Sulfur Dioxide and Trioxide

The United Kingdom, however, still produces 3 x 10 metric tons of sulfur emissions a year, and atmospheric sulfur dioxide causes further problems 

Sulfur dioxide dissolves readily in water to form sulfurous acid, H2SO3, which is a fairly strong acid itself, with pK a(l) - 1.91 and pK a(2) = 7.18, at 25 °C and infinite dilution.  

The sulfur-derived acidity in acid rain, however, is due mainly to H2SO4 rather than H2SO3. The direct reaction of gaseous SO2 with oxygen to give SO3 (a colorless substance, mp 17°C, bp 45 °C), and hence H2SO4 by hydrolysis, is extremely slow.  

Brandt and van Eldik ° review of the chemistry of sulfur(IV) oxidation, with emphasis on the catalytic role of metal ions such as Fe +. We consider here only a simplified summary of the principal atmospheric oxidation processes. It is likely that oxidation is effected primarily through the action of hydrogen peroxide or ozone in water droplets in clouds, through the photochemical effect of ultraviolet light, or by heterogeneous catalysis of the SO2-O2 reaction by dust particles  

Oxidation by H2O2 is probably the dominant process as noted earlier, H2O2 in clouds is formed from hydroxyl radicals, so the importance of OH as the main cleansing agent in the troposphere is again evident. In dry air (i.e., in the absence of clouds), the chief oxidant is probably OH itself (cf. reaction 8.15)  

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Sulfur dioxide and trioxide dissolve to give sulfurous and sulfuric acid. These acids are widely present in aqueous systems, particularly in the atmosphere. It is not possible to prepare pure sulfurous acid, but sulfuric acid can be obtained at high concentration. Sulfurous acid undergoes a range of reactions in atmospheric droplets, which are relevant to its removal from the atmosphere. Many of these reactions oxidize it to the stronger sulfuric acid, which has been a key component of acid rain. 

Sulfuric acid (including sulfur dioxide and trioxide)... 

Because fossil fuels are the remains of once-living organisms, fuels like coal contain sulfur. In electrical power plants, sulfur burns along with the carbon in coal, forming sulfur dioxide (SO ). There also is some sulfur in gasoline, so motor vehicles are also a source of SO3 (not SO2 because catalytic converters convert SO into SO3). Sulfur dioxide and trioxide are both noxious gases (see the section in chapter 3 titled The NO Problem ) and two of the main components of so-called London smog. Oxides of sulfur cause respiratory disease and contribute to the formation of haze that reduces atmospheric visibility. 

Damaging deposits of Na2S04 may originate alone from sea salt contamination of intake air. Sulfur dioxide and trioxide of oil combustion products also contribute, but hot corrosion of marine turbine blades may occur even with use of very low-sulfur fuels [38]. Fligh-chromium alloys are more resistant to hot corrosion than are low-chromium alloys. 

Distinguish between the dry and wet scrubbers used to rid coal-burning power plant effluents of sulfur dioxide and trioxide. 

Additives must rapidly absorb sulfur dioxide and trioxide as it is produced during catalyst regeneratiom The resulting sulfate is then reduced in the reactor and stripper to regenerate the additive and continue the cycle. The reactions taking place are shown schematically in Table 5.14. 

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