Uncatalyzed Oxidation of S IV by

The importance of the reaction of S(IV) with dissolved oxygen in the absence of any metal catalysts (iron, manganese) has been a controversial issue. Solutions of sodium sulfite in the laboratory oxidize slowly in the presence of oxygen (Fuller and Crist 1941 Martin 1984). However, observations of Tsunogai (1971) and Huss et al. (1978) showed that the rate of the uncatalyzed reaction is negligible. The observed rates can be explained by the existence of very small amounts of catalyst such as iron (concentrations lower than 0.01 pM) that are extremely difficult to exclude. It is interesting to note that for real cloud droplets there will always be traces of catalyst present (Table 7.5), so the rate of an uncatalyzed reaction is irrelevant (Martin 1984). 

TABLE 7.5 Manganese and Iron Concentrations in Aqueous Particles and Drops 

This reaction has been the subject of considerable interest (Hoffmann and Boyce 1983 Hoffmann and Jacob 1984 Martin 1984 Hoffmann and Calvert 1985 Clarke and Radojevic 1987), but significantly different measured reaction rates, rate laws, and pH dependences have been reported (Hoffmann and Jacob 1984). Martin and Hill (1987a,b) showed that this reaction is inhibited by increasing ionic strength, the sulfate ion, and various organics and is even self-inhibited. They explained most of the literature discrepancies by differences in these factors in various laboratory studies. 

In the presence of oxygen, iron in the ferric state, Fe(III), catalyzes the oxidation of S(IV) in aqueous solutions. Iron in cloudwater exists both in the Fe(II) and Fe(III) states and there are a series of oxidation-reduction reactions cycling iron between these two forms (Stumm and Morgan 1996). Fe(II) appears not to directly catalyze the reaction and is first oxidized to Fe(III) before S(IV) oxidation can begin (Huss et al. 1982a,b). The equilibria involving Fe(III) in aqueous solution are 

For pH values from 0 to 3.6 the iron-catalyzed S(IV) oxidation rate is first order in iron, is first order in S(IV), and is inversely proportional to [H+] (Martin and Hill 1987a)  

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