Invisible ink
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Invisible ink, also called security ink or sympathetic ink, is a substance used for writing, which is invisible either on application or soon thereafter, and can later be made visible by simple or complex means.
Contents
General
An invisible ink consists of a substance or material that is colorless under standard conditions, which is dissolved in a solution or turned into a colloid when mixed with a (viscous) liquid. This is necessary to be applied on the writing surface. Once dry, the written surface looks as if it were blank, with a similar texture and reflectivity as the surrounding surface.
Types
Since most invisible inks are employed according to their method of development, they will be classified as such.
Inks developed by heat
Some of these are organic substances that oxidize when heated, which usually turns them brown. For this type of "heat fixed" ink, any acidic fluid will work.
- Bodily fluids such as blood serum.
- Cobalt(II) chloride, which turns blue when heated and becomes invisible again after a while (if not overly heated)
- Cola drink
- Honey solution, sugar (sugars turns into caramel by dehydration)
- Lemon, apple, orange or onion juice (organic acids and the paper forms esters under heat)
- Milk (lactose dehydrates)
- Soap solution (carboxylic acids partially oxidize)
- Wine, or vinegar
The writing is rendered visible by heating the paper, either on a radiator, by ironing it, using a hair dryer/heat gun, or by placing it in an oven.
Inks developed by chemical reaction
Substance changes color when mixed with an acid or base, redox indicators or some other chemical mean:
- Ammonia, developed by red cabbage extract.
- Cerium oxalate developed by manganese(II) sulfate and hydrogen peroxide.
- Cobalt(II) chloride, developed by potassium ferricyanide.
- Copper(II) sulfate, developed by sodium iodide, sodium carbonate, aq. ammonia or potassium ferricyanide.
- Iron(II) sulfate, developed by sodium carbonate or potassium ferricyanate.
- Iron(III) sulfate, developed by sodium sulfide.
- Lead(II) nitrate, developed by sodium iodide.
- Lemon juice, developed by iodine solution (ink turns white, paper turns light blue).
- Phenolphthalein, commonly used as a pH indicator, turns pink in the presence of a base such as ammonia fumes or sodium carbonate.
- Sodium chloride (common table salt), developed by silver nitrate, then exposed to light.
- Starch, developed by iodine solution which turns starch dark blue and the paper light blue.
- Vinegar, is revealed by red cabbage water. Vinegar contains acetic acid that affects the pH indicator in red cabbage extract. Vinegar alone may also be developed by heat.
Inks visible under ultraviolet light
Some inks glow faintly (fluorescence) when under an ultraviolet lamp. This is a property of many substances, particularly organic substances and body fluids. Other inks work in a near opposite way by absorbing ultraviolet light but without fluorescing. When these are used on fluorescent paper, the inked areas fluoresce less than the surrounding paper area when under an ultraviolet lamp. This is especially a property of inks with a yellow tint. Common examples include:
- Aromatic compounds, polycyclic aromatic compounds (naphthalene, anthracene, etc.)
- Body fluids, serum, saliva
- Laundry detergents containing optical brighteners
- Lemon juice
- Mold
- Soap
- Sunscreen
- Zinc oxide
Disappearing inks
Disappearing inks typically rely on the chemical reaction between thymolphthalein and a basic substance such as sodium hydroxide. Thymolphthalein, which is normally colorless, turns blue in solution with the base. As the base reacts with carbon dioxide (always present in the air), the pH drops below 10.5 and the color disappears.
Inks which alter the surface of paper
This includes virtually all invisible inks, but pure distilled water can also be used in this way. Application of any fluid will alter the paper surface fibers or sizing. Slightly dampening paper with a sponge or by steam and then drying it before writing a message will prevent writing from being developed by this method, but overdoing dampening will result in telltale paper cockling.
Projects
- Write secret messages
- Demonstration of pH indicators
