Polythionates production

In the interaction of sulphur monochloride with potassium thiosulphate, tetrathionate is obtained as the highest polythionate product.1... 

Attempts to prepare thiosulfuric acid by acidification of stable thiosulfates are invariably thwarted by the ready decomposition of the free acid in the presence of water. The reaction is extremely complex and depends on the conditions used, being dominated by numerous redox interconversions amongst the products these can include sulfur (partly as cyclo-Sf,), SO2, H2S, HiS,. H2SO4 and various polythionates In the absence of water, however, these reactions are avoided and the parent acid is more stable it decomposes quantitatively below 0° according... 

The possible roles in sulfur (and sulfide or sulfane-) oxidation of a sulfur dioxygenase or of electron-transport-linked hydration/dehydrogenation are outlined above, but the fate of the sulfite product may be more complex than previously considered. Vishniac and Santer (1957) showed that S-labeled sulfide was rapidly oxidized first to thiosulfate (and polythionates) and then to sulfate by T. thioparus. This observation was incorporated into the original Peck scheme (Eqs. 15.13-15.17) by Peck and Fisher (1962), who realized that the complete oxidation of thiosulfate (after reductive scission to sulfite and sulfide Eq. 15.3) could be explained if there was recycling of sulfide to produce thiosulfate ... 

Formation.—The free acid is exceedingly unstable and, at best, is obtainable only in very dilute solution many of the older methods described for the synthetic production of aqueous solutions of the acid probably yielded only one or more of the polythionic acids.1... 

When acids act on thiosulphates, polythionic acids are formed.5 This is explained by reactions (b) and (c). The fact that the amount of hydrogen sulphide liberated is very small in proportion to the amount of polythionic acid formed is attributed 8 to the fact that reaction (a) is that which normally occurs to the greatest extent when acid acts upon thiosulphate the hydrogen sulphide is thus liberated in the presence of a large amount of sulphurous acid and hence rapidly destroyed. Trithionic acid is thus a primary product of thiosulphate decomposition, and in its turn decomposes as explained on p. 212. 

On the other hand, much stronger evidence is available in favour of the earlier formula,4 which accords well with the relationship between the acid and the polythionic acids.5 The formation of sodium thiosulphate by Spring s synthesis from sodium sulphide and sodium sulphite (p. 194) is definitely favourable to this constitution, as also especially is the fact that an alkali thiosulphate will react with only an equimoleeular proportion of an organic (alkyl) halide,6 the product... 

Whenever sulphur dioxide, water and nascent sulphur meet, for example in the action of water on sulphur chloride, of mineral acids on a thiosulphate, or of hydrogen sulphide on aqueous sulphur dioxide solution, formation of polythionic acids is likely to occur. Dalton 1 in 1812 demonstrated that the last-named reagents gave rise to an acid liquid, a result which was confirmed later by Thomson 2 in 1846 Wackenroder 3 proved the presence of pentathionic acid in the liquid, since which date the aqueous reaction product has been known as Wackenroder s Solution. ... 

In addition, the further breaking down of these decomposition products results in the formation of polythionates.1... 

Oxidation of sulfide will affect rates of sulfate reduction only if sulfate is the end product of such oxidation. Many compounds with oxidation states intermediate between sulfide and sulfate may be formed instead. Although many details of the oxidation pathways remain to be clarified, evidence suggests that sulfate is formed. Oxidation of sulfide by phototrophic microorganisms results in production of elemental sulfur, sulfate, or polythionates (e.g., 171). Members of each of the three families of phototrophic sulfur-oxidizing bacteria are capable of carrying the oxidation all the way to sulfate elemental sulfur and polythionates are intermediates that are stored until lower concentrations of sulfide are encountered (131, 171). Colorless sulfur... 

S (attributed to polythionates or elemental S and to subsurface production of ester sulfates). Davison and Finlay (47) noted that photosynthetic bacteria, by maintaining low sulfide concentrations, prevented precipitation of FeS in the water column, but they did not evaluate the effect of the bacteria on S speciation in the sediments. Whereas specific S bacteria can occur only in well-defined conditions, a link between S speciation and the presence of these bacteria could be a useful paleolimnological tool for reconstructing previous lake conditions. 

