From About:Chemistry
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.
To find out how disappearing ink works, what you need to make it, and how to do it, follow this link —
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. Neutralizaton 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
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.]
Here’s how to make your own disappearing ink:
Dissolve the thymolphthalein (or phenolphthalein) in the ethyl alcohol.
Stir in 90 ml of water (will produce a milky solution).
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).
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.
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.
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.
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.