TOOS (TOOS sodium salt) is a highly water-soluble aniline derivative widely used in diagnostics and biological experiments. TOOS can be combined with 3-methyl-2-benzothiazolinone hydrazone hydrochloride (MBTH) to form a chromogenic system to measure oxidase activity. In the MBTH-TOOS chromogenic system, MBTH is catalytically oxidized to produce (-NH) free radicals, which react with TOOS to form colorless compounds. Furthermore, the colorless compound undergoes a disproportionation reaction to produce a blue-violet quinoid compound.