Empirical Formula (Hill Notation): C27H28Br2O5S, Molecular Weight: 624.38, ACS reagent, Dye content 95 %, Synonym: 3′,3″-Dibromothymolsulfonphthalein. Bromothymol Blue (BTB) is a pH indicator (pKa approx. 7.1) that is used as a biological slide vital stain to define cell walls or nuclei under the microscope. BTB is also a component of several bacterial growth and detection substrates such as cystine lactose electrolyte deficient (CLED/BROLACIN); polymyxin pyruvate egg yolk mannitol bromothymol blue (PEMBA) and MacConkey agar.