Polysulfide equilibrium

Another feature of the polysulfide equilibrium is that the forward and reverse reactions are not instantaneous. Thus the flowing system can be in meta-stable equilibrium because of the... 

Sulfur is weakly soluble in H2O (10 M at 298 K) [33, 34], but Na2S is very soluble [35]. In deaerated aqueous solutions, the alkali-metal polysulfide system contains, in addition to H2O and alkali-metal cations, OH , H+, H2S, HS , S-, 82 , 83 , 84 , and 85 . It is usually considered that 85 is the least reduced polysulfide in water. However, it has been reported, in several papers [26, 36], that in basic aqueous solutions, at high temperatures, a blue color is observed, suggesting the stability of 83 . The polysulfide equilibrium constants, interrelating polysulfide speciation, A a, K i, and Kq, have been well established [36, 37]. The species in solution are related by the equilibria ... 

Abstract Inorganic polysulfide anions and the related radical anions S play an important role in the redox reactions of elemental sulfur and therefore also in the geobio chemical sulfur cycle. This chapter describes the preparation of the solid polysulfides with up to eight sulfur atoms and univalent cations, as well as their solid state structures, vibrational spectra and their behavior in aqueous and non-aqueous solutions. In addition, the highly colored and reactive radical anions S with n = 2, 3, and 6 are discussed, some of which exist in equilibrium with the corresponding diamagnetic dianions. 

The chemistry of polysulfide dianions is closely related to that of the radical-monoanions S since both types of anions are in equilibrium with each other in solution and in high-temperature melts, e.g. ... 

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. 

Hamilton [13] assumed the presence of all ions with n ranging from 1 to 8 in aqueous polysulfide solutions which is by far the most acceptable model but since there is insufficient experimental data available this model cannot be worked out quantitatively without additional assumptions. The general idea is that those species are most abundant which are close to the average composition of the particular solution, e.g., 84 and 85 for a solution of composition Na284.5, and that the larger and smaller ions are symmetrically less abundant. Equilibrium constants for the various reactions... 

The above-mentioned method is useful but metals that form strong M-S bonds (e.g., Hg, Ag, Sn) do not dissolve in W-Melm solutions of sulfur. This problem has been solved by the addition of Mg to the reaction mixture. Metal polysulfides having a variety of metals can be synthesized by the 7 T-Melm/ M-i-Mg/Sg method (Scheme 11) [48]. For example, a mixture of Mg, Sb powder (1 eq.), Sg (15 eq. as S) and W-Melm is heated at 80 °C for 48 h to afford the orange powder of [Mg(N-MeIm)5]Sb2Sj ( x 15) in 88% yield. Rauchfuss et al. proposed the mechanism of these reactions as follows. First, the reduction of Sg with Mg occurs to give the [Mg(W-MeIm)6] salt of Sg , which is probably in equilibrium with Sg, Ss ", Ss" and other species. Independently, the sulfuration of the thiophilic metal takes place. Next, the polysulfide an-... 

Liquid ammonia solutions of lithium polysulfides have been characterized by Dubois et al. [18]. The least reduced polysulfide was shown to be 8 (not found previously in aquo) lying in a strongly temperature-dependent equilibrium with the radical 83 . 

Although the foregoing reactions involve dehalogenation by reduction or elimination, nucleophilic displacement of chloride may also be important. This has been examined with dihalomethanes using HS at concentrations that might be encountered in environments where active anaerobic sulfate reduction is taking place. The rates of reaction with HS exceeded those for hydrolysis and at pH values above 7 in systems that are in equilibrium with elementary sulfur, the rates with polysulfide exceeded those with HS. The principal product from dihalomethanes was the polythio-methylene HS (CH2-S) H (Roberts et al. 1992). 

Under these circumstances, it seems reasonable to look for alternative explanations for the polysulfide reactions and in this connection it is interesting to note that a bimolecular mechanism is in better agreement with the data reported for the trisulfide disproportionation discussed above. This reaction has been followed by H n.m.r. spectroscopy at different temperatures and initial concentrations. Equimolar amounts of diethyl and di-n-propyl trisulfide were used and after the equilibrium had been established the equilibrium constant... 

This reaction also plays a role in the degradation of polysulfides. A back-biting mechanism as shown in equation 6 results in formation of the cyclic disulfide (5). Steam distillation of polysulfides results in continuous gradual collection of (5). There is an equilibrium between the linear polysulfide polymer and the cyclic disulfide. Although the linear polymer is favored and only small amounts of the cyclic compound are normally present, conditions such as steam distillation, which remove (5), drive the equilibrium process toward depolymerization. 

Equilibrium calculations suggested that Hg complexation varies greatly among redox and pH levels typical of the regions of lakes sampled during this study. In an oxic lake, pore water, and groundwater, Hg complexation with organic matter most likely dominates. Under anoxic conditions in the hypolimnion and pore waters, Hg most likely forms soluble bisulfide and polysulfide complexes. 

One such difficulty is that, while it appears that [Sn2 ] levels may be as high as 400 /iM in some hypoxic environments, the precise number (n) of sulfur atoms in these polysulfide species, or even the range of n, is uncertain. Bouleague (16) discusses some of the thermodynamic equilibrium calculations which may be used to estimate the distribution of s[Sn2 ] among the various polysulfide species. Another difficulty with these data is that the structures of the thiols (RSH) detected are not well known. Mopper and Taylor (14) identified 13 different thiols in slurries of intertidal sediments from Biscayne Bay (FL), and found at least 20 more thiols whose structures could not be determined. Their observations (Table III) indicate that thiols encountered in natural waters will probably exhibit a broad variety of structures in any one location, but that a relatively small number of compounds may dominate the mixture. 

