UV-visible spectrophotometric, adsorptive stripping voltammetric and capillary electrophoretic study of 2-(5'-bromo-2'-pyridylazo)-5-diethylaminophenol and its chelates with selected metal ions: Application to the determination of Co(III) in vitamin B-12

The model ligand 1-(2-pyridylazo)-2-naphthol (PAN) has two pK(a) values of 2.5 and 11.2 corresponding to the pyridinium ion, and the phenolic group, respectively. The related chelating agent, 2-(5'-bromo-2'-pyridylazo)-5-diethylaminophenol (PADAP), has pK(a) values of 1.0, 3.0 and 11.2 corresponding to the 3-bromopyridinium ion, the N,N-diethylanilinium ion and the phenolic group, respectively. On chelation of PADAP with Cd(II), Cu(II), Ni(II), Zn(II) and Pb(II) a 1:1 stoichiometry is found in the intermediate pH range 4-9, indicative of square planar or tetrahedral complexes. Co(II) forms a particularly stable chelate with a 1:2 stoichiometry over the pH range 0-14 with the other chelates showing greater lability when investigated by UV-visible spectrophotometry. Adsorptive stripping voltammetry (AdSV) is compared to capillary zone electrophoresis (CZE) for the detection and determination of trace concentrations of metal ions (Co(II), Cu(II), Cd(II), Zn(II), Ni(II) and PL(II)) as their PADAP chelates. Limits of detection (LODs) for Cd(II), Zn(II), Pb(II) and Co(II) were 8.3, 4.1, 3.0 and 0.5 x 10(-8) mol dm(-3), respectively, using the AdSV method with Cu(II) and Ni(II) not giving reproducible cathodic signals as their respective chelates. CZE was performed using 1 x 10(-4) mol dm(-3) PADAP in the run buffer and gave higher LODs than AdSV but better selectivity. Comparison between the two techniques is made for the determination of Co(III) in vitamin B-12. The effect of the presence of vitamins from the A, B and C groups following destruction of the corrin ring system by UV digestion prior to chelation with PADAP was also investigated by CZE to reveal 100% signal recovery in all cases with 3% relative standard deviation following 5 consecutive 30 s hydrodynamic injections.