Complexation of cobalt by natural organic ligands in the Sargasso Sea as determined by a new high-sensitivity electrochemical cobalt speciation method suitable for open ocean work

Abstract

Abstract A high-sensitivity cobalt speciation method was developed and applied to a profile in the North Atlantic. Method development included examining the redox chemistry of the analytical system and calibrating the electroactive cobalt ligand dimethylglyoxime (DMG) using EDTA as a model ligand. The method was applied to a depth profile at the Bermuda Atlantic Time Series Station (BATS) during a September 1999 cruise. Total dissolved cobalt, measured using adsorptive cathodic stripping voltammetry (ACSV) on ultraviolet light irradiated samples, revealed a nutrient-like profile for cobalt. Co speciation, measured using CLE–ACSV (competitive ligand exchange), showed a cobalt binding ligand concentration that was similar to that of total cobalt throughout the profile. An excess of ligand was observed in the chlorophyll maximum where Prochlorococcus and Synechococcus numbers were highest. A conditional stability constant for CoHDMG2 was measured to be logKCoHDMG2cond=11.5±0.3 at pH 8.0. A pH dependence for KCoHDMG2cond was observed and is consistent with model calculations based on the protonation constants for H2DMG. The conditional stability constant for CoL was determined to be logKCoLcond=16.3±0.9 and total ligand concentrations varied from 9 to 83 pM as calculated by a one-ligand non-linear fit using the Levenberg–Marquardt algorithm. Alternate interpretations of the data are discussed, including the possibility for an underestimation of ligand concentrations and stability constants caused by the existence of Co(III) ligands, and kinetic and thermodynamic competition for natural ligands by Ni and Co(II).