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 38000 Mcm at 595 nm.