1 Describe qualitatively the action of an acid–base indicator.
The chemical formula HIn(aq) simply represents any indicator.
(Colour A) HIn(aq) ↔ H+(aq) + In–(aq) (Colour B)
An indicator is an weak acid or base which has a different colour in its dissociated and undissociated form.
An indicator will be colour A in an acidic solution. It will not want to dissociate as there is a high concentration of H+ ions present in the solution.
However, it will be colour B in an alkali solution as the weak acid will dissociate due to the presence of OH– ions.
2 State and explain how the pH range of an acid–base indicator relates to its pKin value
Similar to weak acids and bases, indicators have a dissociation constant KIn:
KIn = ([H+] x [In–]) / [HIn]
We can assume that if [In–] ≈ [HIn] in a solution, then a colour change from A to B or vice versa will occur as they are almost equal in concentration.
Therefore, if [In–] ≈ [HIn], then from the equation we can also say that the endpoint of an indicator, i.e. when it changes colour is when KIn ≈ [H+].
We can rewrite this as pH ≈ pKIn.
Basically, when the pH of a solution is close to the pKin value of an indicator, it will experience a colour change from A to B or vice versa.
3 Identify an appropriate indicator for a titration, given the equivalence point of the titration and the pH range of the indicator.
Look at the data booklet for values of pKin for indicators.
When you are given the equivalence point of a titration, you want to choose an indicator with a pKIn value close to the equivalence point, making sure that the pH of the equivalence point falls between the pH range of the indicator.
For example, if you have a titration with a strong acid and a equivalence point of 3.8, then you would want an indicator like methyl orange as it has a pH range of 3.1 – 4.4.