Purpose
To illustrate the effect of pH change on indicator color.

Materials

• Thymolphthalein 
• Ethanol 
• Sodium hydroxide solution, NaOH (0.1 M)
• 100 mL beaker or bottle

Safety

• Read the MSDS/WHMIS sheets for all chemicals before using them
• Wear chemical safety glasses
• Sodium hydroxide is caustic, handle with care

Procedure

1. Dissolve a small amount of thymolphthalein in ~10 mL of ethanol.
2. Slowly add 40 mL of water while stirring.
3. Add aqueous sodium hydroxide solution, 0.1 M, dropwise until solution turns dark blue.
4. The resulting solution can be used as disappearing ink.

Results
The ink disappears over time.

Follow-up Teaching Notes

• Thymolphthalein is blue above pH 10 and colorless below pH 9.
• The ink solution becomes colorless as it absorbs carbon dioxide from the air
  2 NaOH + CO₂ -> Na₂CO₃ + H₂O

Connections
acid/base indicators, pH.

Extension
Two common invisible inks are:

• AgNO₃ (aq), which can be developed by light
• CuSO₄ (aq), which can be developed by ammonia

Disposal/Clean-up

• The paper can be disposed of in the garbage.
• Extra ink can be placed in a sealed and properly labeled container for reuse.

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