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 À1.1 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 À1.1 V peak can be assigned to reduction of Fe 2 + 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 (À0.35 to À1.9 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 À0.56 to À0.70, FeS layers are produced additionally by replacement of preformed HgS. Because Fe 2 + is reduced at À1.1 V on FeS layers and at À1.4 V on bare Hg, it may be underdetermined when only the À1.4 V peak is measured.
An association of Cu with sulfide in aerobic natural waters has been attributed to these components' coexistence in clusters of sizes intermediate between mononuclear complexes and colloidal particles. This hypothesis is investigated here. Copper sulfide solid phases display size-related voltammetric behavior at Hg electrodes. Suspensions of copper sulfide powders held at accumulation potentials of 0 to -0.2 V (vs Ag/AgCl) produce voltammetric peaks near -0.15, -0.65, and -0.95 V during subsequent cathodic scans. The first two peaks arise from electrochemically generated Cu-oxyhydroxides and HgS; the -0.95 V peak arises from reduction of sorbed copper sulfide particles. Nanoparticles of radius approximately 10(-8) m produce the third peak even without stirring or accumulation. Still smaller analytes give only the first two peaks. Published evidence alleging production of subnanometer copper sulfide clusters during titrations of Cu2+ and HS- was not reproduced when sulfide oxidation was avoided. Instead, such titrations apparently generate nanoparticles. The titration stoichiometry is 1/1, consistent with previous descriptions of this process: Cu2+ + HS- --> 1/2Cu2S x S0 (brown sol) --> CuS (green sol). Titrating Cu2+ into organic-rich (muscilaginous) Adriatic Sea water, which contains 10(-7) M natural thiols and sulfide, produces solid products. In the future, voltammetry might prove useful for studying semiconductor sulfide nanoparticles in nature.
The reactions of copper on Au(111) electrode in the underpotential deposition (UPD) region from chloride
media were investigated by cyclic voltammetry. Chloride concentrations ranged from trace levels up to
0.55 M. At chloride concentration below 10-3 M three different adlayer structures were detected. At least
one of these structures also contains perchlorate ions besides copper and chloride. At NaCl concentrations
above 5 × 10-3 M, Cu(I) is stabilized by chloride, and the reduction of copper proceeds as one electron
process on the Au(111) surface covered with copper and chloride bilayer but not on Au(111) free of copper.
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