Abstract：A new chromogenic reagent 2-(5-nitro-4-methyl-2-pyridylazo)-5-dimethylaniline (5-NO2-4-Me-PADMA)was synthesized and its color reaction with cobalt was also investigated.It showed that, in HAc-NaAc buffer solution at pH 4.0-7.0, Co reacted with 5-NO2-4-Me-PADMA to form a violet-red complex, whose maximal absorption peak appeared at 568 nm.Cobalt complex kept very stable after formation. When it was acidified with inorganic acid, it was converted into greenish blue double-proton body with high absorption characteristics under the effect of protonation. Its absorption peaks shifted to 620 nm, and its sensitivity was tripled. In the medium of 1.8 mol/L H2SO4, the concentration of Co obeyed Beer'|s law in the range of 0-1.0 μg/mL. The apparent molar absorptivity was 6.02×104 L·mol-1·cm-1. The proposed method was applied to the determination of micro cobalt in ores, whose results were consistent with flame atomic absorption spectrometry. The relative standard deviation (n=6) was less than 2 %.
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