Oxidation in the Aqueous Phase

In the case of S02, oxidation in the aqueous phase, present in the atmosphere in the form of aerosol particles, clouds, and fogs, is also important. Thus S02 from the gas phase dissolves in these water droplets and may be oxidized within the droplet by such species as H202, 03, 02, and free radicals. Oxidation of S02 on the surfaces of solids either present in the air or suspended in the water droplets is also possible. On the other hand, it is believed that HN03 is formed primarily by reaction (10) in the gas phase and subsequently dissolves in droplets. 

FIGURE 7.14 Model-estimated increase in S(IV) oxidation in the aqueous phase of sea salt particles due to the uptake and reactions of NO,. O, taken as 40 ppb, NO, as 0.1 ppb, and H202 as 0.05 ppb. The Y-axis is the calculated ratio of oxidized sulfur, S(VI), formed in droplets when NO, chemistry is included to that when it is not (adapted from Rudich et al., 1998). 

As we shall see in the following sections, these observations are readily understood in terms of the kinetics and mechanisms of oxidation of S02. The oxidation of S02 occurs in solution and on the surfaces of solids as well as in the gas phase. Indeed, under many conditions typical of the troposphere, oxidation in the aqueous phase provided by clouds and fogs predominates, consistent with the observed dependence on these factors. The presence of oxidizers to react with the S02 is, of course, also a requirement hence the dependence on 03 (which is a useful surrogate for other oxidants as well) and sunlight, which is needed to generate significant oxidant concentrations. 

While the emphasis has been on oxidation of DMS and other reduced sulfur compounds in the gas phase, there is some indication that oxidation in the aqueous phase in clouds and fogs should also be considered. For example, Lee and Zhou (1994) have shown that DMS reacts with 03 in aqueous solutions quite rapidly, with a rate constant at 288 K of 4 X 108 L mol-1 s-1. They estimate that at 30 ppb 03, a level found globally, the lifetime for in-cloud oxidation of DMS is about 3 days, of the same order of magnitude as that for the gas-phase oxidation by OH (see Table 8.17). Given the moderately high solubility of not only DMS but other sulfur compounds as well (see Henry s law constants in Table 8.1), this is clearly an area that warrants further research. 

Similarly, the relationship between H and S04 has been shown to be an indicator of the acidity in particles at a variety of sites in Canada. For example, Brook et al. (1997) reported that the amounts of H and S04 in particles were veiy strongly correlated in St. John, New Brunswick (r 0.92), whereas at sites in Ontario and Quebec, the correlations were much weaker. The strong correlation between H and S04 in St. John was attributed to local emissions of S02, followed by uptake into the fogs common to that area, and efficient oxidation in the aqueous phase to H2S04 (see Chapter 8.C.3). At the other sites, there were larger concentrations of NH-, to at least partially neutralize the sulfuric acid. 

However, the situation is not as clear-cut for the liquid-phase oxidation, which, we have seen, predominates in many (perhaps most) situations. In this case, a less than 1 1 relationship between the reduction in H2S04 formed and S02 emitted may result for a number of reasons operating on the microscopic scale. For example, less H202 is available in many clouds than is needed to oxidize all of the S(IV) that is present, and hence the oxidation can be limited by the availability of oxidant (e.g., see Dutkiewicz et al., 1995). Another important factor that comes into play is the interplay between the acidity of the aqueous phase, the reaction kinetics, and the solubility of S(IV). Thus, as seen in Chapter 8, the solubility of S(IV) decreases as the aqueous phase becomes more acidic, limiting the total sulfur available for oxidation. In addition, all oxidations in the aqueous phase except that by H202 are pH dependent as a result, the contribution of... 

The 2,2,6,6-tetramethylpiperidinoxyl (TEMPO) radical was first prepared in 1960 by Lebedev and Kazarnovskii by oxidation of its piperidine precursor. TEMPO is a highly persistent radical, resistant to air and moisture, which is stabilized primarily by the steric hindrance of the NO-bond. Paramagnetic TEMPO radicals can be used as powerful spin probes for investigating the structure and dynamics of biopolymers such as proteins, DNA, and synthetic polymers by ESR spectroscopy [7]. A versatile redox chemistry has been reported for TEMPO radicals. The radical species can be transformed by two-electron reduction into the respective hydroxyl-amine or by two-electron oxidation into the oxoammonium salt [8]. One-electron oxidations involving oxoammonium salts have also been postulated [9]. The TEMPO radical is usually employed under phase-transfer conditions with, e.g., sodium hypochlorite as activating oxidant in the aqueous phase. In oxidations of primary alcohols carboxylic acids are often formed by over-oxidation, in addition to the de-... 

