Seawater, oxidation potential

The chemistries of Ru, Os, Rh, and Ir in seawater are poorly understood. Available evidence suggests that the probable oxidation state of Ru is + IV OSO4 has a large field of stability on oxidation potential vs pH diagrams and can thereby be considered as an important species in seawater. Rh and Ir are expected to exist in lower oxidation states than Ru and Os. Available evidence (Baes and Mesmer, 1976) indicates that the most important oxidation state for Rh and Ir is + III. Rhm and Irm form strong chloride complexes and should, as well, hydrolyse extensively in solution at pH 7.4-8.2. This speciation assessment is consistent with the relative enrichment of Os in seawater compared to Ir. OSO4 should be substantially less reactive towards particles than is the case for Ir(Cl) - and Ir(OH) - species. 

REE patterns in The aqueous geochemistry of the REE is a function of the type of complexes that sea and river the REE may form, the length of time the REE remain in solution in the oceans water (their residence time), and to a lesser extent the oxidizing potential of the water. The topic is well reviewed by Brookins (1989). The REE contents of rivers and seawater are extremely low (Table 4.6), for they are chiefly transported as particulate material. When normalized to a shale composite (Section 4.3.2), REE concentrations in seawater are between six and seven orders of magnitude smaller that the shale value. River wafers are about an order of magnitude higher. 

Anodic-stripping voltaimnetry (ASV) is used for the analysis of cations in solution, particularly to detemiine trace heavy metals. It involves pre-concentrating the metals at the electrode surface by reducmg the dissolved metal species in the sample to the zero oxidation state, where they tend to fomi amalgams with Hg. Subsequently, the potential is swept anodically resulting in the dissolution of tire metal species back into solution at their respective fomial potential values. The detemiination step often utilizes a square-wave scan (SWASV), since it increases the rapidity of tlie analysis, avoiding interference from oxygen in solution, and improves the sensitivity. This teclmique has been shown to enable the simultaneous detemiination of four to six trace metals at concentrations down to fractional parts per billion and has found widespread use in seawater analysis. 

Niobium is used as a substrate for platinum in impressed-current cathodic protection anodes because of its high anodic breakdown potential (100 V in seawater), good mechanical properties, good electrical conductivity, and the formation of an adherent passive oxide film when it is anodized. Other uses for niobium metal are in vacuum tubes, high pressure sodium vapor lamps, and in the manufacture of catalysts. 

Dissolved Minerals. The most significant source of minerals for sustainable recovery may be ocean waters which contain nearly all the known elements in some degree of solution. Production of dissolved minerals from seawater is limited to fresh water, magnesium, magnesium compounds (qv), salt, bromine, and heavy water, ie, deuterium oxide. Considerable development of techniques for recovery of copper, gold, and uranium by solution or bacterial methods has been carried out in several countries for appHcation onshore. These methods are expected to be fully transferable to the marine environment (5). The potential for extraction of dissolved materials from naturally enriched sources, such as hydrothermal vents, may be high. 

Bromine occurs ia the form of bromide ia seawater and ia natural brine deposits (see Chemicals frombrine). Chloride is also present. In all current methods of bromine production, chlorine, which has a higher reduction potential than bromine, is used to oxidize bromide to bromine. 

There have been instances reported in the literature where the breakdown potential for Nb and Ta in seawater has been found to be lower than the generally accepted value of 120 V, with reported values in extreme instances as low as 20- V . This has been attributed to contamination of the niobium surface from machining operations, grit blasting or traces of copper lubricant used in anode manufacture. These traces of impurities, by becoming incorporated in the oxide film, decrease its dielectric properties and thus account for the lower breakdown voltage. Careful control of surface contamination in the manufacture of platinised niobium is therefore essential to minimise the lowering of the breakdown potential of niobium. 

I apply these computational methods to various aspects of the Earth system, including the responses of ocean and atmosphere to the combustion of fossil fuels, the influence of biological activity on the variation of seawater composition between ocean basins, the oxidation-reduction balance of the deep sea, perturbations of the climate system and their effect on surface temperatures, carbon isotopes and the influence of fossil fuel combustion, the effect of evaporation on the composition of seawater, and diagenesis in carbonate sediments. These applications have not been fully developed as research studies rather, they are presented as potentially interesting applications of the computational methods. 

Clearly, we want the net current at the iron to be zero (hence no overall reaction). The rate of corrosion would be enhanced if the power pack supplied an oxidative current, and wasteful side reactions involving the seawater itself would occur if the power pack produced a large reductive current. The net current through the iron can be positive, negative or zero, depending on the potential applied to the rig s leg. The conserver of the rig wants equilibrium, implying no change. 

Molybdenum isotope variations appear to be on the order of 3.5%o in Mo/ Mo ratios, where the largest fractionation is seen between aqueous Mo in seawater and that incorporated in Fe-Mn crusts and nodules on the seafloor (Chapter 12 Anbar 2004). This isotopic contrast is interpreted to reflect fractionation by Mo sorption to Mn oxide-rich sediments relative to aqueous Mo. The 5 Mo values for euxinic sediments in turn are distinct from those of Fe-Mn crusts, highlighting the isotopic contrasts between major repositories of Mo in surface and near-surface environments. As discussed by Anbar (2004) in Chapter 12, a major focus of research on Mo isotopes has been the potential use as a paleoredox indicator in marine systems. 

Bromate has been classified as a human carcinogen by both the I/VRC (International Agency for the Research on Cancer) and the USEPA (United States Environmental Protection Agency) and is known to be toxic to fish and other aquatic life [11, 12]. Bromate could be produced in aquatic systems upon the oxidation of aqueous bromide. Controlled ozonation has been considered as an effective disinfectant tool in aquatic systems [13] but when sea water is subjected to ozonation, oxy-bromide ozonation by-products (OBP) are produced and these are important both in terms of their disinfection ability and also in relation to their potential toxicity. When seawater is oxidized, aqueous bromide (Br-) is initially converted to hypobro-mite (OBr ) which can then either be reduced back to bromide or oxidized further to bromate (Br03-) which is known to be toxic to fish and other aquatic life and classified as a human carcinogen. There has been thus a considerable interest in bromate analysis so that trace analysis of bromate in water has received considerable attention in recent years. 

Calculate the mole fractions of SO j, HSO, and SO3- in solution at a pH of 8.0, equal to that of seawater and therefore expected for airborne particles formed from seawater in clean regions as well as the total concentration of S(IV) in solution for 20 ppb S02 in the gas phase. Comment on the implications for the potential role of sea salt particles in the oxidation of S02 in the marine boundary layer. 

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