Lower oxidation states sulfides

Presence of oxidizable substances in the sample would interfere in the test, thus giving high results. These include S2. S 0,2. and certain metal ions such as Fe2+ in lower oxidation state. Sulfide should be removed by adding 0.5 g zinc acetate, allowing the zinc sulfide precipitate to settle and drawing out the supernatant liquid for analysis. If thiosulfate is present, determine its concentration in an aliquot of sample by iodometric titration using iodine standard. Subract the concentration of thiosulfate from the iodometric sulfite results to calculate the true value of SO,2. ... 

Some metal thiosulfates are inherently unstable because of the reducing properties of the thiosulfate ion. Ions such as Fe " and Cu " tend to be reduced to lower oxidation states, whereas mercury or silver, which form sulfides of low solubiUty, tend to decompose to the sulfides. The stabiUty of other metal thiosulfates improves in the presence of excess thiosulfate by virtue of complex thiosulfate formation. 

The sulfides are fewer and less familiar than the oxides but, as is to be expected, favour lower oxidation states of the metals. Thus manganese forms MnS2 which has the pyrite structure (p. 680) with discrete Mn and 82 ions and is converted on heating to MnS and... 

CO bands near 2190 cm When sulfiding occurs before or during reduction Mo and Co are exposed In lower oxidation states, or even as fully reduced metal atoms. Typically on sulfided samples a strong band, apparently arising from CO on metal sites that are... 

This mechanism as a main cause for epithermal-type Au deposition is supported by sulfur isotopic data on sulfides. Shikazono and Shimazaki (1985) determined sulfur isotopic compositions of sulfide minerals from the Zn-Pb and Au-Ag veins of the Yatani deposits which occur in the Green tuff region. The values for Zn-Pb veins and Au-Ag veins are ca. +0.5%o to -f4.5%o and ca. -l-3%o to - -6%c, respectively (Fig. 1.126). This difference in of Zn-Pb veins and Au-Ag veins is difficult to explain by the equilibrium isotopic fractionation between aqueous reduced sulfur species and oxidized sulfur species at the site of ore deposition. The non-equilibrium rapid mixing of H2S-rich fluid (deep fluid) with SO -rich acid fluid (shallow fluid) is the most likely process for the cause of this difference (Fig. 1.127). This fluids mixing can also explain the higher oxidation state of Au-Ag ore fluid and lower oxidation state of Zn-Pb ore fluid. Deposition of gold occurs by this mechanism but not by oxidation of H2S-rich fluid. 

Chromous chloride is a powerful reductant sometimes used to prereduce analyte to a lower oxidation state. Excess Cr2+ is oxidized by atmospheric 02. Sulfur dioxide and hydrogen sulfide are mild reducing agents that can be expelled by boiling an acidic solution after the reduction is complete. 

VO(OR)3 are very convenient and easily available precursors for the preparation of a wide range of V(V) derivatives and via their reduction by matalla-lkyls even for the derivatives of the lower oxidation states. The complete or partial substitution of OR-groups has led to halides, carboxylales, P-diketo-nates, alkyls, sulfides, azids, complexes with Shiffbases, phenylisocyanate, and so on. Hydrolysis of VO(OR)3 is discussed in Chapter 9. 

As mentioned before, subsequent phosphate treatment does not affect the stable sulfide, and TCLP results show excellent stabihzation of Cr in any oxidation state. Alternatively, a small amount of reductant in the waste will convert chromate into lower oxidation states. Such methods, however, are not preferred, because the reductant may also affect the solubility of other hazardous compounds. The exception is technetium-containing radioactive waste, in which chromate is also a contaminant. As we shall see in Chapter 17, a reductant is essential for stabihzation of technetium, and that will also help in stabilization of chromium. 

The sulfide ion stabilizes chromium in its lower oxidation states. Thus, both Ct2S3 and CrS exist, bnt a chromium(VI) trisulfide does not. Heating of Ct2S3 canses decomposition, leading to CrS by way of a number of phases of intermediate composition. Both Cr2Ss and CrS are semiconductors and exhibit magnetic ordering some of the other snlfides exhibit metallic character. 

Further evidence of specific chemical effects in the hot zone appears in the work of Aten, who investigated the distribution of oxidation states of formed by the n,p reaction in irradiated inorganic sulfur compounds (S). When he irradiated solid potassium sulfate or sodium sulfate decahydrate, only a few per cent of the recoils were in lower (nonphosphate) oxidation states. With sodium sulfite (hydrated) the percentage rose to about 50%, while for sodium sulfide 65% to 95% were in lower oxidation states. He found similar correlations when the P was produced by the Cl (n,a) process. Here, the irradiation of KCIO3 or KCIO4 produced 99% phosphate, while NaCl gave only 35%. 

The lower oxidation states of sulfur ( — II sulfide to +IV sulfite) are electron rich and act as nucleophiles. These species react with alkylhalides to displace the more electronegative halide. As pointed out by Brezonik (1994), such reactions may be significant in anoxic aquatic environments such as sediments and groundwater under landfills. Such reactions (see reaction 2 in Table 11.4) are analogous to hydrolysis and produce thiols. Barbash and Reinhard (1989) have estimated rate constants for such reactions. 

In anaerobic soils, the individual chemistry of the ions is more distinctive. The transition metal ions in the middle of each period of the periodic table—chromium, manganese, iron, nickel, cobalt, and copper—can reduce to lower oxidation states, while the end members—scandium, titanium, and zinc—have only one oxidation state. The lower oxidation states are more water soluble but still tend to precipitate as carbonates and sulfides, or associate with organic matter, thus reducing their movement but increasing then plant availability. 

In comparison with the information about geometrical factors for metallic catalysts, little parallel progress has been made with oxides or sulfides. This is largely due to the uncertainty which exists concerning the precise chemical composition of these catalysts in their active state, and partly to the lack gf reliable data for the crystal structures of some of the lower oxides and sulfides. 

In general, when mineral sulfur in coal is converted to hydrogen sulfide by a coal conversion process, reduction of the metal ion in the sulfide is required. This can occur through reduction of the metal to a lower oxidation state, reduction of the metal ion to the metallic state, or reduction of the metal to the hydride ... 

Sulfur (figure 8.21D) is present in aqueous solutions in three oxidation states (2—, 0, and 6+). The field of native S, at a solute total molality of 10, is very limited and is comparable to that of carbon (for both extension and Eh-pH range). Sulfide complexes occupy the lower part of the diagram. The sulfide-sulfate transition involves a significant amount of energy and defines the limit of predominance above which sulfates occur. 

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