Study on the color reaction of 5-(5-fluoro-2-pyridylazo)-2,4-diaminotoluene with cobalt and its application

A new method for the spectrophotometric determination of cobalt(Ⅱ) has been developed.In a pH 4.7～9.0 medium,5-(5-fluoro-2-pyridylazo)-2,4-diaminotoluene(5-F-PADAT) reacts with cobalt(Ⅱ) to form a violet-red complex exhibiting an maximum absorption at 506 nm.The complex can be changed into another species showing an maximum absorption at 565 nm with addition of mineral acid.This species possesses a higher absorption characteristic.The optimal concentrations of HClO4,H2SO4,HCl and H3PO4 are found to be 0.24～3.6 mol/L,0.16～3.84 mol/L,0.48～2.4 mol/L and 0.64～3.84 mol/L respectively.The apparent molar absorptivity is e565=9.1×104 L·mol-1·cm-1.The Beer′s law is obeyed in the range of 0～0.5 μg/mL.This method has the advantages of high sensitivity and selectivity,and has been successfully applied to the determination of cobalt in vitamin B12 with satisfactory results.