Polarographic behaviour of cobalt(II) ion in the presence of H4-1,3-propanediamine-N,N'-diacetic-N,N'-di-3-propionic acid

Abstract

The complex formation between cobalt(II) ion and the title ligand (abbreviated as H4-1,3-pddadp) has been studied by DC polarographic measurements in 0.1 M KNO3 ionic medium at 298 K, using a dropping mercury electrode. The experimental results obtained in the concentration range [Co2+] = 5.0×10-4 mol/dm3, 1.0 ×10-4 ≤ [H4-1,3-pddadp] ≤ 5.0 × 10-3 mol/dm3 and pH = 5.0 indicate that all the polarographic waves are irreversible with the electron transfer coefficient, αc ranging between 0.56 and 0.70. The half-wave potential of cobalt(II) ion reduction is found at -1.45 (vs. SCE). Upon increasing the concentration of the ligand (pH = 5.0, [Co2+] = 5.0 × 10-4 M) the limiting diffusion current of cobalt(II) decreases, thus indicating the complex formation. The composition of the complex, found by amperometric titration, was Co(H2L) (L = 1,3-pddadp4-). The overall concentration stability constant of this complex, where H2L2- is regarded as component, was found to be: log βc = 0.67 ± 0.04. The possible structure of the complex is discussed.