Bromothymol blue wani sinadari ne da aka saba amfani dashi azaman alamar pH. Yana bayyana azaman foda mai duhu shuɗi ko azaman koren lu'ulu'u lokacin da aka tsarkake. Lokacin da aka narkar da cikin ruwa ko wasu hanyoyin ruwa, yana nuna canjin launi dangane da pH na maganin. A ƙananan matakan pH (yanayin acid), bromothymol blue ya bayyana rawaya, yayin da a mafi girma matakan pH (yanayin alkaline), ya juya blue. Wannan canjin launi yana sa ya zama mai amfani a aikace-aikace daban-daban, kamar a cikin gwaje-gwajen sunadarai, binciken likitanci, da gwajin muhalli.