Electrochemical Investigation of Metal Sulfides at Mercury Electrodes Using Thiourea as a Source of Sulfide Ion

Abstract

AbstractA mercury sulfide layer was prepared at the surface of a mercury electrode by an anodic reaction (+0.1 V vs. Ag|AgCl|KCl(3 M)) in the presence of thiourea at the micromolar concentration level. Electrochemical reactions of mercury sulfide were investigated by a subsequent cathodic scan at near‐neutral pH values and in the absence/presence of a second metal ion. Two different forms of mercury sulfide were evidenced by characteristic cathodic peaks; the first one (in the form of adsorbed molecules) undergoing a reversible reduction at about −0.6 V, whereas the second one (probably as nanoaggregates) being reduced in an irreversible process at −0.9 V. A detailed analysis of the first process was performed by adapting the already existing theoretical approaches. It was proven that the HgS layer prepared by mercury anodization in the presence of thiourea is similar to that prepared by anodization in the presence of sulfide ion. Other metal sulfides were prepared via the exchange reaction of mercury in HgS with a metal ion in solution; namely, Zn2+, Ni2+, and Cd2+; reactions of the latter proceeding in a fair agreement with the theoretical model used (the rapid exchange leading to the single CdS form with no soluble cadmium sulfide species). Evident deviations from this model have occurred with both Ni2+ and Zn2+ due to a sluggish exchange reaction, formation of soluble species, and occurrence of various metal sulfide forms. Two different forms of nickel sulfide were detected by characteristic cathodic reactions, namely i) adsorbed free molecules that are reduced at −1.0 V and are able to take part in a catalytic (EC′) reduction of Ni2+; and ii) an aggregated form that is irreversibly reduced at negative potentials beyond −1.2 V and acts as a catalyst for hydrogen evolution. The method presented herein allows one electrochemical preparation and investigation of metal sulfides with no difficulty due to hydrogen sulfide volatility at pH<12 and emphasizes that this method can be applied to other systems consisting of a chalcogenide anion and a metal (like mercury or silver) being able of undergoing a reversible electrochemical reaction. In addition, this contribution provides an insight to the intricacy of the electrochemical behavior of the mercury electrode/sulfide anion/second metal ion system and mentions a series of sources of errors related to the interpretation of such data obtained with natural water specimens.