Thiosulfates, decompositions

Sulfur Melting. Under the temperature conditions used in the sulfur melting step (>125°C) there is some thiosulfate decomposition. The reaction consumes hydrogen ions and forms sulfur and sulfate as products. The ratio found of 1.37 moles of sulfur formed per mole of thiosulfate reacted suggests that the main form of the decomposition involves formation of sulfur and bisulfite from the thiosulfate, followed by decomposition of the bisulfite (5) via trithionate to sulfate. The sequence is ... 

Purging. In addition to oxidation, sulfate is present from any sulfuric mist removed from the inlet gas and thiosulfate decomposition during the sulfur melting step. While the rate of sulfate formation from all these sources is small, the effect is cumulative and sulfate must be purged from the system. 

Although thiosulfate is one of the few reducing titrants not readily oxidized by contact with air, it is subject to a slow decomposition to bisulfite and elemental sulfur. When used over a period of several weeks, a solution of thiosulfate should be restandardized periodically. Several forms of bacteria are able to metabolize thiosulfate, which also can lead to a change in its concentration. This problem can be minimized by adding a preservative such as Hgl2 to the solution. 

The decomposition of dithionite in aqueous solution is accelerated by thiosulfate, polysulfide, and acids. The addition of mineral acid to a dithionite solution produces first a red color which turns yellow on standing subsequentiy, sulfur precipitates and evolution of sulfur dioxide takes place (346). Sodium dithionite is stabilized by sodium polyphosphate, sodium carbonate, and sodium salts of organic acids (347). 

Acidification of thiosulfate with strong acid invariably leads to decomposition with the formation of colloidal sulfur and sulfur dioxide. The mechanism of this reaction is complex and depends on the thiosulfate concentration and the pH (14). The following reaction explains the formation of the main products ... 

These three methods are employed commercially. In addition, decomposition of polythionates in alkaline solution or their reaction with sulfide or sulfite gives thiosulfates ... 

Aqueous sodium thiosulfate solutions ate neutral. Under neutral or slightly acidic conditions, decomposition produces sulfite and sulfur. In the presence of air, alkaline solutions decompose to sulfate and sulfide. Dilute solutions can be stabilized by small amounts of sodium sulfite, sodium carbonate, or caustic, and by storage at low temperatures away from air and light. Oxidation is inhibited by Hgl2 (10 Ppm) amyl alcohol (1%), chloroform (0.1%), borax (0.05%), or sodium benzoate (0.1%). 

The most common form of calcium thiosulfate is the hexahydrate [10035-02-6] CaS202 6H20, which has triclinic crystals and a density of 1.872 g/cm at 16°C (84). Heating, however, does not give the anhydrous salt because of decomposition at 80°C. At lower temperatures, dehydration stops at the monohydrate [15091-91-5]. The solubiUty of calcium thiosulfates in water is as follows ... 

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... 

Process (3.8) is a total 2e per cadmium atom and indicates that CdS formation occurs via a sulfur atom abstraction from 8203 . This reaction was called for in order to suggest that the reduction of Cd " is the only electrochemical step, whereby charge is consumed, followed by a subsequent chemical step comprising sulfur association to reduced cadmium. Sulfur is generated by the decomposition of thiosulfate. On the other hand, reaction (3.9) corresponds to an overall 4e /Cd process where reduction of S2O3 itself must occur as well as that of Cd ", the former comprising actually the rate-determining step. This route becomes more favorable as pH decreases for it requires additional protons. 

Stablizers. Stabilizers are ingredients added to a formula to decrease the rate of decomposition of the active ingredients. Antioxidants are the principal stabilizers added to some ophthalmic solutions, primarily those containing epinephrine and other oxidizable drugs. Sodium bisulfite or metabisulfite are used in concentration up to 0.3% in epinephrine hydrochloride and bitartrate solutions. Epinephrine borate solutions have a pH range of 5.5 7.5 and offer a more difficult challenge to formulators who seek to prevent oxidation. Several patented antioxidant systems have been developed specifically for this compound. These consist of ascorbic acid and acetylcysteine, and sodium bisulfite and 8-hydroxyquinoline. Isoascorbic acid is also an effective antioxidant for this drug. Sodium thiosulfate is used with sodium sulfacetamide solutions. 

Determination of Additive Effects on the Decomposition of tert-Butyl Hydroperoxide and Hydrogen Peroxide. Solutions of tert-butyl hydroperoxide (1.0 mmol) and 30% aqueous hydrogen peroxide (1.32 mmol) in 5 mL of tert-butyl alcohol with the various additives (Tables 9 and 10) were held at 80°C for 24 hr. Peroxide analyses were obtained by sodium iodide/0.05N sodium thiosulfate titration. 

