Cassandra Krack

Disappearing Ink

Science Concept: This experiment demonstrates an example of neutralization. The indicator (base) that is blue in color will turn colorless when it reacts with the carbon dioxide (acid) in the air.

Materials:

• 125 mL Erlenmeyer flask
• rubber stopper
• 50 mL ethyl alcohol
• less than 1 mL of thymolphthalein indictor
• less than 1 mL of 0.1 M sodium hydroxide
• dropper
• white cotton shirt

Directions:

• Pour 50 mL of the ethyl alcohol in the 125 mL flask.
• Add 2-3 droppers full of thymolphthalein indicator to the ethyl alcohol.
• Add just enough sodium hydroxide (about 2 drops) so a dark blue color is created. Some adjustments may be needed on the amount of sodium hydroxide to add.
• Place the rubber stopper in the flask while the solution is being stored.
• Fill the dropper with some of the blue indicator solution.
• Squirt the solution onto the white cotton shirt and in a few seconds the blue "stain" will disappear. (The residue comes off in the wash. This does not seem to work well with paper).
• Alternative: Experiment with using this on paper as disappearing ink. You may have to blow your breath hard on it or brush over it with a cotton ball of vinegar. spray it with ammonia to make it comeback, etc.

Introduction: Oh no! I accidentally squirted some ink on my favorite shirt! Wait! Where did the stain go? (scratch head) Oh yeah, I know. (explanation).

Explanation: The sodium hydroxide causes the indicator of thymolphthalein solution to turn the dark blue color. When the solution is squirted onto the shirt, it reacts with the carbon dioxide in the air. The carbon dioxides reacts with the water to form an acid called carbonic acid. The carbonic acid then reacts with the sodium hydroxide in a neutralization reaction. This reaction forms sodium carbonate, which is a washing soda, and the stain disappears. The carbon dioxide is the acid that neutralizes the base, sodium hydroxide. The color of the thymolphthalein is colorless in the presence of acid.

Formation of carbonic acid from carbon dioxide in the air.

CO₂ + H₂O ---> H₂CO₃

Neutralization: sodium hydroxide + carbonic acid ---> sodium carbonate + water

2 Na(OH) + H₂CO₃ ---> Na₂CO₃ + 2 H₂O

Safety: The solution of sodium hydroxide is quite weak, but still take care not to get in the eyes.

Waste Disposal: Solution may be kept for later demonstrations, otherwise all materials may go down the drain.

Source: Bilash, Borislaw. A Demo A Day, Flinn Scientific, 1997, p. 184.