What is an indicator?

• An indicator is a substance that undergoes a change when the endpoint of a titration is reached E.g.: acid indicators are used to single the end of acid-base titration.
• Indicators are organic weak acids or bases with complicated structures.
• The chemical substances which are used to indicate the endpoint during titration are called indicators.

Types of indicator:

Self-indicator: A substance is said to be self-indicator if it itself acts as an indicator in the titration. Example: KMnO₄ acts as a self-indicator during the titration of Potassium permanganate (KMnO­­₄) and oxalic acid (COOH)₂.
Internal indicator: The indicator which is used within the solution is called internal indicator. Example: Methyl orange and phenolphthalein can act as an indicator in the acid-base titration.
External indicator: The indicator which is not added to the solution are called external indicator. Potassium Ferrocyanide K₃[Fe(CN)₆] can acts as external indicator during titration of ammonium or ferrous sulphate (FeSO₄) with Potassium dichromate (K₂Cr₂O₇).