Chemical reactions occur in the gas phase and in the aqueous phase (cloud droplets) that both oxidize elemental mercury to divalent mercury and reduce the divalent mercury to elemental mercury. The most important gas phase oxidation pathways are the reactions with ozone (Hall 1995) and OH radicals (Sommar etal. 2001). Small amounts of Hg , which are dissolved in liquid water in the atmosphere (fog or clouds), can also be oxidized by ozone (Munthe 1992) or by OH radicals (Garfeldt et al. 2001). The oxidation in the aqueous phase occurs at a significantly higher rate than in the gas phase but, due to the low solubility of Hg in water and the low liquid water content in the atmosphere, the overall rate of oxidation is comparable to the gas phase oxidation rate (Pirrone etal. 2001). Reduction of divalent mercury back to Hg may also occur by sulfite (S03 ) ions or HO2 radicals furthermore, complexation of divalent mercury with soot may occur to form particulate divalent mercury (figure 17.2). 

In all the reactions considered up to now the reactants and products are gaseous, but porous membranes have also been applied in some three-phase reactions. Reactions tested in this case have included oxidation in the aqueous phase with air or the oxidation of organic substances with H2O2. The oxidation of organic compounds in the aqueous phase has recently attracted the interest of several researchers as a way to remove pollutants. In this kind of reactor the membrane provides an interphase separating the liquid and the gas phase, and in some cases allows the concentration of a gaseous reactant to be handled. 

Manufacture and Processing. Mononitrotoluenes are produced by the nitration of toluene in a manner similar to that described for nitrobenzene. The presence of the methyl group on the aromatic ring faciUtates the nitration of toluene, as compared to that of benzene, and increases the ease of oxidation which results in undesirable by-products. Thus the nitration of toluene generally is carried out at lower temperatures than the nitration of benzene to minimize oxidative side reactions. Because toluene nitrates at a faster rate than benzene, the milder conditions also reduce the formation of dinitrotoluenes. Toluene is less soluble than benzene in the acid phase, thus vigorous agitation of the reaction mixture is necessary to maximize the interfacial area of the two phases and the mass transfer of the reactants. The rate of a typical industrial nitration can be modeled in terms of a fast reaction taking place in a zone in the aqueous phase adjacent to the interface where the reaction is diffusion controlled. 

After epoxidation, propylene oxide, excess propylene, and propane are distilled overhead. Propane is purged from the process propylene is recycled to the epoxidation reactor. The bottoms Hquid is treated with a base, such as sodium hydroxide, to neutralize the acids. Acids in this stream cause dehydration of the 1-phenylethanol to styrene. The styrene readily polymerizes under these conditions (177—179). Neutralization, along with water washing, allows phase separation such that the salts and molybdenum catalyst remain in the aqueous phase (179). Dissolved organics in the aqueous phase ate further recovered by treatment with sulfuric acid and phase separation. The organic phase is then distilled to recover 1-phenylethanol overhead. The heavy bottoms are burned for fuel (180,181). 

As already noted (p. 1073), the platinum metals are all isolated from concentrates obtained as anode slimes or converter matte. In the classical process, after ruthenium and osmium have been removed, excess oxidants are removed by boiling, iridium is precipitated as (NH4)2lrCl6 and rhodium as [Rh(NH3)5Cl]Cl2. In alternative solvent extraction processes (p. 1147) [IrClg] " is extracted in organic amines leaving rhodium in the aqueous phase to be precipitated, again, as [Rh(NH3)5Cl]Cl2. In all cases ignition in H2... 

Poloxamers are used primarily in aqueous solution and may be quantified in the aqueous phase by the use of compleximetric methods. However, a major limitation is that these techniques are essentially only capable of quantifying alkylene oxide groups and are by no means selective for poloxamers. The basis of these methods is the formation of a complex between a metal ion and the oxygen atoms that form the ether linkages. Reaction of this complex with an anion leads to the formation of a salt that, after precipitation or extraction, may be used for quantitation. A method reported to be rapid, simple, and consistently reproducible [18] involves a two-phase titration, which eliminates interferences from anionic surfactants. The poloxamer is complexed with potassium ions in an alkaline aqueous solution and extracted into dichloromethane as an ion pair with the titrant, tet-rakis (4-fluorophenyl) borate. The end point is defined by a color change resulting from the complexation of the indicator, Victoria Blue B, with excess titrant. The Wickbold [19] method, widely used to determine nonionic surfactants, has been applied to poloxamer type surfactants 120]. Essentially the method involves the formation in the presence of barium ions of a complex be-... 

Redox initiation is commonly employed in aqueous emulsion polymerization. Initiator efficiencies obtained with redox initiation systems in aqueous media are generally low. One of the reasons for this is the susceptibility of the initially formed radicals to undergo further redox chemistry. For example, potential propagating radicals may be oxidized to carbonium ions (Scheme 3.44). The problem is aggravated by the low solubility of the monomers (e.g. M VIA. S) in the aqueous phase. 

Under aqueous conditions, flavonoids and their glycosides will also reduce oxidants other than peroxyl radicals and may have a role in protecting membranal systems against pro-oxidants such as metal ions and activated oxygen species in the aqueous phase. Rate constants for reduction of superoxide anion show flavonoids to be more efficient than the water-soluble vitamin E analogue trolox (Jovanovic et al, 1994), see Table 16.1. 

