MEASUREMENT OF THE REDOX SPECIATION OF IRON IN SEAWATER BY CATALYTIC CATHODIC STRIPPING VOLTAMMETRY

Catalytic cathodic stripping voltammetry (CSV) preceded by adsorptive collection of complexes of 1-nitroso-2-napthol (NN) can be used to det ermine iron in seawater. It is shown here that iron(II) is effectively masked in the presence of 2,2-dipyridyl (Dp) so that iron(III) is mea sured selectively. The concentration of iron(II) is then calculated as the difference between the concentrations of reactive iron (Fe-R) in the absence and presence of 2 mu M Dp, Fe-R being defined as that whic h was complexed by 20 mu M NN at pH 6.9 in the presence of 1.8 mM H2O2 and 5 ppm sodium dodecyl sulphate. A 30 min reaction time was allowed for Dp to react with iron(II) in seawater prior to the determination of reactive iron(III) using the same conditions as used for Fe-R. Dete ction limits of 0.08 nM, 0.077 nM and 0.12 nM were obtained for Fe-R, iron(III) and iron(II), respectively, using a 60 s deposition time. Th e method was utilised to determine the redox speciation of iron in the northern North Sea. Concentrations of Fe-R ranged between 0.8 and 3.5 nM with nutrient-like depth profiles. Iron(II) was found to be presen t at concentrations up to 1.2 nM, the highest concentrations occurring in the upper 20 m of the water column.