Determination of organic complexation of cobalt in seawater by cathodic stripping voltammetry

A method was developed to determine the extent of cobalt complexation with organic ligands in seawater, and applied to samples from the North Eastern Atlantic Ocean. The cobalt speciation was determined using catalytic cathodic stripping voltammetry with ligand competition against the adsorptive lig...

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Bibliographic Details
Published in:Marine chemistry 2001-07, Vol.75 (1), p.33-47
Main Authors: Ellwood, Michael J, van den Berg, Constant M.G
Format: Article
Language:English
Subjects:
Cathodic stripping voltammetry
Cobalt
Earth sciences
Earth, ocean, space
Europe, English Channel
Exact sciences and technology
External geophysics
Geochemistry
Mineralogy
Organic complexation
Physical and chemical properties of sea water
Physics of the oceans
Seawater
Silicates
Water geochemistry
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Summary:A method was developed to determine the extent of cobalt complexation with organic ligands in seawater, and applied to samples from the North Eastern Atlantic Ocean. The cobalt speciation was determined using catalytic cathodic stripping voltammetry with ligand competition against the adsorptive ligand (nioxime). Optimised conditions include a nioxime concentration of 200 nM, 0.07 M ammonia/ammonium chloride pH buffer (pH 9.1), and 0.5 M nitrite. The stability of the mixed complex of cobalt, nioxime and ammonia was calibrated against EDTA. The cobalt speciation in the open-ocean surface waters was found to be dominated by complexation to natural organic ligands with conditional stability constants (log K CoL′) ranging between 15.6 and 16.1 and with ligand concentrations between 22 and 60 pM. The cobalt concentrations varied between 25 pM in the open-ocean waters and 103 pM in the English Channel, and were less than the ligand concentrations in many of the surface oceanic waters. The ligand concentration at depths between 30 and 115 m (average 22 pM) was lower than that of cobalt, whereas at other depths (shallower as well as deeper), the ligand concentrations were between 26 and 31 pM, sometimes greater than the cobalt concentrations. The cobalt and ligand concentrations in these waters are finely balanced, strongly binding all cobalt when the ligands are in excess. Free Co 2+ concentrations calculated for the open-ocean surface waters are extremely low (
ISSN:0304-4203
1872-7581
DOI:10.1016/S0304-4203(01)00024-X