Disappearing Ink

A disappearing ink solution is created using the acid-base indicator thymolphthalein. It appears dark blue when basic, but fades to colorless as carbon dioxide from the air lowers the pH. The fading can be sped up with vinegar or slowed down with additional base.

1. Dissolve 1 gram of thymolphthalein powder in 100 mL of ethyl alcohol, stirring to help it dissolve.
2. Add 900 mL of water and stir; the solution may appear cloudy.
3. Carefully add 10 mL of 3 M sodium hydroxide to turn the solution dark blue.
4. Test the solution by applying a small amount to white fabric; watch as the blue stain disappears over time.
5. For a faster effect, apply vinegar or blow on the spot to add carbon dioxide.
6. Store the disappearing ink in a sealed container for later use.

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• Use phenolphthalein instead of thymolphthalein to make pink disappearing ink.
• Adjust the fading time by adding very small amounts of sodium hydroxide.
• For a dramatic effect, trained professionals can use a carbon dioxide fire extinguisher to make the stain vanish instantly.

• Always wear safety glasses when handling chemicals.
• Sodium hydroxide is caustic—handle with care and avoid skin contact.
• Only adults should prepare the sodium hydroxide solution.
• Wash hands thoroughly after handling chemicals.
• Test the ink on fabric before demonstrations, as some fabrics may show residue.
• Do not attempt the fire extinguisher method unless properly trained.

• Why does the ink turn blue when sodium hydroxide is added? (The solution becomes basic, and thymolphthalein turns blue in basic conditions.)
• Why does the color disappear over time? (Carbon dioxide from the air forms carbonic acid, which neutralizes the base and lowers the pH, turning the indicator colorless.)
• How does blowing on the stain make it fade faster? (Exhaled breath contains carbon dioxide, which accelerates the neutralization reaction.)
• What happens if you add too much sodium hydroxide? (The ink will take much longer to fade because the solution remains strongly basic.)