Polysulfide, alkali

The second catalyst paste of the two-paste product is a curing agent. A wide variety of materials convert the Hquid polysulfide polymers to elastomeric products. Alkalies, sulfur, metallic oxides, metallic peroxides, organic peroxides, and many metal—organic salts, ie, paint driers, are all potential curing agents. 

Polysulfide 150 Excellent resistance to oils, gasoline, aliphatic and aromatic hydrocarbon solvents. Very good water resistance, good alkali resistance, fair acid resistance. Poor mechanical properties. 

Nitro-filter cloths are composed of cellulose nitrate, which is an ester of cellulose. Any chemical compound that will saponify the ester will destroy the cloth. Caustic soda or potash in strengths of 2% at 70° C or over alkali sulfides, polysulfides and sulfohydrates or mixtures of ethyl alcohol and ether, ethyl, amyl and butyl acetates, pyridine, ferrous sulfates, and other reducing agents are detrimental to the cloth. 

Reacidification reprecipitates AS2S3 quantitatively. With alkali metal or ammonium polysulfides thioarsenates are formed which are virtually insoluble even in hot cone HCl ... 

Polychalcogenides are less stable than polysulfides (p. 681). Reaction of alkali metals with Se in liquid ammonia affords M2Se2, M2Se3 and M2Se4, and analogous polytellurides have also been reported (see preceding section). However many of these compounds are rather unstable thermally and tend to be oxidized in air. 

In solution this reaction is rather rapid but in the solid state autoxidation takes place much slower. Nevertheless, commercial sulfides and polysulfides of the alkali and alkali earth metals usually contain thiosulfate (and anions of other sulfur oxoacids) as impurities [6]. For all these reasons the chemistry of polysulfides is rather complex, and some of the earlier studies on polysulfides (prior to ca. 1960) are not very rehable experimentally and/or describe erroneous interpretations of the experimental results. 

The reaction at Eq. (12) allows the preparation of Na2S4 and K2S5 from the alkali metals, hydrogen sulfide and sulfur in anhydrous ethanol (ROH). First the metal is dissolved in the alcohol with formation of ethanolate (MOR) and hydrogen. Bubbling of H2S into this solution produces the hydrogen sulfide (MHS). To obtain the polysulfide the solution is refluxed with the calculated amount of elemental sulfur. After partial evaporation of the solvent and subsequent cooling the product precipitates. 

Ionic polysulfides dissolve in DMF, DMSO, and HMPA to give air-sensitive colored solutions. Chivers and Drummond [88] were the first to identify the blue 83 radical anion as the species responsible for the characteristic absorption at 620 nm of solutions of alkali polysulfides in HMPA and similar systems while numerous previous authors had proposed other anions or even neutral sulfur molecules (for a survey of these publications, see [88]). The blue radical anion is evidently formed by reactions according to Eqs. (5)-(8) since the composition of the dissolved sodium polysulfide could be varied between Na2S3 and NaaS with little impact on the visible absorption spectrum. On cooling the color of these solutions changes via green to yellow due to dimerization of the radicals which have been detected by magnetic measurements, ESR, UV-Vis, infrared and resonance Raman spectra [84, 86, 88, 89] see later. 

The blue color of 83 has been observed in numerous experiments. For example, a brilliant blue color occurs if a potassium thiocyanate melt is heated to temperatures above 300 °C [132] or if eutectic melts of LiCl-KCl (containing some sulfide) are in contact with elemental sulfur [132, 133], if aqueous sodium tetrasulfide is heated to temperatures above 100 °C [134], if alkali polysulfides are dissolved in boiling ethanol or in polar aprotic solvents (see above), or if borate glasses are doped with elemental sulfur [132]. In most of these cases mixtures of much 83 and little 82 will have been present demonstrating the ubiquitous nature of these radicals [12]. 

