using chemical reactions, we can design many kinds of invisible ink from scratch, write with a colorless or obscure solution, and then use another substance to change color. In fact, there is another opposite kind of "vanishing ink". It looks very dark just sprinkled on it, but the color will gradually disappear after a while.

recently I have seen a fairly common prank toy like this. At first glance, it looks like dark blue ink, but if you drop it on paper or sprinkle it on cloth, the color will gradually fade until it disappears.

(for the effect of the disappearance of the ink, refer to the mark in the lower right corner. Tecno40)

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how is this kind of "ink" made? Checked the data and found that the principle is actually quite simple, mainly using the discoloration of acid-base indicators.

in blue vanishing ink, the color component is usually thymol phthalein, an acid-base indicator that shows blue when the pH is high enough.

(the discoloration range of thymol phthalein)

(the discoloration range of thymol phthalein)

(the discoloration range of thymol phthalein)

in the solution of disappearing ink, some sodium hydroxide is added to adjust the Thyrophthalein to 10-11, and thymol phthalein will show dark blue like ink. When the ink is applied to the paper or the surface of the clothing, the alkali in the ink begins to react with the carbon dioxide in the surrounding environment and gradually turns into carbonate. In this way, the alkalinity of the solution becomes weaker, crossing the discoloration point of thymol phthalein, and the blue gradually disappears and becomes colorless during placement.

by the way, there is also a discolored solid glue on the market, which also changes from blue to colorless after application, and it is said to use the same principle as vanishing ink. It is also a very convenient design, you can see the place where the glue has been applied, and it does not affect the appearance after drying.

PS: if combined with phenolphthalein, it can also make a two-stage discoloration effect: purple "ink" first turns pink, and then becomes colorless (the pH required for phenolphthalein discoloration is a little lower than thymol, about 8.2).

reference to this article: http://www.chymist.com/Disappearing%20Ink.pdf