weak base, the change in pH is usually 4-5 units.
Suitable indicators have pKa within one unit of the equivalence point pH so that the colour change is close to the equivalence point (as shown in the diagram)..
For a titration of a strong acid with a strong base, the pH change is 8-10 units.
Suitable indicators have pKa within two units of the equivalence point pH.
The volume at which the indicator changes colour is known as the end point in the titration.
A particular indicator can be used to detect the equivalence point only if it has an end point close to the equivalence point for the titration.