Invisible Ink
Objective
To write and visualize words with invisible ink.
Application
Steganography, espionage, anticounterfeiting, property marking, hand stamping for readmission, etc.
Difficulty
Procedure: Easy
Concept: Medium
Concept
Invisible Ink is a substance used for writing, which is either invisible on application, or sooner thereafter, and which later on can be made visible by some means. Once the writing is dried, the paper, or writing surface should appear blank and of similar texture as surrounding material.
There are few types of invisible inks. Different types of Chemicals can be applied and visualzied under heating conditions, ultraviolet(UV)light, fluorecence, and the addition of other solutions that undergoes acid-base reaction by dropping or spraying, fuming of vapours, or other means. One well-known example will be the blueprint. Based on such principles, toy invisible ink pens with two tips, one for invisible ink writing, and another tip for developing the ink, or a decoder tip, was developed and sold on market.
Even though the problem seems to be difficult, the invisible ink can be easily made at home by applying ammonia fumes onto paper written with phenolphthalein. Phenolphthalein is a chemical compound that usually used in titration. It turns colorless in acidic solutions and pink, or even purple in basic solutions depending on the indicator concentration. Ammonia is normally encountered as a gas and very easy to evaporate. Since ammonia acts as a base or a proton acceptor, even though it is colorless in its liquid form, when interacted with phenolphthalein, the solution turns pink. Therefore, by spraying ammonia fumes onto phenolphthalein, the invisible writing appears onto the paper. However, as ammonia fumes evaporates, the invisible writing disappears again.
Materials
- Solid iodine Crystal
- Phenolphhalein solution
- Cotton swab (brushes)
- White Paper (paper towel)
- Beaker
- Ammonia (spray with ammonia)
- Acetic acid (vinegar)
- The sheet of paper may be used repeatedly for writing multiple messages after the firs message disappears.
- All the material used in this experiment are very easy to be acquired. Solutions of acetic acid and ammonia can be stored in suitable containers that prevent evaporation.
Procedure
- Cut out one piece of paper, dip the cotton ball or brush into the phenolphthalein solution and write a message or draw a picture onto the piece of paper. Dry the paper in air.
- Spray the paper towel with Windex that contains ammonia and observe the word appears on the paper then disappears.
- Repeat the same process with aetic acid, and nothing should appear.
Conclusion
Does the writing successfully appear and disappear onto the paper after sprayed with ammonia and acetic acid? If not, does spraying with more solutions help the words to appear? How long does it take the word to appear onto the paper.
Extension
Many compounds can also be applied to produce invisible ink. For example, organic substances, including cola drink, honey, lemon, milk, onion juice, soap water, sugar can all be oxidized to brown color when heated.
