The deprotonation of the neutral form results in a highly conjugated structure, accounting for the difference in color.

[2] The protonated form of bromothymol blue has its peak absorption at 427 nm thus transmitting yellow light in acidic solutions, while the deprotonated form has its peak absorption at 602 nm thus transmitting blue light in more basic solutions.

It is less soluble in nonpolar solvents such as benzene, toluene, and xylene, and practically insoluble in petroleum ether.

[5] Bromothymol blue may be used for observing photosynthetic activities, or as a respiratory indicator (turns yellow as CO2 is added).

The specimen is mixed with blue BTB solution and fixed to a slide by a cover slip.

As vaginal pH normally is acidic, the blue color indicates the presence of amniotic fluid.