Voltammetric study of an FeS layer on a Hg electrode in supersaturated FeS chloride solution

Abstract

Environmental context During the electrochemical analysis of natural waters, the Hg electrode could become the site for surface metal sulfide formation, such as FeS, which can have significant influence on the voltammetric determination of FeII. Reduction of FeII in such conditions can occur both on the bare Hg surface and on the FeS-modified Hg surface. Until recently, measurements of FeII have considered only the signal obtained from reduction on the bare Hg surface, and hence may have underdetermined the true FeII content of natural samples. Abstract Cyclic voltammetry at a Hg electrode was used for the study of an FeS layer formed on the electrode surface during scanning potential in a saturated FeS suspension containing FeS nanoparticles in chloride electrolyte. Newly presented results as a part of comprehensive study on FeS electrochemistry in model and natural samples confirm that the voltammetric peak at –1.1V, which can often be recorded in anoxic waters containing Fe and sulfide species, represents reduction of Fe2+ on the Hg electrode surface modified by the FeS layer. Fe0 deposited on such a modified Hg surface is reoxidisable, often producing two oxidation peaks situated between –0.8 and –0.6V (v. Ag/AgCl). These peaks represent two different Fe0 oxidation products, free Fe2+ and Fe2+ chloro complexes, FeCl+. By scanning the potential from –0.75V in the positive direction, an oxidation exchange peak of Hg and FeS at ~–0.45V occurs, which can be taken as a measure for FeS nanoparticles from bulk solution.