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.

Application

Thymol blue is another weak acid that is used as an indicator. The fully protonated species (abbreviated H2Thy) is red, the monoprotonated species (HThy-) is yellow, and the fully deprotonated species (Thy2-) is blue.