Disappearing ink of thymolphthalein

Blue inks disappear within a few minutes.

Scientific name: Alkaline acid-base indicator (thymolphthalein) solution changes its color when pH decreases.


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Wear eye protection goggles and gloves.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

2NaOH + CO2 → Na2CO3 + H2O

C28H30O4(blue) –(pH change)→ C28H30O4(colorless)

Step-by-step instruction

  1. In order to prepare invisible inks you need the following reagents: thymolphthalein, ethanol, 3 molar sodium hydroxide solution.
  2. Dissolve 0.1 gram of thymolphthalein in 10 ml of ethanol.
  3. Add 90 ml of water and stir the solution. It will be milky-white.
  4. Using a pipette pour about 1 ml of sodium hydroxide solution to the thymolphthalein solution.
  5. The obtained mixture is an invisible ink. If it gets on your clothes or table it will disappear within a few minutes.

Scientific background

Thymolphthalein is an acid-base indicator. It changes color depending on solution pH. In neutral and acidic solutions it is colorless. In alkaline solution it turns dark-blue. pH of invisible inks is alkaline because of sodium hydroxide (NaOH) presence and it is the reason of the dark-blue color of the mixture. When the ink comes in contact with air, sodium hydroxide interacts with carbon dioxide:

2NaOH + CO2 → Na2CO3 + H2O

This reaction leads to pH change. The acidity of the solution increases and thymolphtaleine becomes colorless.

C28H30O4(blue) –(pH change)→ C28H30O4(colorless)


Published on 23 March 2015

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