Indicators are substances that change color, luminescence, or forming a precipitate when the concentration of certain components in solution is changed. Accordingly, there are color, fluorescent and turbidimetric indicators. These indicators are used primarily for establishing of the titration endpoint (or end of any chemical reaction) and to detect pH of solutions.

Area of concentration of solution components, where a color or luminescence change of indicator is detected, is called the transition interval. In accordance with the titrimetric methods, where indicators are used, usually the main acid-based indicators, complexometric indicators, redox indicators and adsorption indicators are applied. Indicators can be reversible and irreversible; the first ones are used in reversible reactions, the second ones are supposed to irreversible chemical transformations.

Even with proper selection of indicators it is possible to have errors in establishing the end point of titration related to the difference between the interval of transition and the equivalence point with limited ability to accurately remember or to compare different colors with a flow rate of a certain volume of titrant in relation of interaction with indicator.