Thymol blue (thymolsulfonephthalein) is a brownish-green or reddish-brown crystalline powder that is used as a pH indicator.
It is insoluble in water but soluble in alcohol and dilute alkali solutions.
At wavelength (378 - 382) nm, extinction coefficient > 8000 and at wavelength (298 - 302) nm, the extinction coefficient > 12000.
[3] Thymol blue has different structures at different pH.
[4] Harmful if swallowed, Acute Toxicity.