Electrochemistry of the Chalcogens

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 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. These diagrams comprise an extremely useful, yet fundamental, 

Bouroushian, Electrochemistry of Metal Chalcogenides, Monographs in Electrochemistry, DOI 10.1007/978-3-642-03967-6 2, 

Starting point for the study of electrochemical systems. Certainly, the ability to predict, understand, and ultimately control electrochemical reactions requires also knowledge of process kinetics. 

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The electrochemistry of S-N and Se-N heterocycles has been reviewed comprehensively. The emphasis is on the information that electrochemical studies provide about the redox properties of potential neutral conductors. To be useful as a molecular conductor the 4-1, 0, and -1 redox states should be accessible and the neutral radical should lie close to the centre of the redox spectrum. The chalcogen-nitrogen heterocycles that have been studied in most detail from this viewpoint... 

It seems useful to understand the mechanisms involved in the chemistry and in the electrochemistry of sulfur and polysulfides. During the last 10 years, more than 90 papers deahng with solvothermal synthesis of chalcogenides MxE or binary chalcogenides MxM Ez (E = S, Se, or Te) have been published. In a typical process, a metal and/or a metallic salt is heated in a solvent (benzene, toluene, pyridine, ethylenediamine, water, etc.) at 100-200 ° C in the presence of an excess of chalcogen (see for instance Ref. 125). These empirical syntheses would benefit from a sound understanding of the involved mechanisms. 

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