Speciation of manganese in Chesapeake Bay waters by voltammetric methods

The Mn(II) to Mn(O) reduction wave (peak) at a mercury electrode was investigated for its analytical usefulness in anoxic Chesapeake Bay waters which contain significant quantities of dissolved and particulate organic matter. The Mn(II) to Mn(O) reduction is characteristic for all Mn(II), MN(III) and MN(IV) complexes and thus represents total dissolved Mn. It does not provide information on only the Mn(II) oxidation state as suggested previously. Inorganic Mn(II) and organic Mn(III) complexes were studied by sampled d.c. polarography, differential pulse polarography, cyclic voltammetry and square wave voltammetry. All methods show that the Mn(II) to Mn(O) reduction is quasi-reversible in sea water. Square wave voltammetry was used for analytical work on field samples. Both Mn(II) and Mn(III) give similar current versus concentration slopes for the Mn(II) to Mn(O) peak when added to Chesapeake Bay samples. The minimum detection limit is near 200 to 300 nM. Comparison of organic free and organic rich laboratory and field samples shows that Ep shifts to more negative potentials for the organic rich samples. Thus, a major finding of this voltammetric study is that manganese can be complexed by organic ligands in marine systems with zones characterized by high organic matter decomposition and low O2 concentrations. Organic complexation of dissolved Mn may have important consequences for Mn chemistry in marine systems characterized by an oxic / anoxic interface.