The present work describes a selective, rapid and economical method for the determination of cobalt(II) using 5-[o-carboxyphenylazo] 2,4-dihydroxybenzoic acid [CPDB] as a spectrophotometric reagent without heating or extraction. The CPDB reacts with Co(II) in presence of Tween 80 surfactant forming a pink complex of a stoichiometric ratio 1:2 absorbed maximally at 555 nm with stability constant amounting to 10.33. Beer's law is obeyed for a concentration of at least 3.50 μg ml with Sandell sensitivity of 3.6 ng cmand molar absorptivity of 1.64×10 l molcm. The relative standard deviation for six replicate sample of 2.0 μg ml is 1.11%. A selective procedure is proposed for cobalt determination in presence of different metal ions in synthetic mixtures and in pharmaceutical formulations with more accurate results.
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