A new reagent, 5-(2'-amino-4'-hydroxyphenylazo)-2,3-dihydro-1,4-phthalazinedione (AHPP), was synthesized by diazotization coupling of luminol with m-aminophenol in weak acidic media (pH 5-7). The structure of AHPP was confirmed with IR, elemental anal. and other phys. methods. AHPP and its Co-complex exhibit max. absorption at 456 and 542 nm, resp. in pH 7.7~9.0 buffered solution (Na2B4O7-HCl). Various parameters such as optimum acidity, amount of AHPP, and effects of surfactants and diverse ions, for cobalt photometric determination were studied. In the complex, the ratio of Co to AHPP is 1:2 using both Job's continuous variation and molar ratio methods. The proposed method was applied to the determination of cobalt is stainless steel and vitamin B12 with satisfactory results.

Key words: COLORIMETRIC METHOD, COBALT, HYDRAZIDE, BENZENE AZO COMPOUNDS, LUMINOL, O-BENZENEDICARBOXYLIC ACID P, FLUORESCENT REAGENT