Mn speciation at nanomolar concentrations with a porphyrin competitive ligand and UV-vis measurements.

We describe the modification of the porphyrin method for low-level determination of the speciation of dissolved Mn in seawater. First, an investigation of sensitivity of the method versus the salinity, the reagent composition and the type of ligands present is described for a few micromolar of dissolved Mn. Then, using certified seawater reference standards, we demonstrate the accuracy of the method for total concentrations of manganese between 1 and 10 nM. To this aim, we report the reactivity of solutions made of Mn(II) and Mn(III) bound to Tiron and DFOB with our optimized porphyrin reagent which includes adding excess Cd, and compare these results with data from representative natural seawater samples. This work describes an improved method that gives the opportunity for future work to investigate Mn speciation at nanomolar concentration in the open ocean.