Chemical bromination of phenol red by hydrogen peroxide is possible in the absence of haloperoxidases

The dye phenol red (phenolsulphonphthalein) was chemically brominated to bromophenol blue (3′,3″,5′,5″-tetrabromophenolsulphonphthalein) directly by hydrogen peroxide (20 mM or higher) under acidic pH (0.5–4.5) and moderate temperature (30°C). Since this bromination reaction takes place in the absence of any chemical or biological catalyst, the results represent an important warning concerning a commonly used assay for detecting or screening haloperoxidases. Bromophenol red (5′,5″-dibromophenolsulphonphthalein) and another unidentified compound were observed as temporary intermediates. Incubation of the reaction mixture with KCl instead of NaBr did not yield any chlorinated products under the same conditions.