The chemical speciation of iron in seawater is determined by cathodic stripping voltammetry using 2,3-dihydroxynaphthalene (DHN) as adsorptive and competing ligand. The optimized conditions include a DHN concentration of 0.5-1 microM, seawater at its original pH of 8, and equilibration overnight. The alpha-coefficient for DHN (=[FeDHN]/[Fe']) was calibrated against EDTA giving values of 166 for 0.5 microM DHN and 366 at 1 microM DHN and a value of 8.51 +/- 0.07 for log K'(Fe'DHN). The dissociation of the natural iron species FeL was found to have a characteristic reaction time of 50 min, indicating that titrations should be equilibrated overnight rather than the shorter periods sometimes used onboard ship. The method was applied to samples from the Pacific giving ligand concentrations of 1.1 and 1.6 nM for deep and surface waters, respectively, with an average value for log K'(FeL) of 11.9 +/- 0.3 compared to a value of 11.5 for the siderophore deferoxamine. The results are similar to those obtained previously for similar samples, but the new method has much greater sensitivity for iron than previous methods, leading to lower limits of detection and shorter analysis time.
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