Bromothymol blue (also known as bromothymol sulfone phthalein and BTB) is a pH indicator. It is mostly used in applications that require measuring substances that would have a relatively neutral pH (near 7). A common use is for measuring the presence of carbonic acid in a liquid. It is typically sold in solid form as the sodium salt of the acid indicator.

Function:
Bromothymol blue acts as a weak acid in solution. It can thus be in protonated or deprotonated form, appearing yellow or blue, respectively. It is bright aquamarine by itself, and greenish-blue in a neutral solution.
The deprotonation of the neutral form results in a highly conjugated structure, accounting for the difference in color. An intermediate of the deprotonation mechanism is responsible for the greenish color in neutral solution.
The protonated form of bromothymol blue has its peak absorption at 427 nm thus transmitting yellow light in acidic solutions, and the deprotonated form has its peak absorption at 602 nm thus transmitting blue light in more basic solutions.[3] Highly acidic Bromothymol blue is magenta in color.
The general carbon skeleton of bromothymol blue is common to many indicators including chlorophenol red, thymol blue, and bromocresol green.
The presence of one moderate electron withdrawing group (bromine atom) and two moderate donating groups (alkyl substituents) are responsible for bromothymol blue's active indication range from a pH of 6.0 to 7.6. While the conjugation is responsible for the length and nature of the color change range, these substituent groups are ultimately responsible for the indicator's active range.