Britannica
Sources used in this answer
How do chemical indicators show when a reaction has reached its end point?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
chemical reaction
Log burning in a fire. Burning wood is an example of a chemical reaction in which wood in the presence of heat and oxygen is transformed into carbon dioxide, water vapor, and ash.
Chemical indicators signal the end point of a chemical reaction—most commonly during titrations—by providing a visible change when a specific chemical species reaches a critical concentration or threshold.12
While a color change is the most frequent indicator, some reactions signal completion through the formation or disappearance of turbidity. The mechanism depends on the type of indicator used:1
- Acid-Base Indicators: These substances exist in different colored forms depending on the acidity or alkalinity of the solution. As the concentration of hydrogen ions shifts, the indicator transitions between these forms.12
- Oxidation-Reduction Indicators: These undergo oxidation or reduction near the end point, resulting in a distinct color shift based on the solution's oxidation potential.12
- Specific-Substance and Adsorption Indicators: These react to the presence of a specific ion or form an adsorbed layer on a precipitate, causing a visible change once the primary reaction is complete and an excess of the titrant is present.12
- Complex-Formation Indicators: These dyes form a colored complex with metal ions; at the end point, the reagent displaces the dye, causing a color change.2