Disappearing Ink
It's a joke shop classic... Disappearing ink. There's just something funny
about squirting dark blue "ink" on someone's shirt only to have it
disappear like magic. Best of all, there's actually some great acid-base
chemistry behind all of the prankster fun.
Watch the Disappearing Ink Video
Materials
It's probably cheaper to buy 99¢ bottles of disappearing ink from the toy
store than to purchase the chemicals required to make the solution.
If you do decide to make your own ink, you'll need:
•thymolphthalein (a common acid-base indicator in the form of a powder)
•ethyl alcohol
•3 molar sodium hydroxide
•water
•safety glasses.
Safety first... Never spray disappearing ink into anyone's face.
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 will drop to a pH of 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. By the way, did
you know that red disappearing ink can be made using phenolphthalein (a
very common acid-base indicator) in place of thymolphthalein.
Experiment From Steve Spangler Science
Another Disappearing Ink Recipe
Materials:
• Ammonia
• Phenolpthalein
• Beaker
• Eye dropper
• White lab coat on your partner
Procedure:
1. Mix a pinch of phenolphthalein with ammonia in a beaker. The solution should turn
pink.
2. Using the dropper, drip some pink solution onto the back of a white lab coat while
showing the
audience.
3. Have your partner turn to face the audience and wait about 30 seconds.
4. Turn your partner with his back to the audience again to show how the ink has
disappeared.
Explanation:
Phenolpthalein is an indicator, which turns pink in the presence of a base. Ammonia is
a base. Cotton, however, is acidic, and will cause the phenolphthalein to turn clear
again.
Questions:
What could this be used for?
Materials:
• Ammonia
• Phenolpthalein
• Beaker
• Eye dropper
• White lab coat on your partner
Procedure:
1. Mix a pinch of phenolphthalein with ammonia in a beaker. The solution should turn
pink.
2. Using the dropper, drip some pink solution onto the back of a white lab coat while
showing the
audience.
3. Have your partner turn to face the audience and wait about 30 seconds.
4. Turn your partner with his back to the audience again to show how the ink has
disappeared.
Explanation:
Phenolpthalein is an indicator, which turns pink in the presence of a base. Ammonia is
a base. Cotton, however, is acidic, and will cause the phenolphthalein to turn clear
again.
Questions:
What could this be used for?
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