The red tetrasulfide radical anion 84 has been proposed as a constituent of sulfur-doped alkali hahdes, of alkah polysulfide solutions in DMF [84, 86], HMPA [89] and acetone [136] and as a product of the electrochemical reduction of 8s in DM80 or DMF [12]. However, in all these cases no convincing proof for the molecular composition of the species observed by either E8R, Raman, infrared or UV-Vis spectroscopy has been provided. The problem is that the red species is formed only in sulfur-rich solutions where long-chain polysulfide dianions are present also and these are of orange to red color, too (for a description of this dilemma, see [89]). Furthermore, the presence of the orange radical anion 8e (see below) cannot be excluded in such systems. 

Licht et al. [17] developed a method of numerical analysis to describe the above-quoted equilibria of the 11 participating species (including alkali metal cations) in aqueous polysulfide solution, upon simple input to the algorithm of the temperature and initial concentration of sulfur, alkali metal hydroxide, and alkali metal hydrosulfide in solution. The equilibria constants were evaluated by compensation of the polysulfide absorption spectrum for the effects of H8 absorption and by computer analysis of the resultant spectra. Results from these calculations were used to demonstrate that the electrolyte is unstable, and that gradual degradation of polysulfide-based PECs (in the long term) can be attributed to this factor (Chap. 5). 

Solutions of alkali metal polysulfides have been investigated by UV/vis and Raman spectroscopy.1 2,33 All species S2 in the range n = 2-5 are present in... 

Similar reducing effects are obtained from alkali sulfides, hydrosulfides and polysulfides [241]. A peculiar reaction believed to be due to sodium polysulfide formed in situ by refluxing sulfur in aqueous-ethanolic sodium hydroxide is a conversion of p-nitrotoluene to p-aminobenzaldehyde [242]. Oxidation of the methyl group by the polysulfide generates hydrogen sulfide which then reduces the nitro group to the amino group. 

The reaction of suitable aliphatic dihalogen compounds with alkali or alkaline earth polysulfides results in the formation of linear, rubbery or resinous, po-ly(alkylene sulfide)s ... 

Only the inorganic polysulfides (and more specifically the alkali polysulfides) will be considered. Furthermore, we shall not review the electrochemical properties of SxOy ions or molecules. It must be noted that a comprehensive review, devoted to the oxidation of sulfur(IV) oxides, oriented toward the atmospherically relevant processes and mechanisms, appeared in 1995... 

The understanding of the reduction process of sulfur requires the identification of the reduced forms of sulfur, that is, polysulfides or S . The phase diagrams of M2S —Sg systems (where M is an alkali-metal cation) have been reviewed [23], as also the properties of polysulfide melts [24]. Many polysulfides have been characterized in the solid state by x-ray diffraction, infrared and Raman spectroscopy [25]. The identification of polysulfides in solution is often rather difficult. The reason is that the dissolution of a M2S polysulfide leads, for most of them, to a disproportionation process of the type ... 

It is supposed to originate from the dissociation of 84 . This dissociation is perhaps significant at high temperatures, like those of the synthesis of ultramarine pigments or those of doping of alkali halide crystals. 83 and 82 radicals have also been observed in the alkali halides doped with sulfur [30, 31]. Another radical anion polysulfide, 84 , has been identified, by EPR experiments, in solution in DMF, originating from the dissociation of 8g , which is the least reduced polysulfide in this solvent [32]. 

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 ,... 

Polysulfide polymers provide inherent resistance to fluel and quite good resistance to alkali. In contrast to the silicone polymers, they have low gas... 

Alkali metal polysulfides are prepared by the direct reaction of an alkali metal with cyclo-Sg. Alternatively, n-butyllithium may be used to generate... 

Sulfur combines direcdy with hydrogen at 150—200°C to form hydrogen sulfide. Molten sulfur reacts with hydrogen to form hydrogen polysulfides. At red heat, sulfur and carbon unite to form carbon disulfide. This is a commercially important reaction in Europe, although natural gas is used to produce carbon disulfide in the United States. In aqueous solutions of alkali carbonates and alkali and alkaline-earth hydroxides, sulfur reacts to form sulfides, polysulfides, thiosulfates, and sulfites. 

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