Complexation of Cobalt by Natural Ligands in Freshwater

Ligand exchange with dimethylglyoxime (DMG) and dif ferential pulse cathodic stripping voltammetry (DPCSV) was applied to determine free Co(II) ionic concentrations [Co 2+ ] and Co(II) complexation by natural organic ligands in freshwater. [Co 2+ ] and the complexing parameters were calculated on the basis of the thermodynamic equilibria between added and natural ligands. The optimal working conditions were carefully evaluated in synthetic solutions and in natural freshwater samples. The results from samples collected at various sites from the Aare and Rhine Rivers and the Biel and Lucerne Lakes, Switzerland, are presented. The total dissolved Co concentrations are in the range of 0.5−6.5 nM, and [Co 2+ ] is in the range of 0.05−0.5 nM. Organic complexes of Co are predominant in most samples with 80−96% of total dissolved Co. A one-ligand model could well fit the titrations of freshwater samples with Co(II) (using the FITEQL program). The conditional stability constants of the natural ligands were computed to be log K = 9.5−11.6 (pH 8.0 ± 0.1), and the ligand concentrations were [L] = 1.4−7.6 nM. The reliability and applicability of the technique and the implications of the results are discussed.