An indicator is a substance that indicates when the concentration of a chemical species has passed a certain threshold value, by a change of color, turbidity, or fluorescence. Indicators are generally used to find the end point of a titration.
The indicator is a substance existing in two visibly different forms in an equilibrium that is the same kind as that of the reaction being followed. Thus to follow an acid-base titration, a conjugate acid-base system is used as an indicator which changes color over a narrow range of pH corresponding to the end-point. (For this to happen, the equilibrium constant K of the indicator must approximately equal the hydrogen-ion concentration of the end-point.) A universal indicator is a mixture of indicators which changes color continuously over a wide pH range, used as a quick guide to acidity. To follow an oxidation-reduction reaction, an indicator is used which exists reversibly in an oxidized or reduced state, its oxidation potential being about the same as that of the reaction. For a precipitating or complexing reaction, an indicator is used which itself forms a colored precipitate or complex with excess added reagent. A good indicator must be visible at such low concentrations that it does not interfere with the reaction.