Alkaline conditions must be used to prevent the decomposition of thiosulfate. The resulting pregnant leach solutions also contain the monothiosulfato complex, [Au(S203)] .49... 

The decomposition of thiosulfate ion is promoted in an acidic solution. If the white solid is Na2SOreaction with strong acids such as HC1. By contrast, if the white solid is Na2S203, S02(g) will be liberated and a pale yellow precipitate of S(s,rhombic) will form upon addition of HCl(aq). 

Cyclic voltammetry of iron(III) porphyrin-sulfate complexes has been described. Thiosulfate can add to iron(III) porphyrins to give an adduct which is high-spin at normal temperatures but low-spin at low temperatures. The tetraphenylporphyrin adduct undergoes decomposition slowly in DMF to give [Fe (tpp)] plus tetrathionate. In DMSO tetraphenylporphyrinatoiron(III) oxidizes thiosulfate by an autocatalytic process. Tetrathiotungstate complexes of iron(III)-tetra-phenylporphyrin undergo spontaneous reduction to iron(II) products with a half-life of about 30 minutes at ambient temperature. " ... 

Sg was first obtained by acid decomposition of aqueous sodium thiosulfate which according to recent results yields a mixture of Sg, S7 and Sg ... 

Ed, the activation energy for thermal initiator decomposition, is in the range 120-150 kJ mol-1 for most of the commonly used initiators (Table 3-13). The Ep and Et values for most monomers are in the ranges 20-40 and 8-20 kJ mol-1, respectively (Tables 3-11 and 3-12). The overall activation energy Er for most polymerizations initiated by thermal initiator decomposition is about 80-90 kJ mol-1. This corresponds to a two- or threefold rate increase for a 10°C temperature increase. The situation is different for other modes of initiation. Thus redox initiation (e.g., Fe2+ with thiosulfate or cumene hydroperoxide) has been discussed as taking place at lower temperatures compared to the thermal polymerizations. One indication of the difference between the two different initiation modes is the differences in activation energies. Redox initiation will have an Ed value of only about 40-60 kJ mol-1, which is about 80 kJ mol-1 less than for the thermal initiator decomposition [Barb et al., 1951], This leads to an Er for redox polymerization of about 40 kJ mol-1, which is about one half the value for nonredox initiators. 

Fig. 5.1.2 Monodispersed BaS04 ellipsoids of 0.14 pm mean size, prepared by homogene-oas precipitation with the slow release of S042- ions through decomposition of thiosulfate ions by hydrogen peroxide in the presence of Ba2+ ions at 20°C. To reduce the mean size, the reaction was carried out with sodium citrate (0.1 mol dm-3). (From Ref. 1.)...

The appearance of turbidity indicates saturation of alkyl halide. In this way both sodium thiosulfate and 2-bromopropane are nearly in a one-phase system, thus shortening significantly the heating period. Furthermore, the competitive hydrogen bromide elimination and the ensuing acid-promoted decomposition of thiosulfate into sulfur and sulfur dioxide are minimized, the checkers added 300 ml. of water over a period of 90 minutes. 

Kinetic Studies. Peracetic Ac id Decomposition. Studies with manganese catalyst were conducted by the capacity-flow method described by Caldin (9). The reactor consisted of a glass tube (5 inches long X 2 inches o.d.), a small centrifugal pump (for stirring by circulation), and a coil for temperature control (usually 1°C.) total liquid volume was 550 ml. Standardized peracetic acid solutions in acetic acid (0.1-0.4M) and catalyst solutions also in acetic acid were metered into the reactor with separate positive displacement pumps. Samples were quenched with aqueous potassium iodide. The liberated iodine was titrated with thiosulfate. Peracetic acid decomposition rates were calculated from the feed rate and the difference between peracetic acid concentration in the feed and exit streams. 

Performic Acid. Contact with the nitrate may lead to explosive decomposition.8 Phenol and Trifluoroacetic Acid. Rapid exothermic reaction occurs in mixture.9 Sodium. An explosive compound is formed by interaction with Na.10 Sodium Thiosulfate or Sodium Phosphinate. Mixture explodes on heating.11... 

It is imperative that an excess of selenious acid or tellu-rous acid be present at every stage of the process, since thiosulfate catalyzes the decomposition of selenopentathio-nate and telluropentathionate. ... 

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