Sulphide ions sulphate

This is a simplified treatment but it serves to illustrate the electrochemical nature of rusting and the essential parts played by moisture and oxygen. The kinetics of the process are influenced by a number of factors, which will be discussed later. Although the presence of oxygen is usually essential, severe corrosion may occur under anaerobic conditions in the presence of sulphate-reducing bacteria Desulphovibrio desulphuricans) which are present in soils and water. The anodic reaction is the same, i.e. the formation of ferrous ions. The cathodic reaction is complex but it results in the reduction of inorganic sulphates to sulphides and the eventual formation of rust and ferrous sulphide (FeS). 

Within soils Hg can exist in the ionic or associated forms, HgCl, Hg(OH)2 and HgCb, as elemental Hg and within various solid phases. The presence of gas-phase Hg in soil is therefore understandable, although it is evident that under conditions of equilibrium the level of free Hg in aerated chloride-containing soil will be low. It can be expected that similar equilibrium conditions apply to the aqueous solutions derived from sulphide oxidation, since the sulphate ion does not interact strongly with the several Hg species. 

A kinetic study of the oxidation of sulphite ions to sulphate ions by gaseous oxygen in melts of lithium and potassium chlorides at temperatures between 414 and 504 °C has been carried out, and a possible reaction mechanism proposed.14 The interaction of oxygen at 20 °C with hydrogen sulphide presorbed at 55 °C and vice versa has been investigated.15... 

Many of physico-chemical processes taking place in the system water-rocks-atmosphere have an oxidation or reduction character. Insoluble sulphides are oxidized in the presence of oxygen into sulphates. The rate of oxidation depends on the grain size of the sulphides, the degree of rock protection, conditions of oxygen supply, etc. Thus, the natural waters are enriched by sulphate ions in the chemical erosion of eruptive rocks. 

Pore-water concentration profiles of redox-sensitive ions (nitrate, Mn, Fe, sulphate and sulphide) and nutrients (ammonium and phosphate) demonstrate the effects of degradation of OM. In freshwater sediments, the redox zones generally occur on a millimetre to centimetre scale due to the high input of reactive OM and the relatively low availability of external oxidators, especially nitrate and sulphate, compared to marine systems. A typical feature for organic-rich freshwater sediments deposited in aerobic surface waters, is the presence of anaerobic conditions close to the sediment-water interface (SWI). This is indicated by the absence of dissolved oxygen and the presence of reduced solutes (e.g. Mn, Fe and sulphides) in the pore water. Secondary redox reactions, like oxidation of reduced pore-water and solid-phase constituents, and other postdepositional processes, like precipitation-dissolution... 

Naturally occurring sulphur-bearing species include native sulphur, the sulphate and sulphide minerals, gaseous H S and SO2 and a range of oxidized and reduced suphur ions in solution. A summary of the isotopic compositions of some major rock types is given in Figure 7.21. 

Figure 8.18. The Na2S04 has penetrated into the alloy preferentially removing chromium and aluminium. Since the Na2S04 layer is thick, virtually no oxygen is supplied by the gas phase. Due to reaction with chromium and aluminium the oxygen pressure is reduced below that to oxidize nickel, and nickel sulphide particles are formed at the surface of the specimen. Figure 8.18(a) and 8.18(b). Within the reaction-product scale sulphate ions provide oxygen to oxidize aluminium and chromium and sulphur to form nickel sulphide. Consequently, the oxide-ion concentration is increased to levels at which the Cr203 and AI2O3 can react with oxide ions as described in Equations (8.14) and (8.15) ...

In the absence of organic sulphur compounds in wort, hydrogen sulphide arises from sulphate ions. The metabolism of sulphate ions by yeast is indicated in Fig. 17.24. Leakage of sulphide ions from the pathway is the origin... 

Heterogeneous Polythene, polypropylene, silicone rubber etc. with various halide, sulphate, phosphate, sulphide or oxalate precipitates form membranes for Ba, SO4, PO4, X, S etc. PTFE + graphite with ionic compounds for range of ions. 

Plants obtain their sulphur as sulphate ions (valency +6) from the soil and the assimilation of sulphate involves its reduction to the level of sulphide (valency —2). Early studies on sulphate utilisation using homogenates of mammalian liver indicated that formation of phenyl sulphates depended on the presence of ATP and involved the intermediate formation of active sulphate , an adenosine phospha-tosulphate analagous to active acetate (see p. 98). The activated sulphate group can be transferred to a variety of acceptors such as the phenols mentioned above or can be reduced as occurs in those organisms which use sulphate as their source of sulphur. 

Low-carbon and chromium-nickel steels, certain copper, nickel and aluminium alloys (which are all widely used in marine and offshore engineering) are liable to exhibit stress-corrosion cracking whilst in service in specific environments, where combinations of perhaps relatively modest stress levels in material exposed to environments which are wet, damp or humid, and in the presence of certain gases or ions such as oxygen, chlorides, nitrates, hydroxides, chromates, nitrates, sulphides, sulphates, etc. 

General corrosion damage was the cause of failure of an A1 alloy welded pipe assembly in an aircraft bowser which was attacked by a deicing-fluid — water mixture at small weld defects . Selective attack has been reported in welded cupro-nickel subjected to estuarine and seawater environments . It was the consequence of the combination of alloy element segregation in the weld metal and the action of sulphate reducing bacteria (SRB). Sulphide-coated Cu-enriched areas were cathodic relative to the adjacent Ni-rich areas where, in the latter, the sulphides were being continuously removed by the turbulence. Sulphite ions seemed to act as a mild inhibitor. 

Composition of the liquid environment The ionic composition, arising from dissolved salts and gases, has a considerable influence on the performance of inhibitors. In near-neutral aqueous systems the presence of certain ions tends to oppose the action of inhibitors. Chlorides and sulphates are the most common examples of these aggressive ions, but other ions, e.g. halides, sulphides, nitrates, etc. exert similar effects. The concentration of inhibitor required for protection will depend on the concentrations of these aggressive ions. Laboratory tests " have given some quantitative relationships... 

A determination of dimethyl sulphoxide by Dizdar and Idjakovic" is based on the fact that it can cause changes in the visible absorption spectra of some metal compounds, especially transition metals, in aqueous solution. In these solutions water and sulphoxide evidently compete for places in the coordination sphere of the metal ions. The authors found the effect to be largest with ammonium ferric sulphate, (NH4)2S04 Fe2(S04)3T2H20, in dilute acid and related the observed increase in absorption at 410 nm with the concentration of dimethyl sulphoxide. Neither sulphide nor sulphone interfered. Toma and coworkers described a method, which may bear a relation to this group displacement in a sphere of coordination. They reacted sulphoxides (also cyanides and carbon monoxide) with excess sodium aquapentacyanoferrate" (the corresponding amminopentacyanoferrate complex was used) with which a 1 1 complex is formed. In the sulphoxide determination they then titrated spectrophotometrically with methylpyrazinium iodide, the cation of which reacts with the unused ferrate" complex to give a deep blue ion combination product (absorption maximum at 658 nm). 

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