Redox potential of an electrode

When we introduce a metal M into a solution containing ions, a thermodynamic equilibrium is established which involves an oxidation reaction between the metal M and a reduction reaction with the electro-active species in solution  

55 More detailed electrochemical discussions can be found in J. Robert, J. Alzieu, Accumulateurs. Considerations theoriques . Techniques de I ingenieur, D 3 351, 2005. 

Finally, by convention, the choice is made to associate the negative sign with the redox potential of reduction reactions. 

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The Fermi level represents in a way the pressure of electrons and is rather similar to the redox potential of an electrode. 

In electrochemistry, the voltage of an electrode is defined versus a reference electrode (see section 2.9.1 Redox potential of an electrode). The voltage between the two electrodes in a battery is therefore equal to the difference between the voltages of those two electrodes versus the reference electrode. 

Another aspect of tuning the redox potential of an electrode material has been demonstrated by Goodenough et al. [3, 29]. They have shown that the use of polyanions (X04) such as (804) , (P04), (As04), or even (W04) lowers 3if-metals redox energy to useful levels compared to the Fermi level of the Li anode. Thus, the most attractive key point of the polyanion frameworks can be seen in the strong X-O covalency, which results in a decrease of the Fe-O covalency. This inductive effect is responsible for a decrease of the redox potential in comparison to the oxides [29, 30]. The polyanion P04 unit stabilizes the olivine structure of LiFeP04 and lowers the Fermi level of the Fe redox couple through... 

Theoretical cell potential The algebraic sum of the individual redox potentials of an electrochemical cell at zero current, i.e. emf = Epositive electrode - negative electrode- In practice, when Current flows in a cell, a liquid junction potential is present, and the cell potential is larger than this theoretical value. 

The redox properties of an electrode are determined by its potential measured relative to some reference electrode. Many different reference electrodes are used in the literature. In order to make cross comparisons easily, most of the electrode potential quoted for reactions have been converted to the scale based on the saturated calomel electrode as reference. Electrode materials and electrolyte solutions used by the original workers are quoted. In many cases, the electrodes could be fabricated from more modem materials without affecting the outcome of the reactions. In the not too distant past perchlorate salts were frequently used as electrolytes. This practise must be discouraged for preparative scale reactions because of the danger of an explosion when perchlorates and organic compounds are mixed. Alternative electrolytes are now readily available. 

The potential of an electrode of the first kind is determined by the redox equilibrium between the cation with its metal. The simplest example is the equilibrium... 

Pt electrodes, respectively (5). Voltage readings from the Pt electrode in the Np suspensions appeared stable within 15 minutes The redox measurements were converted to pe values [pe 16.9 Eh (in volts) at 25°C] because it is convenient to discuss the redox potential of an aqueous solution in terms of the pe (defined as -log Q of the electron activity)(8). 

When the equivalence point is reached, the Fe2+ will have been totally consumed (the large equilibrium constant ensures that this will be so), and the potential will then be controlled by the concentration ratio of Ce3+/Ce4+. The idea is that both species of a redox couple must be present in reasonable concentrations for a concentration to control the potential of an electrode of this kind. If one works out the actual cell potentials for various concentrations of all these species, the resulting titration curve looks much like the familiar acid-base titration curve. The end point is found not by measuring a particular cell voltage, but by finding what volume of titrant gives the steepest part of the curve. 

The ion sensitive field-effect transistor (ISFET) is a special member of the family of potentiometric chemical sensors [6,7. Like the other members of this family, it transduces information from the chemical into the electrical domain. Unlike the common potentiometric sensors, however, the principle of operation of the ISFET cannot be listed on the usual table of operation principles of potentiometric sensors. These principles, e.g., the determination of the redox potential at an inert electrode, or of the electrode potential of an electrode immersed in a solution of its own ions (electrode of the first kind), all have in common that a galvanic contact exists between the electrode and the solution, allowing a faradaic current to flow, even when this is only a very small measuring current. 

