Indicators are substances that change color at or near the end point of a chemical reaction, helping to detect when a reaction is complete.
In volumetric analysis (titration), indicators are used to visually show the end point, making it easy to measure how much titrant has reacted with the analyte.
How Indicators Are Used:
- Acid-base titrations:
- Indicators like phenolphthalein or methyl orange change color depending on the pH.
- Example: Phenolphthalein is colorless in acid and pink in base. When the pink appears, the titration is complete.
- Redox titrations:
- Indicators change color when oxidation or reduction occurs.
- Example: Starch is used with iodine to show the end point by forming a blue-black complex.
- Complexometric titrations:
- Indicators form colored complexes with metal ions to show the end point.
- Example: Eriochrome Black T changes from red to blue when all calcium or magnesium ions react with EDTA.
Key Points:
- The color change signals the endpoint, which should be as close as possible to the equivalence point.
- The choice of indicator depends on the type of reaction and pH or chemical conditions.
In short:
Indicators are color-changing substances that help detect the end of a titration in volumetric analysis.