Standard redox electrode potential

Fig. 6. Band edge positions of several semiconductors ia contact with an aqueous electrolyte at pH 1 ia relation to the redox (electrode) potential regions (vs the standard hydrogen electrode) for the oxidation of organic functional groups (26,27).

Table 3.2 Standard reduction potential ( ) for some redox pairs relative to the standard hydrogen electrode potential 0...

Standard Reduction Electrode Potentials for Inorganic Systems in Nonaqueous Solutions at 25°C Redox Potentials for Some Biological Half Reactions... 

Fig. 16.9 CB and VB energy levels of several semiconductors. (The semiconductors are in contact with aqueous electrolyte at pH 1. The energy scale is indicated in electron volts using either the normal hydrogen electrode (NHE) or vacuum level as reference. On the right the standard potentials of several redox couples are presented against the standard hydrogen electrode potential.) [Reprinted by permission from Macmillan Publishers Ltd [Nature] (Gratzel 2001), copyright (2001)]...

Table 7.3 Standard Equilibrium Electrode Potentials for Some Redox Systems...

Table 3. Standard reduction electrode potentials of some common redox systems at 25°C...

Oxidation Reactions. Potassium permanganate is a versatile oxidizing agent characterized by a high standard electrode potential that can be used under a wide range of reaction conditions (100,133—141). The permanganate ion can participate in a reaction in any of three distinct redox couples. 

To obtain comparative values of the strengths of oxidising agents, it is necessary, as in the case of the electrode potentials of the metals, to measure under standard experimental conditions the potential difference between the platinum and the solution relative to a standard of reference. The primary standard is the standard or normal hydrogen electrode (Section 2.28) and its potential is taken as zero. The standard experimental conditions for the redox... 

Beginning in the early 1980s [20, 21] metallic lithium was replaced by lithium insertion materials having a lower standard redox potential than the positive insertion electrode this resulted in a "Li-ion" or "rocking-chair" cell with both negative and positive electrodes capable of reversible lithium insertion (see recommended papers and review papers [7, 10, 22-28]). Various insertion materials have been proposed for the anode of rechargeable lithium batteries,... 

In redox couple notation, E°(HJ"/H2) = 0 at all temperatures. A hydrogen electrode in its standard state, with hydrogen gas at 1 bar and the hydrogen ions present at 1 mol-L 1 (strictly, unit activity), is called a standard hydrogen electrode (SHE). The standard hydrogen electrode is then used to define the standard potentials of all other electrodes ... 

Defining a reference value for the SHE makes it possible to determine E ° values of all other redox half-reactions. As an example. Figure 19-14 shows a cell in which a standard hydrogen electrode is connected to a copper electrode in contact with a 1.00 M solution of C U . Measurements on this cell show that the SHE is at higher electrical potential than the copper electrode, indicating that electrons flow from the SHE to the Cu... 

The standard redox potential E° for these reactions in solutions at pH 7 (when referred to the potential of the hydrogen electrode in the same solution) varies from -0.218V for reaction (15.71) to +0.170V (for reaction (15.70) (i.e., to a rough approximation are close to zero). 

The titration is represented in Fig. 2.22 by plotting the Pt electrode potential versus the titration parameter k. BB is the voltage curve for titration of Fe2+ with Ce4+ and B B that for titration of Ce4+ with Fe2+ they correspond exactly to the pH curves BB and B B in Fig. 2.18, with the exception that the initial point in Fig. 2.22 would theoretically have an infinitely negative and an infinitely positive potential, respectively. In practice this is impossible, because even in the absence of any other type of redox potential there will be always a trace of Fe3+ in addition to Fe2+ and of Ce3+ in addition to Ce4+ present. Further, half way through the oxidation or reduction the voltage corresponds to the standard reduction potentials of the respective redox couples it also follows that the equivalence point is represented by the mean value of both standard potentials ... 

While the laws governing electrode potentials in non-aqueous media are basically the same as for potentials in aqueous solutions, the standardization in this case is not so simple. Two approaches can be adopted either a suitable standard electrode can be selected for each medium (e.g. the hydrogen electrode for the protic medium, the bis-diphenyl chromium(II)/ bis-diphenyl chromium(I) redox electrode for a wide range of organic... 

Figure 1.1 Some redox potentials (in volts) of iron and copper enzymes and chelates at pH 7 relative to the standard hydrogen electrode. From Crichton and Pierre, 2001. Reproduced by permission of Kluwer academic publishers.

The thin-layer configuration and its associated diffusion problems means that it is possible to oxidise (or reduce) all of the electroactive species in the thin layer before they can be replenished to any marked degree. Consider, for example, the 0"+/0 couple, with a standard redox potential well within the "electrochemical window of the solvent, so that the current in the absence of the couple is small and can easily be accounted for. With the electrode pushed against the window the potential is stepped cathodic enough to ensure the rapid reduction of the 0" + and the current measured as a function of time, the concentration such that the time for the current to reach zero, or a steady residual value, is small. If the area under the I ft curve is A ampere seconds, then the charge passed Q = A coulombs. Thus, the number of moles of 0"+ reduced, N0, is given by ... 

The phenomenological treatment assumes that the Gibbs energies of activation Gox and Gred depend on the electrode potential , but that the pre-exponential factor A does not. We expand the energy of activation about the standard equilibrium potential >0o of the redox reaction keeping terms up to first order, we obtain for the anodic reaction ... 

Electron mediators successfully used with oxidases include 2,6-dichlorophenolindophol, hexacyanoferrate-(III), tetrathiafulvalene, tetracyano-p-quinodimethane, various quinones and ferrocene derivatices. From Marcus theory it is evident that for long-range electron transfer the reorganization energies of the redox compound have to be low. Additionally, the redox potential of the mediator should be about 0 to 100 mV vs. standard calomel electrode (SCE) for a flavoprotein (formal potential of glucose oxidase is about -450 mV vs SCE) in order to attain rapid vectrial electron transfer from the active site of the enzyme to the oxidized form of the redox species. 

Equation 13 0.059, [ox] E — Eq + log n [red] This is the well known Nernst equation. Eo is the potential of the electrode when [ox] = [red]. This potential E0 is called the standard electrode potential (sep) which is a characteristic for a particular redox couple. Table 2-1 gives the sep of a number of redox couples. 

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