How can i make invisible ink?

I don’t know that you can make it, but to get you on the right track, I believe you are actually asking about disappearing ink.

Maybe this is the sort of thing you’re looking for.

I agree with EmpressPixie—you don’t want invisible ink (made from lemon juice usually), you want disappearing ink . . .

How to Make Disappearing Ink (from about.com chemistry)

“Disappearing ink is a water-based acid-base indicator (pH indicator) that changes from a colored to a colorless solution upon exposure to air. The most common pH indicators for the ink are thymolphthalein (blue) or phenolphthalein (red or pink).”

The last part of the article gives this recipe:

“Here’s how to make your own disappearing ink:
1. Dissolve the thymolphthalein (or phenolphthalein) in the ethyl alcohol.
2. Stir in 90 ml of water (will produce a milky solution).
3. Add sodium hydroxide solution dropwise until the solution turns a dark blue or red (might take slightly more or less than the number of drops stated in the Materials section).
4. Test the ink by applying it to fabric (cotton tee-shirt material or a table cloth works well). Paper allows less interaction with air, so the color change reaction takes more time.
5. In a few seconds, the ‘stain’ will disappear. The pH of the ink solution is 10–11, but after exposure to air will drop to 5–6. The damp spot will eventually dry. A white residue may be visible on dark fabrics. The residue will rinse out in the wash.
6. If you brush over the spot with a cotton ball that has been dampened in ammonia the color will return. Similarly, the color will vanish more quickly if you apply a cotton ball dampened with vinegar or if you blow on the spot to improve air circulation.
7. Leftover ink may be stored in a sealed container. All of the materials may be safely poured down the drain.

“Disappearing Ink Safety
* Never spray disappearing ink into a person’s face. Particularly avoid getting the solution in the eyes.
* Preparing/handling the sodium hydroxide (lye) solution requires adult supervision, as the base is caustic. In case of skin contact, immediately rinse well with water. ”

Isn’t it interesting, fellow Flutherites, how much one word changed the meaning of this question— “invisible” rather than “disappearing”? Puts me in mind of all the times I’ve been misunderstood or misinterpreted because I meant one thing but said something else. Not meaning to sound preachy and certainly not criticizing @lexipoorocks. it’s just that as a writer, I’m fascinated by this.

On paper try pee lol I saw it in a documentary on prison life.

@gasman Thanks for posting that! I didn’t know one could make their own disappearing ink. My kids would love to try it out!

@adri027 My kids and I tried out different things to use as invisible ink, and they really wanted to use pee. They were disappointed when I said no. Yuck!