It is worth mentioning that we normally think of calculating the potential of an electrode for a redox system in terms of concentration rather than the other way around. Just as pH is the independent variable in our a calculations with acid/base systems, potential is the independent variable in redox calculations. It is much easier to calculate a for a series of potential values than to solve the expressions for potential given various values of a. 

As already discussed in Section 3.2 the potential across a single solid-liquid interface cannot be measured. One can only measure the potential of an electrode vs. a reference electrode. It has already been shown in Section 3.2 that a certain potential is produced at a metal or semiconductor electrode upon the addition of a redox system, because the redox system equilibriates with the electrons in the electrode, i.e. the Fermi level on both sides of the interface must be equal under equilibrium. It should be emphasized here that the potential caused upon addition of a redox couple to the solution occurs in addition to that already formed by the specific adsorption of, for instance, hydroxyl ions. A variation in the relative concentrations of the oxidized and reduced species of the redox system leads to a corresponding change of the potential across the outer Helmholtz layer, as required by Nernst s law (see Eq. 3.47), which can be detected by measuring the electrode potential vs, a reference electrode. However, there still exists a potential across the inner Helmholtz layer which remains unknown. 

The reaction begins at a certain potential (voltage). As the potential changes, it controls die point at which the redox reaction will take place. Electrodes are placed in an electrolyte solution. The electrolyte contains analyte that will undergo the redox reaction. In CV, the current in the cell is measured as a function of potential. The potential of an electrode in... 

To summarize, you may think of Eh of a solution as either a cell potential or a half-cell potential. It is the potential of a cell having one electrode that responds reversibly to a redox couple (such as Fe +/Fe +) or couples in the solution and the SHE as the other electrode. Or it is the half-cell potential of an electrode responding reversibly to a redox couple or couples in the solution. You may use any kind of electrode as the other side of the cell, as long as you correctly deduce the half-cell potential of the electrode that is responding to conditions in your solution. Half-cell potentials are defined in terms of the SHE by our lUPAC conventions. 

The potential of a half-cell reaction between dissolved species is called the oxidation-reduction potential or redox potential. It describes the oxidative strength of a solution. For example, the electrode reaction (2.88) sets the redox potential of an aqueous solution containing dissolved oxygen. 

The thermodynamics of a redox reaction may be regarded a basic parameter set which defines the theoretical or optimal properties of a system. The theoretical potential of an electrode can be calculated on the basis of the Nernst equation and the Gibbs free energy of the reaction. Such estimations are often done based on literature data of standard enthalpies and entropies of formation of reactants and products. However, there may be deviations in the electrochemical experiment, which may be due to the following reasons ... 

In Chapter 1 we explored the fundamental relationship between the electrode potential and a redox couple in solution. It was also pointed out that if the potential of an electrode is controlled externally, the solution can be made to adjust by electron transfer to approach equilibrium with the electrode potential. In many electrochemical experiments, the solution initially has only one form of a redox couple present, and the electrode is initially set at a potential such that this form does not undergo electron transfer. This ensures that the experiment begins at zero faradaic current. The electrode potential is then changed to a position that favors electron transfer. The manner in which the potential is changed gives rise to a profusion of electrochemical controlled potential techniques. 

Photoexcitation can therefore alter the effective redox potential of an oxidizing or reducing agent. Effects of this kind are now well established. The simplest example is provided by the so-called photovoltaic effect (Fig. 6.20). If we place two platinum electrodes in a solution of a colored compound that can undergo redox reactions and shine light on one of the electrodes, a potential is set up due to the change in redox potential of the solute near the irradiated electrode. 

This value does not exactly equal the redox potential difference in anode and cathode mediators because of a mixed potential, as there is some DET potential at equilibrium even when mediators are present. When the cell is not at equilibrium and current flows, the electrode potentials are separated from the mediator redox potentials by an electrode polarization overpotential anode or /cathode- For example, at the cathode (Equation 9.6),... 

