Sulfur dioxide aqueous phase reactions

Sulfur dioxide (SO ) and nitrogen oxides (NO ) are oxidized to sulfate and nitrate aerosols either homogeneously rn the gas phase or heterogeneously in atmospheric microdroplets and hydrometeors Gas-phase production of nitric acid appears to be the dominant source of aerosol nitrate because the aqueous phase reactions of NO (aq) are slow at the nitrogen oxide partial pressures typically encountered in the atmosphere (5,i5). Conversely, field studies indicate that the relative importance of homogeneous and heterogeneous SO2 oxidation processes depends on a variety of climatological factors such as relative humidity and the intensity of solar radiation (4, -1 ). 

Quantitative analysis of different reaction pathways for the transformation of aquated sulfur dioxide in atmospheric droplet systems has been a major objective of the research conducted in the principal investigator s laboratory for the last four years. Available thermodynamic and kinetic data for the aqueous-phase reactions of SO2 have been incorporated into a dynamic model of the chemistry of urban fog that has been developed by Jacob and Hoffmann (23) and Hoffmann and Calvert (39). The fog and cloud water models developed by them are hybrid kinetic and equilibrium models that consider the major chemical reactions likely to take place in atmospheric water droplets. Model results have verified that... 

Carbon Dioxide/Water Equilibrium 345 Sulfur Dioxide 348 Ammonia/Water Equilibrium 353 Nitric Acid/Water Equilibrium 355 Equilibrium of Other Important Atmospheric Gases Aqueous-Phase Reaction Rates 361 S(IV) to S(VI) Transformation and Sulfur Chemistry 363... 

Continental aerosol particles contain a significant fraction of minerals. The insoluble fraction consists mainly of the major crustal elements silicon, aluminum and trivalent iron, which occur as alumino-silicates, quartz, and iron oxides. Elements that are eluted from minerals by water are sodium, potassium, calcium (inpart), and magnesium. The water-soluble inorganic salt ftaction is dominated by am-monimn sulfate. Again, sulfate arises from the oxidation of sulfur dioxide, both by gas-phase and by aqueous phase reactions. Whereas the mineral components are mainly found in the coarse particle size range, ammonium sulfate resides mainly in the accumulation mode. Nitrate occurs partly in association with ammoniirm in the accumulation mode, and partly together with sodiirm and other cations in the coarse particle mode. Thus, nitrate often shows a bimodal size distribution. 

Snia Viscosa. Catalytic air oxidation of toluene gives benzoic acid (qv) in ca 90% yield. The benzoic acid is hydrogenated over a palladium catalyst to cyclohexanecarboxyhc acid [98-89-5]. This is converted directiy to cmde caprolactam by nitrosation with nitrosylsulfuric acid, which is produced by conventional absorption of NO in oleum. Normally, the reaction mass is neutralized with ammonia to form 4 kg ammonium sulfate per kilogram of caprolactam (16). In a no-sulfate version of the process, the reaction mass is diluted with water and is extracted with an alkylphenol solvent. The aqueous phase is decomposed by thermal means for recovery of sulfur dioxide, which is recycled (17). The basic process chemistry is as follows ... 

Reactions between A -(l-chloroalkyl)pyridinium chlorides 33 and amino acids in organic solvents have a low synthetic value because of the low solubility of the amine partner. A special protocol has been designed and tested in order to circumvent this drawback. Soon after the preparation of the salt, an aqueous solution of the amino acid was introduced in the reaction medium and the two-phase system obtained was heated under reflux for several hours. However, this was not too successful because sulfur dioxide, evolved during the preparation of the salt, was converted into sulfite that acted as an 5-nucleophile. As a result, A -(l-sulfonatoalkyl)pyridinium betaines such as 53 were obtained (Section IV,B,3) (97BSB383). To avoid the formation of such betaines, the salts 33 were isolated and reacted with an aqueous solution of L-cysteine (80) to afford thiazolidine-4-carboxylic acids hydrochlorides 81 (60-80% yields). 

This difference in kinetics was exploited to develop a procedure to determine free and reversibly bound sulfite in food. The mobile phase consisted of an aqueous solution of 0.05 M tetra-butylammonium hydroxide adjusted to the desired pH by the addition of glacial acetic acid (34). Fluorimetric detection is also possible, because a reaction of the formaldehyde-bisulfite complex with 5-aminofluorescein gives a nonfluorescent product. The sulfite is measured indirectly by its suppresion of the fluorescence of the reagent (31). This method is applicable to the determination of S02 at > 10 ppm and is not applicable to dark-colored foods or ingredients where SO, is strongly bound, e.g., caramel color. This method does not detect naturally occurring sulfite. Sulfur dioxide is released by direct alkali extraction. 

Sulfur dioxide is oxidized in the atmosphere mainly within clouds, fogs and other aqueous-phase domains. The primary pathway [111-115] involves oxidation with H2O2 however, other reaction pathways are viable depending on pH. Some of the direct and indirect photochemical reactions of interest related to the fate of SO2 in the atmosphere include ... 

