Blue inks disappear within a few minutes.
name: Alkaline acid-base indicator (thymolphthalein) solution changes its color when pH decreases.
Thymolphthalein is an acid-base indicator. It changes color depending on solution pH. In neutral and acidic solutions it is colorless. In alkaline solution it turns dark-blue. pH of invisible inks is alkaline because of sodium hydroxide (NaOH) presence and it is the reason of the dark-blue color of the mixture. When the ink comes in contact with air, sodium hydroxide interacts with carbon dioxide:
2NaOH + CO2 → Na2CO3 + H2O
This reaction leads to pH change. The acidity of the solution increases and thymolphtaleine becomes colorless.
C28H30O4(blue) –(pH change)→ C28H30O4(colorless)