This disproportionation reaction follows complex kinetics involving formation of polythionates (S3062-, S4062-, and S5062-) as intermediate products. The initial and rather slow decomposition during the induction period is later accelerated by thiosulfate, which functions as an autocatalyst. After a critical thiosulfate concentration has been reached, sulfur is precipitated, and the acidity increases rapidly. 

Alternatively, under less reducing conditions, conversion may involve the addition of sulphur (S) from intermediate sulphur species (e.g. polysulphides, polythionates or thiosulphate (S2( )i )), which are the products and reactants in microenvironmental sulphur redox cycling. The reaction involving S2O5- can be summarized as ... 

Older reports that certain chemical reactions (hydrolysis of S2CI2 [111], reaction of SO2 with H2S [138]) would produce polymeric sulfur are in error since the yellow product also contains long-chain polythionates which make it hydrophilic [139]. 

So-called Raffo or LaMer sulfur sols are used most frequently. These can be prepared by dropwise addition of aqueous sodium thiosulfate to concentrated sulfuric acid followed by coohng and precipitation of the hydrophilic sol by a saturated solution of sodium chloride. The sol originates from the spontaneous decomposition of the primary product thiosulfuric acid (H2S2O3) which disproportionates in a series of complex redox reactions producing elemental sulfur, hydrogen sulfide, sulfur dioxide and polythionic acids ... 

Thiosulfate Solutions. Aqueous thiosulfate is stable at room temperature for several weeks, but it decomposes within days at 100°C. The reaction products depend on the pH. Our Raman study essentially confirms the formation of products found by earlier authors ( 8, jj). Figure 3 summarizes our results. At high pH thiosulfate prevails at pH<7 two different product groups appear either elemental sulfur and sulfate appear, or polythionates rarely both product groups form simultaneously. At 2[Pg.120]

Regeneration. The products formed by treating sulfur dioxide solutions with hydrogen sulfide depend in part on the pH. When hydrogen sulfide is added to a solution of sulfur dioxide in water, a complex mixture is formed that includes polythionic acids, thiosulfuric acid, and colloidal sulfur (in contrast to the crystalline sulfur obtained in the citrate process). This is known as Wackenroder s solution and has been extensively studied. The composition varies with the conditions used. With excess hydrogen sulfide the final product is ultimately approximately 100% sulfur (6). While the overall stoichiometry of the reaction is the same as the gas phase Claus reaction, the chemistry is more complex. 

In Figure 3 the regeneration reactions were studied by measuring the concentrations of the reactants and the products as a function of time in contact with hydrogen sulfide. For phase one, as the bisulfite decreases, there is a simultaneous buildup of polythionate and thiosulfate. 

Regeneration Step Theory. The reaction of hydrogen sulfide and sulfur dioxide in aqueous solution has been studied extensively as the Wackenroder reaction. The consensus of various workers (9) is that the first stage of the reaction is the formation of an unstable intermediate acid that further reacts to produce the products observed, principally sulfur, thiosulfuric acid, and polythionic acids. The most prominent of the suggested intermediates are sulfoxylic acid (S(OH)2) (10) and thiosulfurous acid (H2S202) (11). Both intermediates have schemes to explain all of the various products formed. The major overall reactions are given below. 

For dilute acid, the reaction rate has been studied (17). The main products were reported to be sulfur and sulfite with only small amounts of polythionates. The time of sulfur appearance was measured, and based on this the initial rate of sulfur production was expressed as ... 

Reaction between sulfanemonosulfonates was also postulated to explain the formation of polythionates (sulfanedisulfonates) as a side product in this system. Two molecules of disulfanemonosulfonate could give pentathionate ... 

By bacterial sulfate reduction H S is produced as the extracellular end-product (Widdel and Hansen 1991). During the oxidation of H S, oxic or anoxic, chemical or biological, compounds such as zero-valent sulfur (in elemental sulfur, polysulfides, or polythionates), thiosulfate (S Oj ), and sulfite (SOj ) are produced (Cline and Richards 1969 Pyzik and Sommer 1981 Kelly 1988 Dos Santos Afonso and Stumm 1992). These intermediates may then be further transformed by one or several of the following processes ... 

One function of polythionic acids is to act as cathodic depolarizers, thereby stimulating dissolution of chromium-depleted grain-boundary material. Another possible function is that their cathodic reduction products (H2S or analogous compounds) stimulate absorption to interstitial hydrogen by chromium-depleted... 

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