The oxidation reaction is interrupted by the addition of 10 ml of ethanol. After 10 min, the manganese dioxide is filtered off through a layer of water-washed fumed silica ( Kieselguhr , Fluka AG Switzerland) and washed with a small amount of sodium bicarbonate (1% solution). The aqueous phase is extracted with 2 x 50 ml of diethyl ether, the ether in turn is extracted with 15 ml of sodium bicarbonate (1% solution) and, after adjustment of the pH to 6.5 with 9M sulfuric acid, the combined aqueous extracts are evaporated to a small volume (30 ml). 

The process flowsheet as presently developed is shown in Figure 17. The exothermic Bunsen reaction produces two aqueous solutions of sulfuric acid and hydriodic acid from material feeds of water, sulfur dioxide and iodine. The reaction favors presence of excess water and iodine to make it spontaneous and with iodine rich hydriodic acid (Hix) formed to facilitate subsequent phase separation. The excess of water and iodide, however, imposes heavy process stream loads upon subsequent reactions, particularly so in the HI reaction steps. Though not yet reflected in the present flowsheet, improved reaction conditions are being studied with the goal of significantly reducing excessive reactants in order to simplify overall process and production cost. 

Sulfur dioxide (S02) reacts under tropospheric conditions via both gas-and aqueous-phase processes (see Section X) and is also removed physically via dry and wet deposition. With respect to chemical removal, reaction with the OH radical is dominant ... 

The chemistry that occurs in cloud and fog droplets in the atmosphere has been shown, in the last decade or so, to be highly complex. Most atmospheric species are soluble to some extent, and the liquid-phase reactions that are possible lead to a diverse spectrum of products. The aspect of atmospheric aqueous-phase chemistry that has received the most attention is that involving dissolved S02. Sulfur dioxide is not particularly soluble in pure water, but the presence of other dissolved species such as H202 or 03 displaces the dissolution equilibrium for S02, effectively... 

Atmospheric reactions modify the physical and chemical properties of emitted materials, changing removal rates and exerting a major influence on acid deposition rates. Sulfur dioxide can be converted to sulfate by reactions in gas, aerosol, and aqueous phases. As we noted in Chapter 17, the aqueous-phase pathway is estimated to be responsible for more than half of the ambient atmospheric sulfate concentrations, with the remainder produced by the gas-phase oxidation of S02 by OH (Walcek et al. 1990 Karamachandani and Venkatram 1992 Dennis et al. 1993 McHenry and Dennis 1994). These results are in agreement with box model calculations suggesting that gas-phase daytime S02 oxidation rates are l-5% per hour, while a representative in-cloud oxidation rate is 10% per minute for 1 ppb of H202. 

Instead of the use of sulfuryl chloride, SO2 and CI2, which combine to give sulfuryl chloride, can be employed for vapor-phase reactions. When the ratio of SO2 to CI2 is adjusted, it is feasible to produce sulfonic or chlorosulfonic acid derivatives of saturated paraffins in liquid-phase reactions. According to Lockwood and Richmond, a premixed stream of sulfur dioxide and chlorine can be used for the countercurrent photochlorination of white oil (petroleum fraction, sp gr, 0.8033 bp, 283-324 C) at about 50°C. The product, after hydrolysis with 30 per cent sodium hydroxide, yields an aqueous solution of the sodium salt of a sulfonic or chlorosulfonic acid. An increase in the ratio of SO2 to CI2 greatly increases the yield of solubilized product. When a 3 1 ratio is used, the product contains very little chlorine and has good wetting and detergent characteristics. ... 

Atmospheric reactions modify the physical and chemical properties of emitted materials, changing removal rates and exerting a major influence on acid deposition rates. Sulfur dioxide can be converted to sulfate by reactions in gas, aerosol, and aqueous phases. As we... 

When dissolved into the aqueous phase, the atmospheric constituents alter the chemistry of the aqueous layer through various chemical or electrochemical reactions. One important chemical process is the deposition of sulfur dioxide into the aqueous layer to form bisulfite (HS03 ) ions ... 

The above reaction mechanisms for reduced sitlfmcom-potmds make evident that sulfur dioxide is the principal oxidation product in all cases. The further oxidation of sulfur dioxide to sulfuric acid occurs in the gas phase as well as in the aqueous phase of clouds. Aqueous reactions are discussed in Section XI. The gas-phase oxidation... 

In other systems, similar effects of dissolved gaseous species can be important and may require the use of HT/HP corrosion test procedures to give accurate simulation of service environments. Examples of such conditions are those that contain carbon dioxide, sulfur dioxide, and NOx, which can determine the pH of the aqueous phase and affect the severity of corrosion reactions. 

The Arrhenius theory was the first scientific theory of acidity acids provided hydrogen ions, H (aq), in an aqueous solution (as the only cations) and bases provided hydroxide ions, OH"(aq) in aqueous solution (as the only anions). One of the problems with the Arrhenius theory is that it is rather restrictive since many reactions are carried out in solvents other than water or in the gas phase in the absence of solvent. These non-aqueous solvents include liquid ammonia and liquid sulfur dioxide. 

This latter reaction is similar to the gas-phase reactions between S02" and SO2. Chemical reduction by alkali metals can also be used and work with lithium suggests that the complex 8204 also dimerizes [29]. In non-aqueous solvents, the reaction of sulfur dioxide with superoxide produces the sulfur dioxide radical anion, but with an equilibrium constant of only about =1.1 [30], reaction (16) ... 

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