The measurement of organically complexed FeII in natural waters using competitive ligand reverse titration Original Research Article

Whilst there is increasing evidence for the presence of stabilized FeII associated with organic matter in aquatic environments, the absence of a reliable method for determining FeII speciation in solution has inhibited the study of this aspect of Fe biogeochemistry. A technique is described here for the determination of FeII organic complexation in natural waters that is based on competitive ligand reverse titration and a model fit to experimental results, from which ligand concentration and a conditional stability constant can be obtained. Spectrophotometry was used to detect the Ferrozine (FZ) complex with reactive FeII, which in combination with a liquid waveguide capillary cell (LWCC) enabled high sensitivity and precision measurements of FeII to be made. A series of samples was collected in the Itchen River in Southampton, UK to test the method at a wide range of salinities including river water. Levels of FeII and total dissolved Fe were within previously reported values for this system. FeII was found to occur organically complexed with values for image (conditional stability constant for FeII-natural ligand complexes) of ≈8 at salinities between 0 and 21, whilst no measurable complexation was detected at a salinity of 31. This work demonstrates that spectrophotometry can be used in combination with ligand competition to investigate metal speciation in natural waters.