Used with cresol red and methyl red for urine pH analysis, 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.
Demonstrating Photosynthesis w/ Bromothymol Blue Experiment - Scientific Method
In this activity 'Demonstrating Photosynthesis with Bromothymol Blue' students use Bromo blue to photosynthesis occurring. Students use the indicator to show the presence of Carbon Dioxide and the lack of its presence after it was used by a sprig of elodea.