How to Make Disappearing Ink

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Disappearing Ink Chemistry

Disappearing ink makes a stain when wet, yet vanishes when the ink dries.
Disappearing ink makes a stain when wet, yet vanishes when the ink dries. Southern Stock, Getty Images

Disappearing ink is a water-based acid-base indicator (pH indicator) that changes from a colored to a colorless solution upon exposure to air. The most common pH indicators for the ink are thymolphthalein (blue) or phenolphthalein (red or pink). The indicators are mixed into a basic solution that becomes more acidic upon exposure to air, causing the color change. Note that in addition to disappearing ink, you could use different indicators to make color-change inks, too.

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How Disappearing Ink Works

Several color-change chemistry demonstrations use the same principle as disappearing ink.
Several color-change chemistry demonstrations use the same principle as disappearing ink. Arne Pastoor, Getty Images

When the ink is sprayed onto a porous material the water in the ink reacts with carbon dioxide in the air to form carbonic acid. The carbonic acid then reacts with the sodium hydroxide in a neutralization reaction to form sodium carbonate. Neutralization of the base causes a color change of the indicator and the stain disappears:

Carbon dioxide in the air reacts with water to form carbonic acid:

CO2 + H2O → H2CO3

The neutralization reaction is sodium hydroxide + carbonic acid -> sodium carbonate + water:

2 Na(OH) + H2CO3 → Na2CO3 + 2 H2O

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Disappearing Ink Materials

This is the chemical structure of phenolphthalein.
This is the chemical structure of phenolphthalein. Ben Mills/PD

Here's what you need in order to make your own blue or red disappearing ink:

  • 0.10 g thymolphthalein for blue ink or phenolphthalein for red ink (1/3 of 1/8 tsp)
  • 10 ml (2 tsp) ethyl alcohol (ethanol) [can substitute 14 ml or 3 tsp of ethyl rubbing alcohol]
  • 90 ml water
  • 20 drops of 3M sodium hydroxide solution or 10 drops 6M sodium hydroxide solution [make a 3 M sodium hydroxide solution by dissolving 12 g of sodium hydroxide NaOH (1 level tablespoon of lye) in 100 ml (1/2 cup) of water.]
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Make Disappearing Ink

This is the chemical structure of thymolphthalein.
This is the chemical structure of thymolphthalein. Ben Mills/PD

Here's how to make your own disappearing ink:

  1. Dissolve the thymolphthalein (or phenolphthalein) in the ethyl alcohol.
  2. Stir in 90 ml of water (will produce a milky solution).
  3. Add sodium hydroxide solution dropwise until the solution turns a dark blue or red (might take slightly more or less than the number of drops stated in the Materials section).
  4. Test the ink by applying it to fabric (cotton tee-shirt material or a table cloth works well). Paper allows less interaction with air, so the color change reaction takes more time.
  5. In a few seconds, the 'stain' will disappear. The pH of the ink solution is 10-11, but after exposure to air will drop to 5-6. The damp spot will eventually dry. A white residue may be visible on dark fabrics. The residue will rinse out in the wash.
  6. If you brush over the spot with a cotton ball that has been dampened in ammonia the color will return. Similarly, the color will vanish more quickly if you apply a cotton ball dampened with vinegar or if you blow on the spot to improve air circulation.
  7. Leftover ink may be stored in a sealed container. All of the materials may be safely poured down the drain.

Disappearing Ink Safety

  • Never spray disappearing ink into a person's face. Particularly avoid getting the solution in the eyes.
  • Preparing/handling the sodium hydroxide (lye) solution requires adult supervision, as the base is caustic. In case of skin contact, immediately rinse well with water.