What are the classifications of chemical indicators?

1. Acid-base indicator: It indicates the change of H+ concentration in the solution. It is an organic weak acid or organic weak base, and its acidity and alkalinity have different colors. The dissociation constant of indicator acid HIn in solution Ka=[H+][In-]/[HIn], that is, the color of the solution is determined by [In-]/[HIn], and [In-]/[HIn] determines In [H+]. Taking methyl orange (Ka=10-3.4) as an example, when the pH of the solution is <3.1, it is acidic and red; when the pH is >4.4, it is alkaline and yellow; and when the pH is 3.1 to 4.4, it appears red. The mixed color of yellow and orange is called the color change range of the indicator. Different acid-base indicators have different color changing ranges.
2. Metal indicators: Most of the indicators used in complex titration are dyes, which can complex with metal ions at a certain pH to show a completely different color from the free indicator to indicate the end point.
3. Redox indicator: It is an oxidant or reducing agent. Its oxidized form and reduced form have different colors. When it is oxidized (or reduced) during titration, it changes color, indicating the change in solution potential.
4. Adsorption indicator: It is a type of indicator used in volumetric precipitation method. Generally, they are certain organic dyes that can be adsorbed by the precipitate generated during the titration process and change their color.
5. Precipitation titration indicator: Mainly used for titration of Ag+ and halogen ions, using potassium chromate, ferric ammonium vitriol or fluorescent yellow as indicator.