Polysulfide ions

Barium sulfide solutions undergo slow oxidation in air, forming elemental sulfur and a family of oxidized sulfur species including the sulfite, thiosulfate, polythionates, and sulfate. The elemental sulfur is retained in the dissolved bquor in the form of polysulfide ions, which are responsible for the yellow color of most BaS solutions. Some of the mote highly oxidized sulfur species also enter the solution. Sulfur compound formation should be minimized to prevent the compounds made from BaS, such as barium carbonate, from becoming contaminated with sulfur. 

The sulfur analog of hydrogen peroxide also exists and is an example of a polysulfane, a catenated molecular compound of composition HS—S —SH, where n can take on values from 0 through 6. The polysulfide ions obtained from the polysulfanes include two ions found in lapis lazuli (Fig. 15.15). 

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] ... 

The generated polysulfide dianions of different chain-lengths then establish a complex equilibrium mixture with all members up to the octasulfide at least see Eqs. (5) and (6). For this reason, it is not possible to separate the polysulfide dianions by ion chromatography [6]. The maximum possible chain-length can be estimated from the preparation of salts with these anions in various solvents (see above). However, since the reactions at Eqs. (22) and (23) are reversible and Sg precipitates from such solutions if the pH is lowered below a value of 6, the nonasulfide ion must be present also to generate the Sg molecules by the reverse of the reaction at Eq. (22). The latter reaction (precipitation of Sg on acidification) may be used for the gravimetric determination of polysulfides [11]. There is no evidence for the presence of monoprotonated polysulfide ions HS - in aqueous solutions [67, 72]. 

The composition of sodium polysulfide solutions saturated with sulfur of zero oxidation number (S°) has also been studied at 25 and 80 °C (solutions in contact with elemental sulfur) [76]. In this case the ratio 8° 8 per polysulfide ion increases with increasing alkahnity. The maximum average number of sulfur atoms per polysulfide molecule was obtained as 5.4 at 25 °C and 6.0 at 80 °C and pH values of >12. Equilibrium constants for reactions as in Eqs. (26) and (27) have been derived assuming various models with differing numbers of polysulfide ions present. 

Under special conditions the rapid protonation of aqueous polysulfide ions by hydrochloric acid at temperatures of below 0 °C produces polysulfanes ... 

Sulfite ions react with polysulfide ions at 50 °C in neutral solution to thiosulfate and monosulfide, e.g. ... 

In a similar fashion, cyanide ions desulfurize polysulfide ions at 100 °C to the monosulfide level, e.g. ... 

Aqueous polysulfide solutions are thermodynamically unstable with respect to thiosulfate and sulfide. Therefore, on heating to 150-240 °C under anaerobic conditions polysulfide ions disproportionate reversibly [72, 80, 104], e.g. ... 

The latter reaction has been studied numerous times because of its relevance for the autoxidation of hydrogen sulfide in seawater and other aqueous systems [112, 113]. 8ince the polysulfide ions can be further oxidized to elemental sulfur which precipitates from the solution, these reactions are the basis for several industrially important desulfurization processes (e.g., the 8tretford, 8ulfolin, Lo-Cat, 8ulFerox, and Bio-8R processes) [114] ... 

Dissociation of polysulfide ions into radicals 82 or 83 , and disproportionation into sulfide and thiosulfate become significant at temperatures above 150 C. In fact, in near-neutral solutions, polysulfide ions are stable with respect to this disproportionation up to 240 °C however, at pH > 8 polysulfide ions become metastable, even at room temperature. 

Gillespie RJ, Passmore J (1971) Polycations of Group VI. Acc Chem Res 4 413 19 Giggenbach WF (1974) Equilibria involving polysulfide ions in aqueous sulfide solutions up to 240 °C. Inorg Chem 13 1724-1730... 

