Electrode redox reactions

The spontaneous redox reaction shown in Figure 19-7 takes place at the surfaces of metal plates, where electrons are gained and lost by metal atoms and Ions. These metal plates are examples of electrodes. At an electrode, redox reactions transfer electrons between the aqueous phase and the external circuit. An oxidation half-reaction releases electrons to the external circuit at one electrode. A reduction half-reaction withdraws electrons from the external circuit at the other electrode. The electrode where oxidation occurs is the anode, and the electrode where reduction occurs is the cathode. 

Thus, an electrode redox reaction occurs according to the scheme... 

In this picture, the electron transfer processes mediated by metallic electrodes (redox reactions in a heterogeneous phase) can also be classified to proceed according to outer-sphere or inner-sphere mechanisms (obviously, considering the electrode surface as a reagent). 

It is important to notice that the rate of a given outer sphere electrode redox reaction should be independent of the nature of the metal electrode if allowance is made for electrostatic work terms or double layer effects which will, of course, be dependent on the nature of the electrode material. Inner sphere reactions, on the other hand, are expected to be catalytic with kinetics strongly dependent on the electrode surface due to specific adsorption interactions. 

In - voltammetry, the electrode redox reaction necessarily involves a change in ionic strength near the electrode (i.e., an ion is consumed, produced, or undergoes a change in charge number). Therefore, the activity coefficients of the electroactive species vary as a function of time and distance from the electrode. In this case, the presence of an excess of a - supporting electrolyte serves as an ionic strength buffer [iv],... 

Electrodialysis can be performed with two main cell types multi-membrane cells for dilution-concentration and water dissociation applications, and electrolysis (or electro-electrodialysis [EED]) cells for oxidoreduction reactions. In multimembrane cells, only the membrane transport phenomena intervene, while electrochemical reactions occurring at the electrodes do not interact with the separation process the electrodes are simple electrical terminals immersed in electrolytes allowing the current transfer. The electrolysis cell operates with only one membrane that separates two solutions circulating in each electrode compartment. This application is based on electrode redox reactions, which are electrolysis specific properties. The anode induces oxidations, and reductions occur at the cathode [4]. 

In the domain of food industries, EED was used to reduce oxygen in fruit juice [46], to extract cytoplasmic proteins from alfalfa [47,48], to coagulate proteins [49], and to reduce disulfide bonds in proteins [50]. These applications are based on the electrode redox reactions coupled with monopolar membrane action. 

Nevertheless, it is convenient to be able to compare the relative affinities of redox couples for electrons as a tool for predicting reaction directions. This comparison is typically done by comparing the electron affinity of a couple to that of the Standard Hydrogen Electrode (SHE). As illustrated in Fig. 3.12, the SHE consists of a platinum electrode (redox reaction site) that is immersed in a water solution at 25 °C containing H" ions at an activity of 1 (pH = 0). Pure hydrogen gas is bubbled around the platinum electrode at a partial pressure of 1 atm, so that H2 also has an activity of 1. The reaction occurring in the SHE is the reduction of H+ to gaseous H2 ... 

We have examined certain approaches to the investigation of redox reactions on membranes. Just as in the case of any electrode redox reaction, a potential arises on the membrane. In the simplest case the potential follows the Nernst equation. In other, more complex cases, when simultaneously ionic permeability is possible or when... 

For such a type of closed electrochemical cell with no convection, it is possible to obtain a non-zero-current steady state (in the strictest sense of the term as regards the electrolyte) If all the electroactive species present in the electrolyte have the same stoichiometries in both electrode redox reactions. The average composition of the electrolyte does not change over time, as illustrated in the example in figure 4.36 which shows concentration profiles for a chronopotentiometry with a fast and nernstian redox system In the presence of a supporting electrolyte with no convection... 

The secondary electrode redox reaction is chosen to be a system where there is little perceptible visible color change or as an electrochromic system where the color change is complementary to that of the color change at the primary electrochromic electrode. 

Amperometric detection makes use of three electrodes the auxiliary, the reference and the working electrode. The first two mentioned are used to monitor the potential in the solution, and the working electrode is the detection electrode. A constant potential is kept between the solution in the cell and the working electrode. Redox reactions of the analyte at the working electrode produce the detection signal. The generated current, I, is proportional to the amount of redox reactions, and thus to the concentration of the analytes [3]... 

So far, we have spoken about a redox reaction between two ions in a solution. In principle, the same applies to an electrode redox reaction also. In this case, we must replace the particle A by the electrode. Of course, for an electrodg reaction the equilibrium values for q and U, and hence E and E, will be different. 

Electron transfer can be established experimentally in reactions involving only ions in solution. Inert electrodes, made from platinum, are used to transfer electrons to and from the ions. The apparatus used is shown in Figure 4.3. the redox reaction being considered... 

The potential of a metallic electrode is determined by the position of a redox reaction at the electrode-solution interface. Three types of metallic electrodes are commonly used in potentiometry, each of which is considered in the following discussion. 

