Sulfide polysulfide couple

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. 

Next, photogeneration of electron-hole pairs leads to the formation of quasi-levels of minority and majority carriers, Fp and F , as shown in Fig. 12. Since, at the surface, Fp < Fs -/sl and F > Fs -/sl, illumination results in the acceleration of both forward and reverse reactions in a sulfide polysulfide couple. If the circuit is closed on an external load R, the anodic and cathodic reactions become separated the holes are transferred from the semiconductor photoanode to the solution, so that ions are oxidized to 82 , and the electrons are transferred through the external circuit to the metal counterelectrode (cathode) where they reduce S2 to The potential difference across a photocell is iphR, where iph is the photocurrent, and the power converted is equal to /phF. 

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... 

The synthesis of polysulfide elastomers involves the use of a small amount of trichloroalkane in addition to dichloroalkane and sodium sulfide in order to form a branched polymer. The prepolymer is treated with a mixture of sodium hydrosulfide and sodium sulfite followed by acidification to convert all end-groups to thiol groups. Further polymerization and crosslinking is achieved by oxidative coupling of the thiol end-groups by treatment with lead dioxide, p-quinone dioxime, or other oxidizing agent... 

As Berner (36) pointed out in his classic work, the formation of pyrite is coupled to a process in which free sulfide is oxidized to form polysulfides, which again react with FeS to form pyrite. In this study elemental sulfur was the oxidant. However, elemental sulfur was always less than 1% of the total sulfur content in the study of White et al. (35). The findings of the experimental studies discussed on the interaction between H2S and ferric oxides (20-23), in combination with the field observations, suggest a mechanism in which ferric iron oxides are the oxidants to form polysulfides and subsequently pyrite. 

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... 

Two highly stable aqueous phase redox couples, iodide and polysulfide, are used in these studies to further this bipolar indirect multiple band gap photoelectrochemistry. Platinum and cobalt sulfide, respectively, provide an effective (low overpotential) electrocatalyst for a wide range of iodide and polysulfide electrolytes [2]. These electrolytes have also been utilized in bipolar band gap solar cells. Table 1 summarizes the results of a variety of bipolar two band gap solar cells [2, 3]. In this table, comparison of the solid-state and direct photoelectrochemical cells shows that energy conversion efficiency of the photoelectrochemical cell approach that of the solid-state device. In the bipolar direct MPEG cell, the majority of the photopower loss in these photoelectrochemical cells was attributed to polaiTzation... 

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