Electrochemical characterization of Cobalt(II)-Complexes involved in marine biogeochemical processes. I. Co(II)-4-nitrocatechol and Co(II)-Humate

Cobalt (II) as an important micronutrient for numerous biological processes in marine environment, has been tested in model aqueous solutions with naturally occurring ligands 4-nitrocatecol (4NC) and humic acid (HA) by voltammetry and spectrophotometry. Irreversible, two-electron reduction processes of Co(II)–4NC and Co(II)-HA complexes adsorbed on the mercury drop surface, depending on accumulation time and composition of the solution (cCo, c4NC, cHA and pH), were detected. Complexes Co:L = 1:1 stability constants formed only in the solution, were determined by spectrophotometry and amout to log KCo(II)4NC = 3.98 (pH = 8.2) and log KCo(II)4NC = 5.76 (pH = 6.5), and log KCo(II)HA = 3.80 (pH = 8.2). Co:L = 1:2 complexes Co(II)-(4NC)2 at pH 6.5 and 8.2 and Co(II)-(HA)2 at pH 8.2, formed at the mercury drop electrode surface as hydrophobic specia, were detected only by voltammetric measurements. Conditional stability constants were calculated by the CLE/ACSV method: log KCo(II) (4NC)2 = 21.86 (pH = 8.2), log KCo(II) (4NC)2 = 21.11 (pH = 6.5) and log KCo(II) (HA)2 = 11.32 (pH = 8.2).