Definition: Indicators are substances that change color in response to a change in pH.

Purpose: Used to determine the endpoint of a titration and to gauge the pH of a solution.

How Indicators Work

• Color Change: Indicators change color because they are weak acids or bases that dissociate differently at different pH levels, altering their color.

• HIn ↔ H⁺ + In⁻ Process: In a general form, an indicator (HIn) dissociates into a hydrogen ion (H⁺) and an anion (In⁻). The color of HIn differs from In⁻.

Types of Indicators

• pH Indicators: Change color over a range of pH values. Example: litmus, phenolphthalein.

• End-Point Indicators: Used in titrations to signal the completion of the reaction.

• Redox Indicators: Change color in response to oxidation or reduction.

Choosing an Indicator

• Based on pH Range: The pH range over which an indicator changes color should match the expected pH change in the reaction.

• Type of Titration: For acid-base titrations, the indicator should change color at the equivalence point.

Common Indicators and Their pH Ranges

• Litmus: Red in acidic solutions and blue in basic solutions. pH range: about 5.5 (acidic) to 8.0 (basic).

• Phenolphthalein: Colorless in acidic and neutral solutions, pink in basic solutions. pH range: 8.2 - 10.0.

• Methyl Orange: Red in acidic solutions and yellow in basic solutions. pH range: 3.1 - 4.4.