Solution oxidation potential

These elements are noble metals and, as such, can be dissolved only with great difficulty. The usual leaching agent is hydrochloric acid, with the addition of chlorine to increase the solution oxidation potential. This strong chloride medium results in the almost exclusive formation of aqueous chloroanions, with, under certain circumstances, the presence of some neutral species. Very seldom are cationic species formed in a chloride medium. However, these elements do possess a range of easily accessible oxidation states and, with the possibility of a number of different anionic complexes that are dependent on the total chloride concentration, this provides a very complicated chemistry. A summary of the most important chloro complexes found in these leach solutions is given in Table 11.6, from which the mixed aquochloro and polynuclear species have been omitted. The latter are found especially with the heavier elements. 

The solution oxidation potentials for radicals 3 and 7 show that both are powerful electron donors, although not as strong as the neutral dithiopentalenyl radical 37 <1997JA2633>. 

For reductions in acid solution, oxidation potentials show that barium metal is a better reducing agent than calcium. The reverse is true in basic solution. Explain. 

The identity of the radical ions formed upon steady-state radiation of DNA in low-temperature glasses has been established by means of electron paramagnetic resonance (EPR) spectroscopy [53]. EPR analysis indicates that electrons and holes are localized on a single nucleobase rather than being delocalized over several stacked bases at low temperatures. Radical ion formation is presumed to occur randomly at all four nucleosides. However, EPR studies establish that the electron holes are localized predominately on guanine, which has the lowest gas phase ionization potential and solution oxidation potential (Tables 1 and 3). Yan et al. [54]... 

Rationalization via a thermochemical cycle is not straightforward here. In the case of the largely used silanes, for example, the required oxidation potential of a trialkylsilyl radical is not known. One can resort to the use of gas phase measured ionization potentials instead of the solution oxidation potentials (Eq. 5 instead of Eq. 2) - ... 

Much rarer are used redox-couples of anions like NO /NH, HCO / CH, SO "/HS , etc. Their equilibrium ratio is reached much slower. The reason is that these reactions have high activation energy associated with the destruction of strong covalent bonds. For this reason, many of them are implemented only on the accoimt of biochemical catalysis (for instance, NO " NOj ). Rates of such processes are very low and imstable as they depend on the environment, nature and abundance of microbial populations and sufficiency of substrate for them. Many redox reactions of these couples are essentially irreversible. Their half-life may reach several years. The only relatively rapid process is oxidation of sulphide sulphur in alkaline medium. Besides, concentrations of anions often depend on pH value. For this reason. Eh value of individual anion redox-couples rarely may be a criterion of the solutions oxidation potential as a whole. 

Table 1. C-C ring bond distance differences (8r), from crystal structure determinations of thiophene-based monomers, and corresponding solution oxidation potentials (OP, Volts versus SCE) ... 

A variant of the above procedures involving sorption of the oxidant is simple addition of monomer into a solution of the host polymer + oxidant, with the solution oxidation potential such that immediate polymerization occurs. The composite is... 

The largest division of interfacial electrochemical methods is the group of dynamic methods, in which current flows and concentrations change as the result of a redox reaction. Dynamic methods are further subdivided by whether we choose to control the current or the potential. In controlled-current coulometry, which is covered in Section IIC, we completely oxidize or reduce the analyte by passing a fixed current through the analytical solution. Controlled-potential methods are subdivided further into controlled-potential coulometry and amperometry, in which a constant potential is applied during the analysis, and voltammetry, in which the potential is systematically varied. Controlled-potential coulometry is discussed in Section IIC, and amperometry and voltammetry are discussed in Section IID. 

W. M. Latimer, The Oxidation Potentials of the Elements and their Potentials in Aqueous Solutions, Prentice-HaH, Inc., Englewood Chffs, N.J., 1938. 

The oxidation potentials for rhenium in aqueous acidic solution are summarized in the following diagram (6). 

In most ores, sufficient Fe is already present. For some ores, it is necessary to add metallic iron. In practice, the oxidation potential of the solution can be monitored and controlled using the Fe /Fe ratio. Very high leaching efficiencies with H2SO ate common, eg, 95—98% dissolution yield of uranium (39). If acid consumption exceeds 68 kg/1 of ore treated, alkaline leaching is preferred. The comparative costs of acid, sodium hydroxide, and sodium carbonate differ widely in different areas and are the determining factor. 

Electrochemical Potentials. The oxidation potential of a solution containing a metal in two of its valence states, and... 

Some hypochlorites, either as solutions or soflds, are much more stable than hypochlorous acid, and because of thek high oxidation potential and ready hydrolysis to the parent acid, find wide use in bleaching and sanitizing appHcations. One of the novel uses of hypochlorites was for disinfection of ApoUo Eleven on its return from the moon (136). 

Sodium hydrosulfite or sodium dithionate, Na2S204, under alkaline conditions are powerful reducing agents the oxidation potential is +1.12 V. The reduction of -phenylazobenzenesulfonic acid with sodium hydrosulfite in alkaline solutions is first order with respect to -phenylazobenzenesulfonate ion concentration and one-half order with respect to dithionate ion concentration (135). The SO 2 radical ion is a reaction intermediate for the reduction mechanisms. The reaction equation for this reduction is... 

O = Oxidizing potential R = Reducing potential T = Temperature S = Salts in solution F = Fluid flow conditions A = Agitation... 

The tendency for F2 to give F" ions in solution is also much greater than for the other halogens as indicated by the steady decrease in oxidation potential (E°) for the reaction X2(soln) -I-2e" 2X" (aq) ... 

Periodates are both thermodynamically potent and kinetically facile oxidants. The oxidation potential is greatest in acid. solution (p. 855) and can be progressively dimini.shed by increasing the pH of the solution. In acid solution it is one of the... 

Five oxidation states of At have been definitely established (-1, 0, +1, V, VII) and one other (III) has been postulated. The standard oxidation potentials connecting these states in 0.1 M acid solution are E°fV) ... 

Figure 4-421. Corrosion characteristics of an active passive metal as a function of solution oxidizing power (eiectrode potential). (From Ref. [183].)...

Pure tin is completely resistant to distilled water, hot or cold. Local corrosion occurs in salt solutions which do not form insoluble compounds with stannous ions (e.g. chloride, bromide, sulphate, nitrate) but is unlikely in solutions giving stable precipitates (e.g. borate, mono-hydrogen phosphate, bicarbonate, iodide) . In all solutions, oxide film growth occurs and the potential of the metal rises. Any local dissolution may not begin for several days but, once it has begun, it will continue, its presence being manifested... 

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