Invisible Ink

Mysterious 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.

BluePrint, an example of invisible inks
Invisible inks with UV-light
Invisible ink pen

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

Procedure

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.

Reference

  • http://en.wikipedia.org/wiki/Invisible_writing
  • http://www.52huaxue.com.cn/ArticleContent.asp?ID=1678
  • http://www.mcbworld.com/mcbworld/images/phpThumb.php?src=photos/uvpen1.jpg
  • Pictures used in the experiment are all from http://tieba.baidu.com/f?kz=287557108