Bromophenol blue is 3H-2,1-Benzoxathiole 1,1-dioxide in which both of the hydrogens at position 3 have been substituted by 3,5-dibromo-4-hydroxyphenyl groups. It is used as a laboratory indicator, changing from yellow below pH 3 to purple at pH 4.6, and as a size marker for monitoring the progress of agarose gel and polyacrylamide gel electrophoresis. It has also been used as an industrial dye. It has a role as a dye, an acid-base indicator and a two-colour indicator. It is a member of phenols, an organobromine compound, a 2,1-benzoxathiole, a sultone and an arenesulfonate ester.