Polysulfides of several metals can be prepared by reaction of the metals with excess sulfur in liquid NH3 (group IA metals) or by heating sulfur with the molten metal sulfide. The polysulfide ion binds to metals to form coordination compounds in which it is attached to the metal by both sulfur atoms (as a so-called bidentate ligand). One example is an unusual titanium complex containing the S52-ion that is produced by the following reaction (the use of h to denote the bonding mode of the cyclo-pentadienyl ion is explained in Chapter 16) ... 

Carbon-sulfur hybrid materials, i.e., porous carbons [58,59] or CNTs having nanosized S in the pores or channels, are the most promising solution for the Li-S battery to increase the electronic and ionic conductivity of sulfur or sulfide, and prevent, to a great extent, the solubility of the polysulfide ions formed on reduction of S or upon oxidation of insoluble sulfides [60]. An intimate contact between carbon and sulfur is essential [61]. 

Obviously, the donor activity of the nucleophile, that is, the sulfide ion is enhanced as the negative charge is dispersed along the polysulfide ion produced from the sulfide on the addition of elemental sulfur. This increases the mobility of electrons and facilitates electron transfer. That is why this reaction can be initiated in such a simple way as the addition of elemental sulfur. 

Free polysulfide ions consist of sulfur chains. The atoms of an chain are necessarily coplanar. Longer chains, however, exhibit different possibilities of isomerism. For example, by addition of one sulfur atom d- and can be obtained (Fig. 1). To describe the structxures, in... 

Fig. 1. Idealized stereochemistry (dihedral angle equal to 90°) for polysulfide ions (191). (a), (b), d- and (c), (d), cis- and trons-S, (e), trans,trans-S ...

In this review, we shall mainly consider the electrochemical behavior of sulfur and polysulfide ions (i.e. the reduced forms of sulfur) in solution. Recent works (see Sect. 8.3.1) gave a better understanding of the elementary steps leading from sulfur Sg to polysulfide ions S (or S ) in non-aqueous solvents. This has been achieved by using spectroscopic techniques for the identification of chemical species, the direct coupling of spectroscopic and electrochemical techniques, and by using digital simulation calculations for the validation of the proposed models. 

The electrochemical behavior of sulfur, sulfide (H8 , S ) and polysulfide ions in water is much less documented than for nonaqueous solvents. Experimental studies are less numerous and do not include a systematic study versus the stoichiometry n of polysulfides M28 . The conclusions of these investigations are often speculative, since the experimental curves do not display strong evidence for chemical species involved in the proposed mechanisms. Moreover, the very low solubility of sulfur in water does not allow the study of its electrochemical reduction in water. 

The ground mixture is heated to about 750 °C under reducing conditions, normally in a batch process. This can be done in directly fired kilns with the blend in lidded crucibles of controlled porosity, or muffle kilns. The heating medium can be solid fuel, oil, or gas. The sodium carbonate reacts with the sulfur and reducing agent at 300 °C to form sodium polysulfide. At higher temperatures the clay lattice reforms into a three-dimensional framework, which at 700 °C is transformed to the sodalite structure, with entrapped sodium and polysulfide ions. 

Oxidation. The furnace is allowed to cool to ca. 500 °C when air is admitted in controlled amounts. The oxygen reacts with excess sulfur to form sulfur dioxide, which exothermically oxidizes the di- and triatomic polysulfide ions to S and S, free radicals, leaving sodium sulfoxides and sulfur as byproducts. When oxidation is complete, the furnace cools and is unloaded - a full kiln cycle can take several weeks. The raw ultramarine product typically contains 75 wt % blue ultramarine, 23wt% sodium sulfoxides, and 2wt% free (uncombined) sulfur with some iron sulfide. 

Dinuclear complexes with polysulfide ions as bridging ligands 542... 

The addition of aromatic thiols, ArS-, to cyanamide, NCNH2, is general acid catalysed, giving isothiourea as product.311 A significant movement of a hydron in the TS to the cyano nitrogen atom is indicated. The reactivity of sulfur towards thio-carboxylate ions (341 R = Ph, Me, Bu ) has been looked at and among the species formed are Sj/Sf polysulfide ions (342) and (343).312... 

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