Experiment

1. Add 1 gram of thymolphthalein into 100 mL of ethyl alcohol. The solution will require stirring to dissolve all of the powder.
2. Add 900 mL of water to the solution and stir. Don't worry if the solution looks white – that's because the thymolphthalein indicator is not soluble in water. No worries, the next step will fix everything.
3. Slowly add 10 mL of 3 molar sodium hydroxide to the solution to turn the liquid a dark blue.

Caution: Sodium hydroxide (commonly known as lye) is a caustic solution and must be handled by an adult. Once the sodium hydroxide is diluted with water, the solution is safe to use as disappearing ink, but care must be taken because the pH of the ink solution is about 10 (basic). Be sure to wash your hands well with water after handling the more concentrated solution.

Always test the disappearing ink on a small piece of white fabric to make sure that it actually disappears. In a few seconds, the ink stain will disappear. The color will vanish more quickly if you apply a cotton ball dampened with vinegar or if you blow on the spot (the carbon dioxide in your breath is the secret!). The pH of the ink solution is 10-11, but after exposure to air the pH will drop to 5-6. The damp spot will eventually dry. A white residue may be visible on some dark fabrics. Be sure to store the disappearing ink solution in a sealed container. All of the materials may be safely poured down the drain.

How Does It Work?

The secret to making the ink disappear is carbon dioxide in the air which reacts with the water in the solution to form carbonic acid. The carbonic acid then reacts with the sodium hydroxide in a neutralization reaction to form sodium carbonate. This lowers the pH of the solution with the alcohol acting as an acid to turn the indicator colorless and the ink stain magically disappears. The “fading time” can be prolonged by adding a small amount (use drops to make these adjustments) of sodium hydroxide. But care should be taken not to add too much sodium hydroxide. Here's a fun fact… red disappearing ink can be made using phenolphthalein (a very common acid-base indicator) in place of thymolphthalein.

In Steve's version of the demonstration, he used a carbon dioxide fire extinguisher to make the stain vanish very quickly. This should not be done by anyone who has not been properly trained in how to select the right fire extinguisher and how to use it for this purpose. Thanks to Lee Marek for inspiring thousands of science teachers to use a carbon dioxide fire extinguisher with the disappearing ink demonstration.

Additional Info

Read Steve's blog to find out what really happened behind the scenes on television. Thanks to David Katz for publishing much of what we know about disappearing ink and hundreds of other science-related toys.