Halogens oxidation-reduction potentials

The Co(bipy)3+ ion is a useful catalyst for a number of borohydride reductions, e.g., organic nitro compounds are reduced smoothly to amines at pH 6.5-7 the true reducing agent is Co(bipy)3+. The oxidation-reduction potential for Co(I)/Co(II) is 0.91 volt (vs. standard calomel electrode in 50% aqueous ethanol) and this should fall between the potentials of the other reactants (709). Catalytic reductions of organic halogen compounds may be achieved (436), and the system is reactive to small molecules such as NgO (38). 

The effectiveness of the purification by the halogens depends on their oxidation-reduction potentials and decreases from chlorine to iodine. In addition, the latter can disproportionate ... 

Chemical oxidizers used to disinfect RO systems include hydrogen peroxide (peroxide), halogens, and ozone. Although halogens (and specifically chlorine) are the most popular oxidizers using in conjunction with RO pretreatment, they do not have the highest oxidization-reduction potential (ORP). Table 8.8 lists the ORP for several oxidizers. As the table shows, ozone and peroxide have nearly twice the ORP or oxidative power as chlorine. Despite the relatively low ORP, chlorine is the most commonly used disinfectant in brackish water RO pretreatment due to its ease of use and its ability to provide residual disinfection (for seawater desalination using RO, bromine (as HOBr)... 

Although it is only slowly oxidized in moist air at ambient temperature, cadmium forms a fume of brown-colored cadmium oxide [1306-19-0] CdO, when heated in air. Other elements which react readily with cadmium metal upon heating include the halogens, phosphoms, selenium, sulfur, and tellurium. The standard reduction potential for the reaction... 

F2 has the most positive standard reduction potential and therefore is the strongest of all common oxidizing agents. Oxidizing strengths of the diatomic halogen molecules decrease down Group 7A. 

Radical cations can be generated by many chemical oxidizing reagents, including Brpnsted and Lewis acids, the halogens, peroxide anions or radical anions, metal ions or oxides, nitrosonium and dioxygenyl ions, stable aminium radical cations, semiconductor surfaces, and suitable zeolites. In principle, it is possible to choose a reagent with a one-electron redox potential sufficient for oxidation-reduction, and a two-electron potential insufficient for oxidation-reduction of the radical ion. 

The standard reduction potentials for the main species formed by the Group 17 elements in aqueous solution are given in Tables 6.16 and 6.17, for pH values 0 and 14, respectively. Irrespective of the pH of the solution, the halogen elements range from the extremely powerful F2 (which has the potential to oxidize water to dioxygen), through the powerful oxidants Cl2 and Br2, to 12, which is a relatively weak oxidant. 

C. E. Ophardt, "Redox Demon-f strations and Descriptive Chemistry Part 2. Halogens/ /. Chem. Educ., Vol. 64,1987,807. Using an abbreviated table of reduction potentials as a predictive tool, reactions of bromine and iodine in various oxidation states are demonstrated. 

The reduction potentials for various alkyl halides range from +0.5 to +1.5 V therefore, when Fe° serves as an electron donor, the reaction is thermodynamically favorable. Because three reductants are present in the treatment system (Fe°, H2, and Fe2+), three possible pathways exist. Equation (13.9) represents the oxidation of Fe° by reduction of a halogenated compound. In the second pathway, the ferrous iron behaves as a reductant, as represented in Equation (13.10). This reaction is relatively slow because the ability to reduce a pollutant by ferrous iron is dependent on the speciation ferrous ions, which is determined by the ligands present in the system. The third possible pathway, Equation (13.11), is dehalogenation by hydrogen. This reaction does not occur easily without a catalyst. In addition, if hydrogen levels become too high, corrosion is inhibited (Matheson and Tratnyek, 1994) ... 

A few typical reactions of l,4-dihydro-l,2,4,5-tetrazines are listed in Scheme 55.1,4-Dihydrotetra-zines (2) are easily oxidized to 1,2,4,5-tetrazines (1) by nitrous acid, nitric acid, oxygen, halogens, iron(III) chloride, hydrogen peroxide, and lead tetraacetate <78HC(33)1077>. In contrast, 1,2,4,5-tetrazines are strong oxidants, as the half wave reduction potentials of Table 4 show. The redox system is quite mobile. 

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