Nonaqueous Systems In nonaqueous (nonpolar) solvent systems, nitrosatlon also proceeds. In these solvents, alpha-tocopherol acts as a lipid soluble blocking agent in much the same fashion as ascorbic acid functions in the aqueous phase. Alpha-tocopherol reacts with a nitrosating agent and reduces it to nitric oxide. At the same time, alpha-tocopherol is oxidized to tocoquinone, which is the first oxidation product of vitamin E and also a normal metabolite in vivo. 

The selectivity issue has been related to multi-phase processing [31]. Nitrations include both organic and aqueous phases. Oxidation to phenol as one side reaction takes places in the organic phase, whereas all other reactions occur in the aqueous phase and are limited by organic solubility. For this reason, enhancing mass transfer by large specific interfaces is a key to affecting product selectivity. 

FIG. 2 Cyclic voltammogram of the ferricenium transfer across the water-DCE interface at lOmVs. The electrochemical cell featured a similar arrangement to Fig. 1(b), but the organic phase contained 2mM of ferrocene. Heterogeneous oxidation of Fc occurred in the presence of 0.2mM CUSO4 in the aqueous phase. Supporting electrolytes were lOmM 02804 and lOmM BTPPATPBCl. The transfer of the standard tetramethylammonium (TMA+) under the same condition is also superimposed. 

FIG. 19 Dependence of the half-wave potentials for Fc (curve 1) and ZnPor (curve 2) oxidation in benzene on CIO7 concentration in the aqueous phase. In these measurements, half-wave potentials were extracted from reversible steady-state voltammograms obtained at a 25 pm diameter Pt UME. The benzene phase contained 0.25 M tetra-w-hexylammonium perchlorate (THAP) and either 5 mM Fc or 1 mM ZnPor. All potentials were measured with respect to an Ag/AgCl reference electrode in the aqueous phase. (Reprinted from Ref. 48. Copyright 1996 American Chemical Society.)... 

In these studies, CIO4 was used as the potential-determining ion, with 0.1 M THAP employed as the supporting electrolyte in the DCE phase, together with various concentrations of NaC104 in the aqueous phase. In contrast to studies of ZnPor oxidation [80],... 

The ET reaction between aqueous oxidants and decamethylferrocene (DMFc), in both DCE and NB, has been studied over a wide range of conditions and shown to be a complex process [86]. The apparent potential-dependence of the ET rate constant was contrary to Butler-Volmer theory, when the interfacial potential drop at the ITIES was adjusted via the CIO4 concentration in the aqueous phase. The highest reaction rate was observed with the smallest concentration of CIO4 in the aqueous phase, which corresponded to the lowest driving force for the oxidation process. In contrast, the ET rate increased with driving force when this was adjusted via the redox potential of the aqueous oxidant. Moreover, a Butler-Volmer trend was found when TBA was used as the potential-determining ion, with an a value of 0.38 [86]. 

FIG. 23 SECM approach curves for the reaction between tip-generated aqueous IrClg (via the oxidation of IrClg ) and DMFc in DCE obtained with a 25 pm diameter Pt UME. The potential across the ITIES was established with 0.1 M NaC104 in the aqueous phase and 0.1 M TEIAP in the DCE phase. The aqueous phase also contained 0.1 M NaCl and 1 mM IrClg. The solid curves are the experimental data for K. = 2 (upper curve) and 1 (lower curve), and the dashed theoretical curves are for ki2 = 180 cm s M for these two cases. The dotted theoretical curves are shown for ki2 = 1000 cm s M with K = 2 (top curve) and 1 (bottom curve) which is close to the diffusion-limited behavior. (Reprinted from Ref 86. Copyright Elsevier Science.)... 

The driving force for the transfer process was the enhanced solubility of Br2 in DCE, ca 40 times greater than that in aqueous solution. To probe the transfer processes, Br2 was recollected in the reverse step at the tip UME, by diffusion-limited reduction to Br . The transfer process was found to be controlled exclusively by diffusion in the aqueous phase, but by employing short switching times, tswitch down to 10 ms, it was possible to put a lower limit on the effective interfacial transfer rate constant of 0.5 cm s . Figure 25 shows typical forward and reverse transients from this set of experiments, presented as current (normalized with respect to the steady-state diffusion-limited current, i(oo), for the oxidation of Br ) versus the inverse square-root of time. 

MEMED has also been used to investigate the nature of coupled ion-transfer processes involved in spontaneous electron transfer at ITIES [80]. In this application, a key strength of MEMED is that all of the reactants and products involved in the reaction can be measured, as shown in Figs. 19 and 20. The redox reaction studied involved the oxidation of either ferrocene (Fc) or decamethylferrocene (DMFc) in a DCE phase (denoted by Fcdce) by either IrCle or Fe(CN)g in the aqueous phase (denoted by Ox ) ... 

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