An indicator is used to follow the progress of an acid-base titration and the end point of the reaction is signalled by a change in the indicator colour. Ideally, we would choose an indicator with an end point that matches the equivalence point of the acid-base reaction i.e. the indicator changes colour at the same pH as the equivalence point.
Indicators are weak acids or weak bases which have different colours in their neutral / uncharged and their ionic forms.
Imagine we are using phenol red as an indicator in a titration between hydrochloric acid and sodium hydroxide. If we add a few drops of phenol red to HCl in the conical flask, the presence of the H+ ions from HCl shifts the equilibrium position of the indicator to the left and the indicator turns yellow. As the titration proceeds, NaOH is added from the burette and the OH– ions react with the H+ ions present in the solution. The indicator’s equilibrium position shifts to the right as more HA dissociates to attempt to replace the neutralised H+ ions. The end point is reached when we have equal concentrations of the neutral form of the indicator (HA) and the ionic form (A–), and we are half way between yellow and fucshia. The indicator has turned a pinkish-red colour.
So, acid-base indicators are weak acids. HA is often written as HIn for indicators.
which means that Ka = [H+] and, taking the logarithm of both sides, pKa = pH.
The pH of the end point is the same as the pKa value of the indicator. This allows us to choose suitable indicators for different titrations.
- Strong acid – strong base titrations:
The equivalence point is pH7 so we need an indicator with a pKa ∼ 7. Phenol red has a pKa 7.9 (it changes colour over the pH range 6.7-8.1 which matches the steepest part of the titration curve).
2. Weak acid – strong base titrations:
The equivalence point is ∼ pH9, so phenolphthalein is a good choice of indicator as it has a pKa 9.4. Methyl orange would be a poor choice as it has a pKa 3.4 so its colour change / end point would not correspond with the steepest part of the titration curve (the equivalence point).
3. Weak base – strong acid titrations:
The equivalence point is ∼ pH4 so methyl orange is a good choice here with a pKa 3.4.
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