Redox Electrodes Electrodes of the first and second kind develop a potential as the result of a redox reaction in which the metallic electrode undergoes a change in its oxidation state. Metallic electrodes also can serve simply as a source of, or a sink for, electrons in other redox reactions. Such electrodes are called redox electrodes. The Pt cathode in Example 11.1 is an example of a redox electrode because its potential is determined by the concentrations of Ee + and Ee + in the indicator half-cell. Note that the potential of a redox electrode generally responds to the concentration of more than one ion, limiting their usefulness for direct potentiometry. 

Selecting a Constant Potential In controlled-potential coulometry, the potential is selected so that the desired oxidation or reduction reaction goes to completion without interference from redox reactions involving other components of the sample matrix. To see how an appropriate potential for the working electrode is selected, let s develop a constant-potential coulometric method for Cu + based on its reduction to copper metal at a Pt cathode working electrode. 

Influence of the Kinetics of Electron Transfer on the Faradaic Current The rate of mass transport is one factor influencing the current in a voltammetric experiment. The ease with which electrons are transferred between the electrode and the reactants and products in solution also affects the current. When electron transfer kinetics are fast, the redox reaction is at equilibrium, and the concentrations of reactants and products at the electrode are those specified by the Nernst equation. Such systems are considered electrochemically reversible. In other systems, when electron transfer kinetics are sufficiently slow, the concentration of reactants and products at the electrode surface, and thus the current, differ from that predicted by the Nernst equation. In this case the system is electrochemically irreversible. 

Nonfaradaic Currents Faradaic currents result from a redox reaction at the electrode surface. Other currents may also exist in an electrochemical cell that are unrelated to any redox reaction. These currents are called nonfaradaic currents and must be accounted for if the faradaic component of the measured current is to be determined. 

Determining Concentration bet s assume that the redox reaction at the working electrode is... 

Determining Equilibrium Constants for Coupled Chemical Reactions Another important application of voltammetry is the determination of equilibrium constants for solution reactions that are coupled to a redox reaction occurring at the electrode. The presence of the solution reaction affects the ease of electron transfer, shifting the potential to more negative or more positive potentials. Consider, for example, the reduction of O to R... 

A compound which is a good choice for an artificial electron relay is one which can reach the reduced FADH2 active site, undergo fast electron transfer, and then transport the electrons to the electrodes as rapidly as possible. Electron-transport rate studies have been done for an enzyme electrode for glucose (G) using interdigitated array electrodes (41). The following mechanism for redox reactions in osmium polymer—GOD biosensor films has... 

Electroanalytical chemistry is one of the areas where advantage of the unique properties of SAMs is clear, and where excellent advanced analytical strategies can be utilized, especially when coupled with more complex SAM architectures. There are a number of examples where redox reactions are used to detect biomaterials (357,358), and where guest—host chemistry has been used to exploit specific interactions (356,359). Ion-selective electrodes are an apphcation where SAMs may provide new technologies. Selectivity to divalent cations such as Cu " but not to trivalent ions such as Fe " has been demonstrated (360). 

Pseudocapacitance is used to describe electrical storage devices that have capacitor-like characteristics but that are based on redox (reduction and oxidation) reactions. Examples of pseudocapacitance are the overlapping redox reactions observed with metal oxides (e.g., RuO,) and the p- and n-dopings of polymer electrodes that occur at different voltages (e.g. polythiophene). Devices based on these charge storage mechanisms are included in electrochemical capacitors because of their energy and power profiles. 

For an interfering redox reaction at an ion-selective membrane, the overpotential t B can be easily determined experimentally. It is the potential difference between the ion-selective membrane and an inert redox electrode in the same solution containing the measured ion and an interfering redox system. 

In principle at least, any spontaneous redox reaction can serve as a source of energy in a voltaic cell. The cell must be designed in such a way that oxidation occurs at one electrode (anode) with reduction at the other electrode (cathode). The electrons produced at the anode must be transferred to the cathode, where they are consumed. To do this, the electrons move through an external circuit, where they do electrical work. 

In normal battery operation several electrochemical reactions occur on the nickel hydroxide electrode. These are the redox reactions of the active material, oxygen evolution, and in the case of nickel-hydrogen and nickel-metal hydride batteries, hydrogen oxidation. In addition there are parasitic reactions such as the corrosion of nickel current collector materials and the oxidation of organic materials from separators. The initial reaction in the corrosion process is the conversion of Ni to Ni(OH)2. 

In redox flow batteries such as Zn/Cl2 and Zn/Br2, carbon plays a major role in the positive electrode where reactions involving Cl2 and Br2 occur. In these types of batteries, graphite is used as the bipolar separator, and a thin layer of high-surface-area carbon serves as an electrocatalyst. Two potential problems with carbon in redox flow batteries are (i) slow oxidation of carbon and (ii) intercalation of halogen molecules, particularly Br2 in graphite electrodes. The reversible redox potentials for the Cl2 and Br2 reactions [Eq. (8) and... 

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