Sulfur polysulfide ions

This review has shown the complexity of the chemistry and the electrochemistry of sulfur, polysulfide ions, and sulfur cations. This complexity originates from the ability of sulfur to form catenated species, which leads to disproportionation and dissociation equilibria. 

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 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. 

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] ... 

The various oxidation states of sulfur have been determined by polarography. The electrochemical oxidation of sulfide ions in aqueous solution may lead to the production of elementary sulfur, polysulfides, sulfate, dithionate, and thiosulfate, depending on the experimental conditions. Disulfides, sulfoxides, and sulfones are typical polarographically active organic compounds. It is also found that thiols (mer-captans), thioureas, and thiobarbiturates facilitate oxidation of Hg resulting thus in anodic waves. 

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... 

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. 

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... 

Bisulfide and Polvsulfide as Nucleophiles. Hydrogen sulfide (H2S) and bisulfide ion (HS ) are probably the primary sulfur nucleophiles (i.e. species with a lone pair of electrons on sulfur) in reducing sediments. However, several environmental factors such as diffusion of oxygen, the presence of Fe(III) may cause incomplete oxidation of sulfide to form polysulfide ions (S,2-, where n>l) (21)-... 

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. 

The Michael addition mechanism, whereby sulfur nucleophiles react with organic molecules containing activated unsaturated bonds, is probably a major pathway for organosulfur formation in marine sediments. In reducing sediments, where environmental factors can result in incomplete oxidation of sulfide (e.g. intertidal sediments), bisulfide (HS ) as well as polysulfide ions (S 2 ) are probably the major sulnir nucleophiles. Kinetic studies of reactions of these nucleophiles with simple molecules containing activated unsaturated bonds (acrylic acid, acrylonitrile) indicate that polysulfide ions are more reactive than bisulfide. These results are in agreement with some previous studies (30) as well as frontier molecular orbital considerations. Studies on pH variation indicate that the speciation of reactants influences reaction rates. In seawater medium, which resembles pore water constitution, acrylic acid reacts with HS at a lower rate relative to acrylonitrile because of the reduced electrophilicity of the acrylate ion at seawater pH. 

Kinetic data show that in seawater medium S42 reacts about 20-30 times faster than HS with acrylic acid, whereas the reaction of S42 with acrylonitrile is only about 4-6 times higher than with HS. However, in any environment, the importance of polysulfide versus bisulfide reactions is also dependent on their relative concentrations. In a situation where polysulfide and bisulfide ions are present in similar concentrations, our results imply that polysulfide ions, rather than bisulfide, are the important sulfur nucleophiles for reactions with activated unsaturated molecules having a terminal carboxyl group (e.g. acrylic acid, cinnamic acid). However, for neutral molecules such as fucoxanthin, in addition to polysulfide ions, reactions with bisulfide ions will also be of importance. 

Sulfide ion in alkaline solution reacts with solid sulfur to form polysulfide ions having the formulas S2-, S3-, S4-, and so on. The equilibrium constant for the formation of S2- is 12, for S2- is 130, and both are formed from S and S2-. What is the equilibrium constant for the formation of S2- from S2- and S ... 

The elemental sulfur (Sq) in Equation 3 is generally dissolved as a polysulfide ion. In iron-poor sediment or sediment in which iron resides largely in refractory minerals, only small amounts of sulfide minerals form. Excess H2S in these sediments slowly reacts with organic matter to form organosulfur. 

One of the most interesting characteristics of solutions of the metal sulfides is their ability to dissolve sulfur with the formation of polysulfide ions, S 2 (in most cases, n < 6). Some solid compounds containing polysulfide ions can be isolated, especially with large cations (see Chapter 15). 

Sulfur has a strong tendency to catenation, which manifests itself not only in the many forms of the element that all contain S rings of various sizes, but in polysulfide ions S - which may be discrete in highly ionic salts or serve as chelating ligands towards transition metals, sulfanes (XS X) (where X... 

When aqueous sulfide solutions are heated with sulfur, solutions containing largely S2 and S2" are obtained. These polysulfide ions are the only ones stable in solution but a number of crystalline compounds with Sj ions from n = 3 to n = 8 can be prepared, especially by using large cations (e.g., Cs+, NH4 and enH + and R3NH+). In all those with n = 4 to 8 the S—S distances run from 2.00 to 2.11 A and the S—S—S angles are ca. 110°. Structures of some S2" ions are shown in Fig. 12-3. 

There are large numbers of compounds with the simple sulfide ion (S2 ) as a ligand comparable to O2- that are mononuclear like S=WC13 or bi- or polynuclear with sulfur bridges. These and related species derived from polysulfide ions (S2 ) have been intensively studied in part because sulfur bridged species occur in Nature in ferredoxins and related compounds. 

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