Wiktionary
thymolphthalein
n. A pH indicator that can also be used to make invisible ink.
Wikipedia
Thymolphthalein
Thymolphthalein is an acid– base ( pH) indicator. Its transition range is around pH 9.3–10.5. Below this pH, it is colorless; above, it is blue. Thymolphthalein is used as a visual indicator for the pH of swimming pools, changing the water blue when the pH increases, it can be used to detect the presence of urine being introduced into the normally neutral pH of the water. The molar extinction coefficient for the blue thymolphthalein dianion is 38,000 M cm at 595 nm.
Usage examples of "thymolphthalein".
But you can do the same thing with thymolphthalein indicator and sodium hydroxide.