Reduction-oxidation potentials Defined

The midpoint potential of a half-reaction E, is the value when the concentrations of oxidized and reduced species are equal, [Aox] = [Aredl- In biological systems the standard redox potential of a compound is the reduction/oxidation potential measured under standard conditions, defined at pH = 7.0 versus the hydrogen electrode. On this scale, the potential of 02/water is +815 mV, and the potential of water/H2 is 414 mV. A characteristic of redox reactions involving hydrogen transfer is that the redox potential changes with pH. The oxidation of hydrogen H2 = 2H + 2e is an m = 2 reaction, for which the potential is —414 mV at pH 7, changing by 59.2 mV per pH unit at 30°C. 

The reduction-oxidation potential (typically expressed in volts) of a compound or molecular entity measured with an inert metallic electrode under standard conditions against a standard reference half-cell. Any oxidation-reduction reaction, or redox reaction, can be divided into two half-reactions, one in which a chemical species undergoes oxidation and one in which another chemical species undergoes reduction. In biological systems the standard redox potential is defined at pH 7.0 versus the hydrogen electrode and partial pressure of dihydrogen of 1 bar. 

The oxidation potential of carbanions, ox> or the reduction potential of carbocations, red> could be a practical scale of stability as defined by (3). These potentials can be measured by voltammetry, although the scale is subject to assumptions regarding elimination of the diffusional potential and solvation effects. 

The formal potential of a reduction-oxidation electrode is defined as the equilibrium potential at the unit concentration ratio of the oxidized and reduced forms of the given redox system (the actual concentrations of these two forms should not be too low). If, in addition to the concentrations of the reduced and oxidized forms, the Nernst equation also contains the concentration of some other species, then this concentration must equal unity. This is mostly the concentration of hydrogen ions. If the concentration of some species appearing in the Nernst equation is not equal to unity, then it must be precisely specified and the term apparent formal potential is then employed to designate the potential of this electrode. 

The loss of an electron by M, M + + e, is the process of oxidation in electrochemistry. The electron is then accepted by an electrode of well defined potential, so that the oxidation potential Eox is the free energy of the reaction, as was seen in Figure 4.1. Similarly the reduction potential Ered is the energy of the reduction reaction, e.g. N + e - N. By definition the molecule, which is oxidized, is the donor (M in this case), and the molecule, which is reduced, is the acceptor. The electron transfer from M to N is therefore equivalent to the combined oxidation of the donor and reduction of the acceptor, so that the energy balance is... 

In the second case the standard hydrogen electrode is placed on the right-hand side of the representation of the cell, and the other electrode would be placed on the left-hand side. The emf of the cell would then be written as = i a — ij/c. The value of if/f is defined to be zero and the potential of the electrode on the left, t a, is the emf of the cell. The symbol ip is called the oxidation potential. When all of the reacting substances of the electrode are in their standard states, then // would become ip° and would be called the standard oxidation potential. This terminology is that of Latimer and emphasizes the nature of the reaction taking place at the electrode. We present it here for completeness, knowing that reduction potentials are now the standard convention, but that some of the older literature used oxidation potentials. 

Another point is that the reduction and oxidation potential limits (electrochemical window) are defined as the potentials at which the current density reaches a predefined value that is arbitrarily chosen [40, 48], Ue et al. also mention that the same problem arises in the choice of the sweep rate [40]. For example Egashira and coworkers obtained a log I- U line shifted to a higher position at a faster potential scan in comparison to a slower scan because of non-Faradaic currents such as the larger charging currents of the double-layer, and the decomposition of impurities [41]. The last factor affecting the electrochemical window is the electrode itself, its composition and its morphological surface structure, which defines the electrocatalytic properties [40]. 

Reduction-oxidation reactions are mediated by micro-organisms and involve the transfer of electrons between reactants and products. Free electrons do not exist in solution, so an oxidation reaction (loss of electrons) must be balanced by a reduction reaction (gain of electrons). Redox potential is defined by the Nemst equation and is the energy gained in the transfer of 1 mol of electrons from an oxidant to H2. 

Decomposition potential (voltage) — The onset voltage for electrochemical decomposition of the electrolytic solution or the electrodes. The decomposition can take place due to either oxidation or reduction, or both. The decomposition potentials define the electrochemical window of the system. Its value depends on the salt, solvent, electrode material, temperature, and the existence of materials that can catalyze decomposition reactions, such as Lewis acids. Exact decomposition voltages are hard to reproduce as the onset current of the process is very sensitive to the experimental conditions (e.g., scan rate, temperature, type of electrode, etc.). Decomposi-... 

The fixed potential of the band edges of a given semiconductor can be established independently by many physical or electrochemical methods. These band-edge positions define the limits of the attainable oxidation and reduction half reactions that can be achieved on this surface. As long as the oxidation potential of the adsorbed donor (hole trap) is lower than the valence band edge, and the reduction potential of the adsorbed acceptor (electron trap) is lower than the conduction-band edge. 

The ket values can be calculated from the AG°et and AG o values by use of Eqs 2 and 3. The AG°et value is obtained from the one-electron oxidation potential of an electron donor E°ofj and the one-electron reduction potential of an acceptor ( °red), because AG°et = F E°ox E°rti)-The reorganization energy of electron transfer (k) is defined as 4AG o- The k value was evaluated as 47 kcal mol for the postulated electron transfer from N,N-dimethylanilines to [(TPFPP)Fe" =0] + by fitting the data to Eqs 2 and 3. Essentially the same k value can be obtained by fitting the data to the Marcus equation (Eq. 4) [91] except for the region where the rate becomes close to a diffusion-limited value [81, 92]. 

Because of thermodynamic and electrochemical conventions, standard potentials are defined in the direction of reduction, independently of the respective chemical stabilities of the molecules involved. Thus for the oxidation of toluene to its cation radical, E° refers to the reduction of the highly unstable cation radical into the highly stable toluene. To overcome such a priori chemical nonsence, E is frequently designated as the standard oxidation potential of toluene for example. However, such a term should not be accepted according to canonical rules because it formally implies that the cell now operates in a driven mode, that is, is connected to an external power supply [19]. Thus in this chapter we prefer to use the denomination standard reduction potentials, rather than the usual temi standard potential, as a reminder of the E° definition, although such as expression is basically a pleonasm. 

To convert these oxidation potentials to electrode potentials as defined by the lUPAC convention, one must mentally (1) express the half-reactions as reductions and (2) change the signs of the potentials. 

Define the following terms anode, cathode, electromotive force, standard oxidation potential, standard reduction potential. 

Determining Ehaif-ceii The Standard Hydrogen Electrode What portion of ceii for the zinc-copper reaction is contributed by the anode half-cell (oxidation of Zn) and what portion by the cathode half-cell (reduction of Cu ) That is, how can we know half-cell potentials if we can only measure the potential of the complete cell Half-cell potentials, such as Ezine and °opper. are not absolute quantities, but rather are values relative to that of a standard. This standard reference halfcell has its standard electrode potential defined as zero (E fereiice — 0.00 V). The standard reference half-cell is a standard hydrogen electrode, which consists of a specially prepared platinum electrode immersed in a 1 M aqueous solution of a strong acid, H (fl ) [or H30 (a )], through which H2 gas at 1 atm is bubbled. Thus, the reference half-reaction is... 

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