Characterization of Iron Sulfide Species in Model Solutions by Cyclic Voltammetry. Revisiting an Old Problem

A long-standing problem associated with voltammetric determination of iron and sulfide in reduced natural waters has been the nature of the presumed analyte responsible for a reduction peak at -1.1 V vs. Ag/AgCl. Cyclic voltammetry at the Hg electrode is used here to study solutions with different Fe(II) to sulfide ratios in chloride and acetate electrolytes (pH 6-7). The results indicate that the -1.1 V peak can be assigned to reduction of Fe2+ or its labile complexes on FeS layers that partially cover the Hg electrode. Hg electrodes covered with FeS act like FeS solid electrodes over a very wide potential range (-0.35 to -1.9 V). Two mechanisms for forming FeS layers on Hg are described. Over the broadest deposition potential range, the dominant mechanism involves attachment at the Hg surface of FeS nanoparticles, which are generated quickly in initially supersaturated mixtures of Fe(II) and S(-II). In a narrow deposition potential range, roughly -0.56 to -0.70, FeS layers are produced additionally by replacement of preformed HgS. Because Fe2+ is reduced at -1.1 V on FeS layers and at -1.4 V on bare Hg, it may be underdetermined when only the -1.4 V peak is measured.