2,6-Dibromo-4-(chloroimino)cyclohexa-2,5-dienone (NSC 528) is a colorimetric dye used for the detection of phenolic compounds. 2,6-Dibromo-4-(chloroimino)cyclohexa-2,5-dienone reacts with phenolic compounds at a pH of about 9.4 to produce an indigo dye. 2,6-Dibromo-4-(chloroimino)cyclohexa-2,5-dienone has been used as a Gibbs reagent for the colorimetric detection of phenolic compounds. 2,6-Dibromo-4-(chloroimino)cyclohexa-2,5-dienone has also been used in a colorimetric reaction for the quantitative colorimetric detection of aflatoxins, turning the colorless aflatoxins green with an absorption band at 673 nm.