An indicator in a titration is a substance that signals the end of a chemical reaction by changing color. It helps determine the endpoint—the point at which the reaction between the titrant and the analyte is complete.
Key Points:
- Purpose: To show visually when the titration is complete.
- How it Works:
- Indicators change color depending on the pH (for acid-base titrations) or the chemical environment.
- The color change occurs very close to the equivalence point, where the amount of titrant added is stoichiometrically equal to the substance in the flask.
- Examples of Indicators:
- Phenolphthalein: Colorless in acid, pink in base (used in strong acid vs strong base titrations).
- Methyl orange: Red in acid, yellow in base (used in strong acid vs weak base titrations).
- Bromothymol blue: Yellow in acid, blue in base, green at neutral.
- Selection of Indicator: Depends on the type of reaction and the pH at equivalence point. Choosing the wrong indicator can give inaccurate results.
In short: An indicator is a visual “signal” that tells you when the titration is complete.