Thymol blue 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. It transitions from red to yellow at pH 1.2–2.8 and from yellow to blue at pH 8.0–9.6. It is usually a component of Universal indicator.

Formula: C27H30O5S