New approach for determination of Fe in presence of high concentrations of natural organic matter (NOM) by the modified ferrozine method

The analysis of iron in natural waters is difficult due to the low concentration of Fe coupled with its high affinity to form colloids complexes with organic matter. The complexities of measuring Fe in presence of natural... [ view full abstract ]

The analysis of iron in natural waters is difficult due to the low concentration of Fe coupled with its high affinity to form colloids complexes with organic matter. The complexities of measuring Fe in presence of natural organic matter (NOM) mainly result from their competitive role for Fe acquisition coupled with their dual behavior acting as both a reductant and an oxidant. For these reason, the commonly used methods have been mostly relied on determination of total dissolved iron. Although these methods are efficient under different conditions, they have been always associated with some uncertainties. Here, we have developed a new approach by combining a modified ferrozine method (FZ), semi-flow injection analysis and spectrophotometry with Liquid waveguide capillary cell to determine total dissolved Fe at nM level in water samples containing high concentrations of NOM. To achieve this aim, we exclusively measured the duration required for complete Fe dissociation from desferrioxamine B (DFB), reduction of Fe(III) by Na2SO3 and complexation between FZ and Fe. Our results indicate that storage of the samples at a very low pH (close to1) for 10 days can result in fully dissociation of Fe in presence of 50µM of DFB. Moreover, the results show that addition of FZ shortly after reduction step (15min) and considering a mixing time of at least 24h for ferrozine can dramatically stabilized coloured-complex (FeII-FZ) over time and accordingly improve the reliability of method. This approach was also successfully applied to natural samples collected from Trichodesmium erythraeum cultures and coastal waters.