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Indicators
An indicator is a weak acid or base added in a very small quantity to the titration solution before the experiment begins. Usually they are weak acids.
The indicator has a special property: its acid and conjugate base forms appear as different colours in solution
Example:
HIn + H2O ⇌ In- + H3O+
(red) (blue)
Wize Concept
The pKa value of the chosen indicator should be close to the pH of the solution at the equivalence point (ideally within 1 pH unit of the equivalence point)
We want the indicator to signal a colour change close to the equivalence point.
When the pH of the solution=pKa of the indicator, the indicator's acid and conjugate base components will have an equal concentration [HIn]=[In-] and we will see a mixture of the two colors!
Example:
In our example above, we'd see a
purple
colour Equivalence Point vs Endpoint
Equivalence point: moles of H+=moles of OH-
End point: when we see a colour change because of the indicator
Example:
Look at methyl orange in the table below. Indicate an endpoint and a possible equivalence point.
HIn + H2O ⇌ In- + H3O+
(red) (blue)
1) When pH=pKa of indicator:
- [HIn]=[In-]
- red + blue → we'd seepurple!
2) When pH < pKa
- [HIn] > [In-]
- we'd seered!
3) When pH > pKa
- [HIn] < [In-]
- we'd seeblue!
Practice: Indicators
The indicator cresol red has Ka = 5.0 x 10-9. Over what approximate pH range does it change colour?