Electrochemistry is an analytical tool that can be used to determine redox potentials of an analyte as well as the fate of a molecule upon addition or removal of electrons. Of particular importance to photochemists is the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Cyclic voltammetry is one of the most commonly used electrochemical techniques and is based on the change in potential as a linear function of time. An electrochemical reaction is reversible if = 1 and AEp = 59/n mV, where ip is the anodic peak current, ip is the cathodic peak current, and A p (AE), = A p — Ep ") is the potential peak separation for the anodic ( ), ) and cathodic Ep ) peaks. The oxidation or reduction potential for a reversible electrochemical process is given by 1/2 = Ep + Ep jl and is recorded vs. a reference electrode. All electrochemical data provided herein are converted to V vs. saturated calomel electrode (SCE) to make the comparison more facile. A reversible redox couple implies that the complex undergoes facile electron transfer with the electrode and that no chemical reaction follows the electrochemical step. 

Electrode reaction is usually composed of charge transfer, adsorption/desorption, and mass transport parts that are present at the low Hz-pHz range of the impedance spectrum. Electrode "polarization" occurs whenever the potential of an electrode V is forced away from its equilibrium value at an open circuit l g. When an electrode is polarized, it can cause current to flow via electrochemical reactions that occur at the electrode surface at characteristic redox potential or charge the interface with overabundant species that cannot discharge due to kinetic restrictions, such as sluggish electrode reaction, adsorption, or diffusion limitations [9, p. 100],... 

Under certain conditions, it will be impossible for the metal and the melt to come to equilibrium and continuous corrosion will occur (case 2) this is often the case when metals are in contact with molten salts in practice. There are two main possibilities first, the redox potential of the melt may be prevented from falling, either because it is in contact with an external oxidising environment (such as an air atmosphere) or because the conditions cause the products of its reduction to be continually removed (e.g. distillation of metallic sodium and condensation on to a colder part of the system) second, the electrode potential of the metal may be prevented from rising (for instance, if the corrosion product of the metal is volatile). In addition, equilibrium may not be possible when there is a temperature gradient in the system or when alloys are involved, but these cases will be considered in detail later. Rates of corrosion under conditions where equilibrium cannot be reached are controlled by diffusion and interphase mass transfer of oxidising species and/or corrosion products geometry of the system will be a determining factor. 

Stationary microwave electrochemical measurements can be performed like stationary photoelectrochemical measurements simultaneously with the dynamic plot of photocurrents as a function of the voltage. The reflected photoinduced microwave power is recorded. A simultaneous plot of both photocurrents and microwave conductivity makes sense because the technique allows, as we will see, the determination of interfacial rate constants, flatband potential measurements, and the determination of a variety of interfacial and solid-state parameters. The accuracy increases when the photocurrent and the microwave conductivity are simultaneously determined for the same system. As in ordinary photoelectrochemistry, many parameters (light intensity, concentration of redox systems, temperature, the rotation speed of an electrode, or the pretreatment of an electrode) may be changed to obtain additional information. 

It is now 20 years since the first report on the electrochemistry of an electrode coated with a conducting polymer film.1 The thousands of subsequent papers have revealed a complex mosaic of behaviors arising from the multiple redox potentials and the large changes in conductivity and ion-exchange properties that accompany their electrochemistry. 

Rotating-disk voltammetry is the most appropriate and most commonly employed method for studying mediation. In most systems that have been studied, there has been little penetration of the substrate in solution into the polymer film. This can be demonstrated most easily if the polymer film is nonconductive at the formal potential of the substrate. Then the absence of a redox wave close to this potential for an electrode coated with a very thin film provides excellent evidence that the substrate does not penetrate the film significantly.143 For cases where the film is conductive at the formal potential of the substrate, more subtle argu-... 

By setting the ratio of the oxidized and reduced forms of a redox couple in an electrode coating film to unity, the potential of this electrode in an inert electrolyte is poised at the half-wave potential of the couple. This has indeed been shown for platinum wires coated with polyvinylferrocene or ferrocene modified polypyrrole But the long term stability of these electrodes during cell connection... 

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