Redox sulfide/polysulfide

The study of the electrochemical properties of the sulfide/ polysulfide redox couple have been achieved by measurements of pH, Eh (the potential of a platinum electrode) and [the potential... 

Since the sulfide/polysulfide couple is electroactive and the reaction at the surface of the platinum is rapid ( ], the potentials measured in the H2S-S8-H2O system must be characteristic of the equilibria between sulfide and polysulfide species. These equilibria correspond to the redox level within the environment since the reactions betweeh sulfide and polysulfide are rapid ( 5, 17]. In this instance, the redox potential can be directly interpreted in terms of chemical and electrochemical equilibria of the reduced sulfur species. Thus one can employ the notion of pe (pe = Eh/(2.3 xRx T X ] ] when in the presence of polysulfide ions. 

Field measurements of the pH-Eh-E32- relations in reducing environments where hydrogen sulfide, polysulfides and elemental sulfur were found were in good agreement with the redox level estimated from the concentrations of the sulfur species ... 

However, very often there are deviations from this linearity due to a shift of the position of the band edges under illumination or as a consequence of the reaction with the redox system In order to prevent corrosion, compound semiconductors have often been employed in a saturated solution of their ionic components like sulfides in sulfide solutions with the redox system polysulfide/sul-fide or selenides in the redox system polyselenide/selenide. However, it was found that even under these conditions the crystalline semiconductors can decompose and are transformed either into a surface with numerous lattice defects or into a different polycrystalline material. 

Figure 3.109 Principle of operation of the quantum-dot-sensitised solar cell (QDSSC). Charge injection from excited CdSe QDs into Ti02 nanoparticles (a) is followed by collection of charges at the electrode surface (b). The redox electrolyte e.g. sulfide/polysulfide) scavenges the holes and thus ensures regeneration of the CdSe. Reprinted with permission from Kamat, 2008. Copyright (2008) American Chemical Society...

Therefore, polysulfide ions play a major role in the global geological and biological sulfur cycles [1, 2]. In addition, they are reagents in important industrial processes, e.g., in desulfurization and paper production plants. It should be pointed out however that only sulfide, elemental sulfur and sulfate are thermodynamically stable under ambient conditions in the presence of water, their particular stabihty region depending on the redox potential and the pH value [3] ... 

An interesting idea has been to prepare the photosensitive electrode on site having the liquid play the dual role of a medium for anodic film growth on a metal electrode and a potential-determining redox electrolyte in the electrochemical solar cell. Such integration of the preparation process with PEC realization was demonstrated initially by Miller and Heller [86], who showed that photosensitive sulfide layers could be grown on bismuth and cadmium electrodes in solutions of sodium polysulfide and then used in situ as photoanodes driving the... 

The standard potentials of sulfur compounds are mostly calculated thermodynamically because the redox systems - except that of sulfite-dithionite - are irreversible. In excess sulfide, sulfur dissolves in the form of polysulfides. 

Bacterially produced elemental sulfur can also react with hydrogen sulfide form polysulnde ions. Thus, polysulfide ions should constitute a significant fraction of sulfur nudeophiles in reducing sediments especially where sulfide oxidation is incomplete, such as in intertidal and salt marsh sediments (31321. The polysulfide ions should also be important at redox boundaries (anoxic/ suboxic) in the water column of marine anoxic basins, such as the Black Sea. 

Note that at pH > 7 the redox potentials vary only slightly. The H2S-S0-H2O system behaves as a redox buffer. This effect can be of importance in waters where sulfide and polysulfide are found. 

The computerized aqueous chemical model of Truesdell and Jones (, 3), WATEQ, has been greatly revised and expanded to include consideration of ion association and solubility equilibria for several trace metals, Ag, As, Cd, Cu, Mn, Ni, Pb and Zn, solubility equilibria for various metastable and(or) sparingly soluble equilibrium solids, calculation of propagated standard deviation, calculation of redox potential from various couples, polysulfides, and a mass balance section for sulfide solutes. Revisions include expansion and revision of the redox, sulfate, iron, boron, and fluoride solute sections, changes in the possible operations with Fe (II, III, and II + HI), and updating the model s thermodynamic data base using critically evaluated values (81, 50, 58) and new compilations (51, 26 R. M. Siebert and... 

Less common among the metastable sulfur species are the polythionates (S Oi" with n = 4 to 9). TTiey are formed, for example, in volcanic lakes by the bubbling of H2S gas through sulfurous acid (Takano 1987). The polythionates break down eventually into sulfate and elemental sulfur. The polysulfides and polythionates are themselves metastable relative to sulfite, thiosulfate, S , and the sulfide species. In order to examine the redox-stability environments of these species, it is useful to construct an Eh-pH diagram ignoring sulfate, which is relatively inert. Such a plot is given in... 

A great variety of binary Mo-S complex anions is formed and can be isolated in reactions of the tetrathiomolybdate anion [Mo 4] with various sulfide and polysulfide anions. The nature of the anionic products that can be isolated from these reactions depends on (a) the amount of excess sulfur used (and the types of ligands present in the reaction mixtures), (b) the type of counterion used in the isolation of the complex anions, and (c) the type of solvent employed in the synthetic procedure. In a recent article, we described a scheme that interrelates the various [Mo2(S)n(S2)6.n] anions. In this scheme (Fig. 1), any of the six homologs can hypothetically be obtained from any other by either the addition of sulfur, or the abstraction of sulfur by triphenylphosphine. Experimentally, the correctness of this scheme has been verified by the successful synthesis of most of the [Mo2(S),(S2)6-,] complexes, or of their internal redox isomers. In the [Mo2(S), (S2)6-,] series, the homologs with = 4, 5, and 6 have been characterized structurally. Those with n — 2 and 3 have been characterized structurally as the internal-redox isomers, [(S4)Mo(S)(/i-S)2(S)Mo(S2)] " (ref. 3) and... 

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