Lower oxidation states standard reduction potentials

From Table 19-1, which provides the values at 25°C, 1 atm, and standard states, the (Fe3+ Fe2+) couple has a lower standard reduction potential, 0.771 V, than the (Br2 Br ) couple, 1.065 V. Therefore Fe2 can reduce Br2, but Br- cannot reduce Fe3+. This also means that Fe3+ cannot oxidize Br . 

The standard electrode potential has proved a useful parameter to correlate with metal toxicity (Turner et al. 1983 Lewis et al. 1999). Regarding the interaction of metals in a biological milieu, it can be stated generally that the moderate reduction potential of a biological system tends to convert metals to lower oxidation states. 

If you select any two half-reactions from the chart of standard electrode potentials, the half-reaction higher on the list will proceed as a reduction, and the one lower on the list will proceed in the reverse direction, as an oxidation. Beware Some references give standard electrode potentials for oxidation half-reactions, so you have to switch higher and lower in the rule stated in the preceding sentence, though this is not common. 

In relation to the reduction potentials and to the energy of the singlet excited state of electron acceptors, we can set, applying Weller s equation, the maximum oxidation potential of electron donors to efficiently quench the fluorescence of each one of the electron acceptors listed above. In other words, with the most commonly used electron acceptor, DC A, its fluorescence will be efficiently quenched only by donors with oxidation potentials lower than 2 V vs SCE (calomel standard electrode). Instead, with 2,6,9,10-tetracyanoanthracene (TCA), which has a singlet energy similar to that of DCA, but is much easier to reduce, its fluorescence will be quenched by donors with oxidation potentials as great as 2.5 V vs SCE. 

Lower rare earth oxides, those corresponding to the occurrence of the (2+) oxidation state, are also known. The information currently available about them has been recently reviewed [6]. In accordance with the standard redox potentials reported in [24], for Ln /Ln pairs (Ln Sm, Eu, Tm and Yb), in solution, the Ln (2+) ions typically behave as reducing species, Eu being by far the less reductant of them (e° Eu /Eu -0.35 V to be compared with those for Sm VSm - 1.5 V Tm /Tm - 2.3 V Yb " Alj - 1.1 V). This observation may qualitatively be extended to the oxides, LnO, that of Eu showing the highest stability [6]. 

Another electrochemical parameter, the standard reduction-oxidation potential (AEP) represents the absolute difference in electrochemical potential between an ion and its first stable reduced state, or in other terms, the ability of an ion to change its electronic state. This parameter is seldom used alone in the studies concerning the toxicity of metal ions, but is usually combined with AN/AIP (where AN=atomic number, AIP=the difference in the ionization potential (in eV) between the actual oxidation number (O.N.) and the next lower one (O.N.-1) or with log AN/AIP (Kaiser 1980 Kaiser 1985 McCloskey et al. 1996 Enache et al. 2003). 

A compound with a more positive potential wiU oxidize the reduced form of a substance of lower potential with a standard free energy change AG° = -nP AE° = -n A ° x 96.49 kj moH where n is the number of electrons transferred from reductant to oxidant. The temperature is 25°C unless otherwise indicated. E° refers to a standard state in which the hydrogen ion activity = 1 E° refers to a standard state of pH 7, but in which all other activites are unity. 

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