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Bromothymol blue is a pH indicator for weak acids and bases. In neutral solution, it appears bluish-green. At pH above 7.6 it appears blue, and at pH below 6.0 it appears yellow.
Bromothymol blue (BTB) is an anionic dye. It can be protonated or deprotonated to generate yellow or blue. BTB helps to make ion-pair complex, that is used to identify several pharmaceutical compounds with the help of extractive spectrophotometric methods.

Pale purple crystalline powder

As indicator, pH 6.0 yellow; pH 7.6 blue.

Bromothymol blue is a chemical indicator for weak acids and bases.

Bromothymol blue has various applications in biology and medicine. It is suitable for demonstrating fungal hyphae within plant roots. It is used as a vital stain to trace the movement of fluids from the lymph. It is added to an agar gel microbial culture medium for enumeration of Bacillus cereus. Bromothymol blue is also used as a pH indicator due to its precise color transition properties at neutrality.
Suitable for use:
In preparations of pH indicator solutions.
In phospholipid staining on tissues and cells prepared on microscopy slides.

ChEBI: A member of the class of 2,1-benzoxathioles that is 2,1-benzoxathiole 1,1-dioxide in which both of the hydrogens at position 3 have been substituted by 3-bromo-4-hydroxy-5-isopropyl-2-methylphenyl groups.

Bromothymol blue is synthesized by adding elemental bromine to the thymol blue in a solution of glacial acetic acid.
To prepare a solution that is used as a pH indicator, we should dissolve 0.10 g in an 8.0 cm3 N/50 NaOH and then dilute it with water to 250 cm3. To prepare a solution used as an indicator in volumetric work, we should dissolve 0.1 g in the 100 cm3 of 50% (v/v) ethanol.

Dissolved the dye in aqueous 5% NaHCO3 solution and precipitate it from the hot solution by dropwise addition of aqueous HCl. Repeat this until the extinction at max 420nm does not increase. It is an indicator: aqueous solutions are yellow at pH 6.0, and blue at pH 7.6. [Beilstein 19/3 V 461.]