Chalcogens and Metal Chalcogenides.- Electrochemistry of the Chalcogens.- Electrochemical Preparations I (Conventional Coatings and Structures).- Electrochemical Preparations II (Non-conventional).- Photoelectrochemistry and Applications.- Electrochemical Processes and Technology.
"Chapter 2 Electrochemistry of the Chalcogens (S. 57)
2.1 General References
Because of their multiple oxidation states, the chalcogens, particularly sulfur, can engage in numerous redox couples participating in acid–base, oxidation–reduction, precipitation, and complexation equilibria. In the anion electrochemical series, sulfur, being the less noble element compared to its heavier congeners, occupies an intermediate position between iodine and selenium [(+)F, Cl, Br, I, S, Se, Te(–)]. Selenium, regarded as a metalloid, is a relatively noble element.
Tellurium is rather an amphoteric element: it can enter into solution in the form of both cations and anions. Regarded as a metal, i.e., with respect to its cations, tellurium occupies a position between copper and mercury. Regarded as a metalloid, i.e., with respect to its anions, it is located on the extreme right of the above series. A comprehensive survey of the classical electrochemical facts for sulfur, selenium, and tellurium, as documented until about 1970, can be found in the reviews of Zhdanov [1], wherefrom we cite the lists of standard and formal potentials for aqueous solutions, in Tables 2.1, 2.2, and 2.3, respectively. Many of these potentials have been calculated thermodynamically since the experimental determinations are few.
The listed data are largely drawn from the monograph by Pourbaix (below), and the interested reader should validate the measurement conditions for the indicated potentials or the scatter in their values (not given here in detail). In these tables, the redox systems are assorted by decreasing formal valency of chalcogen in the oxidized state, while at a given valency of the oxidized state they appear in the order of decreasing valency of chalcogen in the reduced state.
Latimer [2] has compiled useful aqueous redox transition potential diagrams (reproduced also in Zhdanov’s monographs) that are convenient as a quick guide in practical problems and for perceiving the oxidation–reduction properties of some chalcogen hydride and oxychalcogenide species. Standard potentials of chalcogens in non-aqueous media are generally not known, at least in a systematic manner. A standard approach for the theoretical presentation of electrochemical equilibria is the use of Pourbaix, or potential–pH predominance area diagrams, which incorporate chemical and electrochemical thermodynamics simultaneously in a straightforward manner."
