Indicators are weak acids/bases whose conjugate acid-base forms have different colors. They change color over a specific pH range.

Phenolphthalein: Colorless in acid (pH < 8.2), pink in base (pH > 10). Transition range: 8.2-10.0. Best for titrations where equivalence point is basic (weak acid + strong base: CH3COOH + NaOH → equivalence at pH ~8.7).

Methyl orange: Red in acid (pH < 3.1), yellow in base (pH > 4.4). Transition range: 3.1-4.4. Best for titrations where equivalence point is acidic (strong acid + weak base: HCl + NH3 → equivalence at pH ~5.3).

Selection rule: The indicator's transition range must overlap with the steep part of the titration curve at the equivalence point.

Strong acid + strong base: Steep curve spans pH 4-10 → either indicator works. Weak acid + strong base: Equivalence at pH > 7 → phenolphthalein only. Strong acid + weak base: Equivalence at pH < 7 → methyl orange only. Weak acid + weak base: No steep region → no simple indicator works → use pH meter.

Universal indicator: Mixture of indicators that shows continuous color change across full pH range (1-14). Used for approximate pH estimation, not for precise titrations.