It is commonly used as a pH indicator (yellow below pH 6.0 blue above pH 7.6) in aquaria to monitor CO levels via color change in medical diagnostics for bacterial growth (e.g. differentiation media) and as a tracking dye in gel electrophoresis.

Its primary purpose is to detect pH changes near neutrality through protonationdeprotonation of phenolic groups creating distinct yellow (acidic) green (neutral) and blue (basic) colors due to extended conjugation differences.

Bromothymol blue (C27H28Br2O5S) is a synthetic sulfophthalein pH indicator dye appearing as a white to beige crystalline powder featuring a central benzoxathiole sultone core with two brominated thymol-derived phenolic rings molecular weight 624.38 gmol density 1.25 gcm and melting point 202C.