COMPLEXATION OF COBALT BY NATURAL LIGANDS IN FRESH-WATER

Ligand exchange with dimethylglyoxime (DMG) and differential pulse cat hodic stripping voltammetry (DPCSV) was applied to determine free Co(l l) ionic concentrations [Co2+] acid Co(ll) complexation by natural org anic ligands in freshwater. [Co2+] and the complexing parameters were calculated on the basis of the thermodynamic equilibria between added and natural ligands. The optimal working conditions were carefully eva luated in synthetic solutions and in natural freshwater samples. The r esults from samples collected at various sites from the Aare and Rhine Rivers and the Biel and Lucerne Lakes, Switzerland, are presented. Th e total dissolved Co concentrations are in the range of 0.5-6.5 nM, an d [Co2+] is in the range of 0.05-0.5 nM. Organic complexes of Co are p redominant in most samples with 80-96% of total dissolved Co. A one-li gand model could well fit the titrations of freshwater samples with Co (II) (using the FITEQL program). The conditional stability constants o f 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 reliab ility and applicability of the technique and the implications of the r esults are discussed.