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 ... 

At low cure times, only Ale and A2c polysulfidic structures (50 ppm) are observed. At longer cure times, Ale and A2c polysulfidic structures reduce in sulfur rank to monosulfide (45 ppm), and Blc (58 ppm). Bit (64 ppm) and Clc (45 ppm) polysulfidic structures are observed. A small amount of ds-to-trans isomerisation was observed, which increased with sulfur content. The reversion reactions of TBSI-accelerated systems result in a lower degree of sulfurisation as opposed to TBBS-accelerated samples. Based on the equilibrium swelling measurements, TBSI is found to be a less efficient accelerator than TBBS. 

In solution, sulfides and polysulfides exist in an equilibrium between their basic and protonated forms. Since the equilibrium position is dependent upon the pH of the solution (Figure 5.7), this nature of the reaction with hydrogen peroxide is also influenced by pH.33 Under acidic to neutral conditions, sulfidic species are oxidized predominantly to sulfates. The key reactions that occur are outlined in Figure 5.8. 

There is a range of polysulfide dianions 8 that can be obtained by reaction of sulfur with simple sulfides, by high-temperature reaction of an alkali metal with sulfur, or by reaction of alkali metals with sulfur in liquid ammonia. Often, once formed, the anions are in equilibrium in solution, but individual anions can be complexed successfully. 8tructures of the free poly sulfide anions are shown in Figure 31. 

The distribution of the sulfur species between sulfide and polysulfide at equilibrium with elemental colloidal sulfur is represented in Figure 1. 

Thus the PH-E32- relations obtained in the H2S-H20 and H2S-Sq-H2(] systems are characteristic of these systems and they can be employed in natural waters to characterize the presence or the absence of polysulfide ions. However, owing to the lack of sensitivity of the Ag/Ag2S electrode to small variations of (S "] the pH-Es2- relations cannot be employed to detect slight departures from equilibrium in the above systems. 

Teder A., The equilibrium between elementary sulfur and aqueous polysulfide solutions. Acta Chem. Scandinavia 25, 1722-1728... 

Giggenbach W., Dptical spectra and equilibrium distribution of polysulfide ions in aqueous solutions at 20°. Inorg. Chem. 11, 1201-1207 [1972). 

Polysulfides and Sulfide. The polysulfide complexes of Ag and Cu have been added to the model in an attempt to reduce the apparent oversaturation with Ag2S(s) calculated for San Francisco Bay waters (12). Calculation of the activity of polysulfide iotis requires the assumptions 1) the quantity of Sg (free sul-fur) is not a limitation on its reaction with bisulfide (HS ) to form polysulfides and 2) polysulfides are in equilibrium with bisulfide. 

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... 

Most polysulfides with a sulfur rank higher than three, are mixtures where the different sulfur ranks coexist in equilibrium (Table 4). In many cases elemental sulfur is also involved in the above equilibrium. Changes in temperature and pressure sometimes alter this equilibrium and precipitate sulfur. Viscosity of the polysulfides also is a function of temperature, increasing dramatically with decreasing temperature. In most cases heating polysulfides results in their decomposition to the alkyl mercaptans. Some of the aromatic polysulfides are tacky solids. Many trisulfides can be isolated as pure compounds, and exhibit unique chemical properties. They are the only polysulfides that are not corrosive to copper. 

Polysulfides can be generated via two major pathways. First, polysulfides can be formed by the oxidation of dissolved sulfide and sulfide minerals(l, 2). Second, they can be formed by the reaction of elemental sulfur with bisulfide ion(35). Polysulfide levels can be predicted for the second process as described in previous studies(, 36-38). Equilibrium calculations as described in a previous study(22) were performed for the polysulfide levels in these samples. The ratio of S(0) experimental to S(0) calculated for all samples from Great Sippewissett were 0.145 (4-8 cm), 0.137 (8-13 cm) and 0.128 (23-28 cm). Because these ratios are less than 1.0, these results indicate that polysulfides should form primarily from the reaction of bisulfide ion with elemental sulfur(5) rather than sulfide oxidation. This data set is... 

The water dissociation equilibrium constant, A w, and the first acid dissociation constant of H28, Ki, are both well documented. 8everal studies have confirmed that K2, the second acid dissociation constant of H2S, has a value of 10 [38-43], and not as previously assumed [38]. This results in a substantial revision of known metal sulfide solubilities [44]. Noticeably, the concentrations of 82 , 83 , 84 , and 85 polysulfides depend on pH and their stabilities increase with pH. A distribution of polysulfides in aqueous solutions, as a poten-tial-pH diagram, has been proposed [45]... 

Since the aqueous sodium polysulfide contains already several polysulfide anions in equilibrium and since the acidification results in some interconversion reactions, a sulfane mixture H2S c is obtained rather than pure H2S4. This mixture nevertheless reacts in dry CS2/Et20 mixture at 20 °C with dichlorodisulfane, besides other products, to S12 which has been isolated in 4% yield by extraction with CS2 and fractional crystallization [41] ... 

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