H2S—See Hydrogen sulfide

Elemental composition K 54.18%, S 44.42%, H 1.40%. An aqueous solution may be analyzed for potassium by various methods (see Potassium). The compound on exposure to air evolves H2S which can be detected from its odor, as well as by various tests (see Hydrogen Sulfide). 

Elemental composition Na 58.93%, S 41.07%. An aqueous solution is analyzed to determine sodium content. Also, an aqueous solution may be analyzed for sulfide by methylene blue colorimetric test or by iodometric titration (APHA, AWWA, and WEF. 1999. Standard methods for the Examination of Water and Wastewater, 20 ed. Washington, DC American Pubhc Health Association). The methylene blue test is based on reacting sulfide, ferric chloride and dimethyl-p-phenylenediamine to produce methylene blue. Also, sulfide can be measured by using a sdver-sdver sulfide electrode. Quahtatively, sulfide may be identified from the hberation of H2S on treatment with acid. The H2S turns the color of paper soaked with lead acetate black (See Hydrogen Sulfide). 

The H2S sulfanes are the subject of several reviews (129,133). Except for hydrogen sulfide these have no practical utiUty. Sodium tetrasulfide [12034-39-8] is available commercially as a 40 wt % aqueous solution and is used to dehair hides in taimeries, as an ore flotation agent, in the preparation of sulfur dyes (qv), and for metal sulfide finishes (see Leather Mineral recovery and processing). 

Sulfides with widely different solubilities and solubility products can be selectively precipitated by adding S2 ions to the solution removed from the chlorides in the first step (see Fig. 11.20). Some metal sulfides (such as CuS, HgS, and Sb2S3) have extremely small solubility products and precipitate if there is the merest trace of S2" ions in the solution. Such a very low concentration of S2 ions is achieved by adding hydrogen sulfide, H2S, to an acidified solution. A higher hydronium ion concentration shifts the equilibrium... 

In Chapter 3 we described the possible external sources of energy required for life. Here we shall assume at first that the most primitive form was not light but the chemical energy stored in unstable minerals. Such minerals were the metals and metal excess sulfides and iron sulfide in their reactions with water or hydrogen sulfide to produce hydrogen (see Wachtershauser in Further Reading) or were stores in the out of balance of states of non-metals such as S /H2S. 

H2S. See also Hydrogen sulfide entries absorption of, 23 598 direct oxidation of, 23 616-617 reduction of tail gas sulfur species to, 23 617-618... 

Hydrogen-storage alloys, 13 626, 627 Hydrogen storage properties, of rare earths, 14 652 Hydrogen streams, 13 461 Hydrogen sulfide, 23 568, 569, 629-638. See also H2S... 

Elemental sulfur1-4 occurs naturally in association with volcanic vents and, in Texas and Louisiana, as underground deposits. The latter are mined by injecting air and superheated water, which melts the sulfur and carries it to the surface in the return flow (the Frasch process). Most of the sulfur used in industry, however, comes as a by-product of the desulfurization of fossil fuels. For example, Albertan sour natural gas, which often contains over 30% (90%, in some cases) hydrogen sulfide (H2S), as well as hydrocarbons (mainly methane) and small amounts of C02, carbonyl sulfide (COS), and water, is sweetened by scrubbing out the H2S and then converting it to elemental S in the Claus process.5 The Claus process is applicable in any industrial operation that produces H2S (see Section 8.5) it converts this highly toxic gas to nontoxic, relatively unreactive, and easily transportable solid sulfur. 

At this point, we might wonder how reasonable it is to use (W) values as given in Tables 14.2 and 14.3 for assessing whether a reaction will occur spontaneously in a given natural system. The species involved will not, of course, be present at standard concentrations. To evaluate this problem, let us compare the En value of a 1CT4 M aqueous hydrogen sulfide (H2S) solution at pH 8 with the ]](W) value of reaction 10 in Table 14.2 (-0.27 V). The calculated value (see Illustrative Example 14.3) is -0.18 V, which is still in the same ballpark. Of course, if we want to evaluate the free... 

Let us now turn to some kinetic considerations of NAC reduction. As an example, consider the time courses of nitrobenzene (NB) concentration in 5 mM aqueous hydrogen sulfide (H2S) solution in the absence and presence of natural organic matter (Fig. 14.7). As is evident, although reduction of NB by H2S to nitrosobenzene and further to aniline (Eq. 14-31) is very favorable from a thermodynamic point of view (see Fig. 14.4), it seems to be an extremely slow process. However, when DOM is added to the solution, reduction occurs at an appreciable rate (Fig. 14.7). In order to understand these findings, some general kinetic aspects of redox reactions involving NACs should be recognized. 

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