Disappearing ink is a classic joke store product based on acid-base chemistry. Typically, the prank involves a pen that you use to squirt blue ink onto white or light-colored cloth. The ink starts out deep blue, but disappears after a few seconds. Here’s how to make your own disappearing ink in blue, red, and purple and a look at the chemistry of how disappearing ink works.
To make disappearing ink, you need a pH indicator pigment, alcohol, water, and sodium hydroxide solution. Use thymolphthalein for blue ink, phenolphthalein for red/pink ink, or a mixture of the two powders to make purple ink.
- 0.10 g (a third of 1.8 tsp) thymolphthalein for blue ink or phenolphthalein for red ink
- 10 ml (2 tsp) ethanol [can substitute 14 ml or 3 tsp of rubbing alcohol]
- 90 ml water
- 20 drops of 3 M sodium hydroxide solution or 10 drops 6 M 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.]
Find ethanol or rubbing alcohol in the pharmacy section of a store. Thymolphthalein, phenophthalein, and sodium hydroxide are readily available as lab chemicals or online. If you’re doing this project with young children, you can substitute 0.1 M sodium hydroxide, which is 0.4 g NaOH in 100 ml of water.
Make Disappearing Ink
It’s easy to make homemade disappearing ink:
- Dissolve the thymolphthalein (or phenolphthalein or both) in the alcohol.
- Stir in 90 ml of water. This makes a milky solution.
- Add sodium hydroxide solution dropwise until the solution turns a dark blue, red, or purple. Use the number of drops needed for the color change, which might be a bit more or less than the amount listed in the “materials” section.
- Test the ink by applying it to fabric, like a white cotton t-shirt. You can test the ink using paper, but it’s less porous so there is less interaction with air and the color reaction takes more time.
- In a few seconds, the ink “stain” disappears. The pH of the original ink solution is 10-11, but air exposure drops the pH to 5 or 6 and decolorizes the dye. The damp spot dries colorless on light fabrics, but a white residue may be visible on dark fabrics. Any residue rinses out in the wash.
- Before you wash it away, you can reveal the invisible phase of disappearing ink. Simply 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. Blowing on the spot speeds the color change because your breath is enriched with carbon dioxide.
- Store leftover ink in a sealed container. All of the materials may be safely poured down the drain.
How Disappearing Ink Works
Disappearing ink is a simple acid-base indicator reaction. The indicator is colored at a high pH (alkaline) and colorless at a lower pH. Carbon dioxide in air reacts with the ink to form carbonic acid. Carbonic acid (an acid) reacts with sodium hydroxide (a base) in a neutralization reaction to make sodium carbonate (a salt) and water. Neutralizing the base changes the color of the indicator, so the ink “disappears.”
Here are the chemical reactions:
Carbon dioxide in the air reacts with water to form carbonic acid:
CO2 + H2O → H2CO3
Sodium hydroxide reacts with carbonic acid to make sodium carbonate and water:
2 Na(OH) + H2CO3 → Na2CO3 + 2 H2O
Fun Things to Try
Disappearing ink is used in pranks, but it has practical applications, too. Disappearing ink and invisible ink are used to write secret messages.
One fun application of disappearing was the color-change hair of Hollywood Hair Barbie and her Magic Hair Mist. You can color any blonde synthetic hair on a doll or wig to get this effect.
- For pink hair, treat the hair with a 1% solution of phenophthalein in alcohol.
- For blue hair, treat the hair with a 1% solution of thymolphthalein in alcohol.
- For purple hair, mix equal amount of phenolphthalein and thymolphthalein solution in alcohol.
The hair dries to its normal color. Reveal the color by spraying it with homemade Magic Hair Mist. Prepare this by mixing 10 ml alcohol, 90 ml water, and about 10 drops of 3 M sodium hydroxide solution. Don’t use the solutions on your own natural hair.
You can use leftover phenolphthalein for a pink color change chemistry demonstration.
Disappearing Ink Safety
The dyes used in disappearing ink are non-toxic, but the sodium hydroxide (lye) solution is caustic. So, making disappearing ink requires adult supervision. Never spray disappearing ink into someone’s eyes. Rinse ink off skin with water. It’s safe to wash exposed clothing and rinse leftover materials down the drain.
- MacRakis, Kristie; Bell, Elizabeth K.; Perry, Dale L.; Sweeder, Ryan D. (2012). “Invisible Ink Revealed: Concept, Context, and Chemical Principles of “Cold War” Writing.” Journal of Chemical Education. 89 (4): 529–532. doi:10.1021/ed2003252
- Nagy, John A. (2009). Invisible Ink: Spycraft of the American Revolution. Westholme Publishing. ISBN 978-1594161414.