engleski

Karakterizacija kobalt(II)-4-nitrokatekol i kobalt(II)-humusna kiselina kompleksa u modelnoj otopini mora

Cobalt (II) as an essential element and important for biochemical processes in the marine environment, is bioavailable in its dissolved form. It is known that in seawater cobalt is partially, but strongly complexed to natural organic ligands. It has an important influence on the growth of coccolithophores, diatoms and cyanobacteria. Dissolved cobalt(II) is very important element for all marine organisms (biochemical and physiological functioning). Despite its abudance in the environment, dissolved Co2+ in the oceans is present at low concentrations, <10-9 mol dm-3, mostly in organic complexes. By its complexing with 4-nitrocatechol and humic acid, complex components of the natural organic matter present in seawater, cobalt(II) forms soluble complexes in aqueous solutions. Electrochemical characterization of cobalt (II) complexes with 4-nitrocatechol and humic acid in aqueous solution (0.55 mol dm-3 NaCl) using square wave and cyclic voltammetry, was performed. Cobalt(II) concentrations varied from 1 to 5 x 10-5 mol dm-3, 4-nitrocatechol form 1-5 x 10-5 and humic acid from 0.1 to 0.9 mg dm-3. First reduction peak of Co(II)-4-nitrocatechol was recorded at Ep -0.8 V while the second one at -1.1 V. At the same potential was registered reduction peak of Co(II)-humic acid complex (Figure 1. A and B). That suggests that nitro group is the binding functional group for cobalt with the humic acid ligand. (Figure 1 ; inset). Investigated cobalt(II)-complexes reduction were recorded in the pH range from 5 to 9.5. Reduction currents reached maximum at ≈ pH 6.5. Further investigations were performed at pH of the seawater. The reduction mechanism of both complexes was investigated by square- wave voltammetry with variation of parameters: frequency, amplitude and step increment. Based on the theory of the reduction current and potential dependencies on SW frequency, it was confirmed that the redox process of both Co(II)-complexes is irreversible. SW amplitude showed exponential increase characteristic for irreversible redox process with the reactant adsorption. CV voltammograms recorded irreversible cathodic peaks. Using NTA as competing ligand, a stoichiometry of the Co(II)-4-nitrocatechol complex (at ≈ -0.8 V) was presumed to be 1:1, 1:2 and 1:3. EDTA was used as competing ligand for titration with humic acid and a stoichiometry of the Co(II)- humic acid complex (at ≈ -1.1 V) was presumed to be 1:2. UV/VIS spectra were recorded under the same conditions experimental conditions. Titration data (changes in complete spectra) were processed by multivariate non-linear least- square fitting program. Obtained results show that 4-nitrocatechol and humic acid form soluble Co2+ complexes in the pH range from 5 to 9 that remain in the solution for a sufficient period of time to be available for phytoplankton.

cobalt ; complexation ; voltammetry ; 4-nitrocatechol ; humic acid

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