Sulfur oxidation state

At ID21 beamline p-XANES and p-XRF maps have been obtained along the same transects analyzed at ID18F. The maps were carried out in area of 100 x 100 pm with a resolution of 1 x 1 pm. The sulfur oxidation state was determined by scanning the energy of the exciting beam across the sulfur absorption K-edge (total-S = 2.55 KeV sulfide-S = 2.472 KeV sulfate-S = 2.482 KeV). 

The effect of the sulfur oxidation state on the proton NMR resonances of 6-methylthiopurine deoxyribonucleosides has been reported <1996T10737>, showing that the chemical shifts of the ring protons moved downfield as the oxidation state increased and the substituent became more electron-withdrawing (Figure 1). Similarly, the effect on purine proton resonances of 8-fluoro substitution has been detailed <1996JA10408>. 

The reactions that involve sulfur oxidation states leave isotopically fractionated sulfur and hence sulfide, a target for investigation by the geologist. Though there is controversy about sulfur isotope fractionation (Farquhar et al, 2000), the strong fractionation of seen in... 

M HCl. The insets show the first derivatives of the pre-edge feature (left) and the K-edge (dashed lines fit). The bottom trace is the difference between the experimental and the fit. Reproduced from P. Frank et al, Coord. Chem. Rev., 237, 31-39. Copyright (2003), with permission from Elsevier. Right top, absorption spectrum bottom, first derivative. The arrows indicate disulfide, sulfonate and sulfate. The K-edge maxima of the various sulfur oxidation states are marked on the central horizontal line. Reproduced from P. Frank et al., Inorg. Chem. 38, 260-270. Copyright (1999), with permission from the American Chemical Society. 

The most stable and predominant form of dissolved phosphate in marine sediments is orthophosphate (henceforth referred to as phosphate) with an oxidation state of -E5. Phosphate, unlike nitrogen (oxidation state varying between — 3 and -E 5) and sulfur (oxidation state —2 to -E6), is not directly involved in redox reactions. Phosphate in solution is chiefly present as ion pairs with the major cations of seawater (Kester and Pytkowicz, 1967). In sediments it is also found in organic matter, adsorbed on hydrous ferric oxides (Mortimer, 1941, 1971 Mackereth, 1966 Stumm and Leckie, 1970), adsorbed on clay minerals (Chen, 1972), and as various forms of apatite. 

Sulfate is the stable sulfur oxidation state in aerobic soils, and sulfide is stable in anaerobic soils. Sulfur changes its oxidation state by microbial catalysis and the changes seem to be much more reversible than nitrogen and carbon reactions. Elemental sulfur is rare naturally in soils but is sometimes added to soils as an amendment, and sulfides are common in many mining wastes. When elemental sulfur and sulfides are exposed to oxygen, they oxidize to H2SO4. Soil acidities as high as pH 2 may persist until the sulfide or sulfur has all been oxidized and leached away. 

Figures 4.4 and 4.5 show the equilibrium relations of the sulfur oxidation states— sulfate, elemental sulfur, and sulfide—which are stable within the stability limits of water. The stable oxidation states are similar to those of nitrogen. Elemental sulfur, however, is much less stable than is elemental nitrogen and is stable only under acid conditions.

Table 2.1 lists atmospheric sulfur compounds. The principal sulfur compounds in the atmosphere are H2S, CH3SCH3, CS2, OCS, and SO2. Sulfur occurs in five oxidation states in the atmosphere. (See Box) Chemical reactivity of atmospheric sulfur compounds is inversely related to their sulfur oxidation state. Reduced sulfur compounds, those with oxidation state -2 or —1, are rapidly oxidized by the hydroxyl radical and, to a lesser extent, by other species, with resulting atmospheric lifetimes of a few days. The water solubility of sulfur species increases with oxidation state reduced sulfur species occur preferentially in the gas phase, whereas the (+6) compounds often tend to be found in particles or droplets. Once converted to compounds in the S(+6) state, sulfur species residence times are determined by removal by wet and dry deposition. 

Vairavamurthy A (1998) Using X-ray absorption to probe sulfur oxidation states in complex molecules. 

With an intermediate valence of 4 and a sulfur oxidation state of 44, SO2 can act as both a reducing agent and an oxidizing agent. The reducing properties, however, are more commonly of interest. Thus, aqueous SO2 (or H2SO3) reduces both bromine and... 

The free dithionous acid, H2S2O4, is not known, but its salts (sulfur oxidation state = +3) can be prepared by reducing sulfites (S = +4) with zinc dust, as shown in Equation (17.19) ... 

Effect of Sulfur Oxidation State on Hydrogels Properties... 

---