Determination of complexation of iron(III) with natural organic complexing ligands in seawater using cathodic stripping voltammetry

A method is presented to determine the extent of iron complexation by natural organic ligands in seawater. Catalytic cathodic stripping voltammetry (CSV) is used to take advantage of ligand competition between the added ligand, 1-nitroso-2-napthol (NN), and natural ligands present in seawater. The conditional stability constant for the complexation of iron by NN was calibrated for salinities between 1 and 36 using ligand competition with EDTA. The values of log K′Fe(NN)3 (valid for pH 6.9 seawater) were found to vary linearly with log salinity according to log K′Fe(NN)3 = −1.04 ± 0.08 log(salinity) + 30.12 ± 0.09. The detection window, defined by the iron complexing ability of NN was altered by varying the concentration of NN. Preliminary measurements of iron complexing ligands in samples from coastal and open oceanic origin revealed the presence of natural complexing ligands at concentrations higher than that of total dissolved iron. The stability constants for the complexes were high, log K′FeL falling within the range of 18.8–21.2, indicating that by far the greatest component (99%) of the dissolved iron occurs organically complexed in pH 6.9 seawater. Model calculations showed that it is possible that the organic fraction may be somewhat less at a pH value near 8.