How do you choose a suitable indicator for a titration?

A suitable indicator for a titration is chosen based on the pH at the equivalence point of the reaction.

In a titration, an indicator is used to signal the point at which the reaction has been completed, known as the equivalence point. The choice of indicator is crucial as it determines the accuracy of the titration results. The most suitable indicator is one that changes colour at a pH close to the equivalence point of the reaction.

The equivalence point is the point in the titration where the amount of titrant added is enough to completely neutralise the analyte solution. The pH at the equivalence point depends on the nature of the acids and bases involved in the reaction. For a strong acid-strong base titration, the pH at the equivalence point is 7. For a strong acid-weak base titration, the pH is less than 7, and for a weak acid-strong base titration, the pH is more than 7.

Indicators are weak acids or bases that have different colours in their acidic and basic forms. The point at which an indicator changes colour is called its end point. The closer the end point of an indicator is to the equivalence point of the reaction, the more accurate the titration results will be.

For example, phenolphthalein is an indicator that changes colour between pH 8.2 and 10. It is suitable for titrations involving strong acids and strong bases, or weak acids and strong bases, as the pH at the equivalence point for these reactions is close to the pH range of phenolphthalein. On the other hand, methyl orange, which changes colour between pH 3.1 and 4.4, is suitable for titrations involving strong acids and weak bases.

In conclusion, the choice of indicator for a titration depends on the pH at the equivalence point of the reaction. The most suitable indicator is one that changes colour at a pH close to this point.