18.5 – Indicators
18.5.1 – Describe qualitatively the action of an acid-base indicator
Indicators change their colour depending on the pH of the solution. This is because they change forms depending on the H+ concentration, according to Le Chatelier’s principle.
To maintain equilibrium, the indicator will favour a certain form:
[H+] increases – shift to the left to favour HIn
[H+] decreases – shift to the right to favour In–
18.5.2 – State and explain how the pH range of an acid-base indicator relates to its pKa value
The pH at the endpoint of the indicator is approximately equal to the pKa of the indicator, pKin. A colour change can only be perceived by humans when the concentration of one colour is ten times greater than the other. The pH range is calculated according to the equation:
Since different indicators have different pKa values, the pH of their endpoint is different.
18.5.3 – Identify an appropriate indicator for a titration, given the equivalence point of the titration and the pH range of the indicator
The equivalence point is where the ratio of moles of acid to moles of base is equal to the stoichiometric ratio. This means that neither reactant is in excess. The endpoint of the reaction is when the indicator changes colour. This should be as close to the equivalence point as possible.
When selecting an appropriate indicator for a reaction from the data booklet, select the one that has a pH range that encompasses the equivalence point of the reaction.