Indicators

Rucete ✏ Chemistry In a Nutshell

1. What Is an Indicator?

• An indicator is a chemical compound that changes color depending on the pH of the solution it's in.
• Used in titrations to visually detect the equivalence point.

2. How Indicators Work

• Indicators are weak acids or bases themselves.
• They exist in equilibrium between two forms:
  • One form dominates in acidic conditions (shows one color)
  • The other dominates in basic conditions (shows a different color)

3. Color Change Range

• An indicator changes color over a specific pH range, not at a single value.
• The pH range corresponds to the region where both acidic and basic forms coexist.

4. Common Indicators and Their pH Ranges

Indicator	pH Range	Color Change (Acid → Base)
Thymol Blue	3.0 – 4.0	Red → Yellow
Chlorophenol Red	4.8 – 6.4	Yellow → Red
Cresol Red	7.2 – 8.8	Yellow → Red
Phenolphthalein	8.3 – 10.0	Colorless → Pink

5. Choosing the Right Indicator

• Choose an indicator with a pH transition range that matches the equivalence point of the titration.
• Example:
  • Strong acid + strong base → use phenolphthalein or bromothymol blue
  • Weak acid + strong base → use phenolphthalein
  • Strong acid + weak base → use methyl orange