Sulfides, organic reaction with iron

The Gibbs phase rule is the basis for organizing the models. In general, the number of independent variables (degrees of freedom) is equal to the number of variables minus the number of independent relationships. For each unique phase equilibria, we may write one independent relationship. In addition to this (with no other special stipulations), we may write one additional independent relationship to maintain electroneutrality. Table I summarizes the chemical constituents considered as variables in this study and by means of chemical reactions depicts independent relationships. (Throughout the paper, activity coefficients are calculated by the Debye-Hiickel relationship). Since there are no data available on pressure dependence, pressure is considered a constant at 1 atm. Sulfate and chloride are not considered variables because little specific data concerning their equilibria are available. Sulfate may be involved in a redox reaction with iron sulfides (e.g., hydrotroilite), and/or it may be in equilibrium with barite (BaS04) or some solid solution combinations. Chloride may reach no simple chemical equilibrium with respect to a phase. Therefore, these two ions are considered only to the... 

Lead enters surface water from atmospheric fallout, run-off, or wastewater. Little lead is transferred from natural minerals or leached from soil. Pb ", the stable ionic species of lead, forms complexes of low solubility with major anions in the natural environment such as the hydroxide, carbonate, sulfide, and sulfate ions, which limit solubility. Organolead complexes are formed with humic materials, which maintain lead in a bound form even at low pH. Lead is effectively removed from the water column to the sediment by adsorption to organic matter and clay minerals, precipitation as insoluble salt (the carbonate, sulfate, or sulfide) and reaction with hydrous iron, aluminum, and manganese oxides. Lead does not appear to bioconcentrate significantly in fish but does in some shellfish such as mussels. When released to the atmosphere, lead will generally occur as particulate matter and will be subject to gravitational settling. Transformation to oxides and carbonates may also occur. 

Concentrations of H2S typically are maintained low by a variety of reactions including oxidation, precipitation with iron, and interactions with organic matter. Sulfide may be oxidized biologically or abiotically. Elemental S reacts readily with H2S to form polysulfides (175) and is oxidized and reduced by a variety of bacteria. In radiotracer experiments, S0 was in... 

Redox Reactions. Aquatic organisms may alter the particular oxidation state of some elements in natural waters during activity. One of the most significant reactions of this type is sulfate reduction to sulfide in anoxic waters. The sulfide formed from this reaction can initiate several chemical reactions that can radically change the types and amounts of elements in solution. The classical example of this reaction is the reduction of ferric iron by sulfide. The resultant ferrous iron and other transition metals may precipitate with additional sulfide formed from further biochemically reduced sulfate. Iron reduction is often accompanied by a release of precipitated or sorbed phosphate. Gardner and Lee (21) and Lee (36) have shown that Lake Mendota surface sediments contain up to 20,000 p.p.m. of ferrous iron and a few thousand p.p.m. of sulfide. The biochemical formation of sulfide is undoubtedly important in determining the oxidation state and amounts of several elements in natural waters. 

The most familiar complexes in this class are Roussin s red and black salts140 obtained as a mixture from the reaction of iron(III) sulfide with nitric oxide and an alkali metal sulfide.141 The dimeric and diamagnetic red salt, K2[Fe(NO)2S]2 gives rise to a series of stable esters [Fe(NO)2SR]2 (R = Me, Et, Ph etc.) which are also diamagnetic, dark red and soluble in organic solvents. Some preparative routes142 to the red salt and its esters are shown in Scheme 3. 

The reaction of phosgene with cadmium sulfide is said to be a good method for the preparation of COS (see Section 9.5.7) [885], used in the manufacture of organic thio compounds, whilst reaction of COCl, with iron(III) phosphate at 300-350 C has been proposed as a synthetic method for POCI3 (see Section 9.4.5) [885]. 

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