Some Electrode Reactions

As shown in Section 3.2, the basic voltammetric reaction is oxidant + ne reductant 

This reaction has three components (oxidant)b ik (oxidant) .,, + ne 

The first two of these components are reactions which proceed with finite rates and are mass-transfer controlled. 

In an electrode-electrolyte system, oxidation occurs at the anode and reduction at the cathode (Skoog and West, 1963). When a reaction takes place at an electrode, a change in potential energy occurs energy is either required for the reaction to proceed, or energy is released. At the standard conditions of 1 atm, with all dissolved substances in 1 molal concentrations and at a temperature of 298°K, the reaction potential is defined as . 

Under other than standard conditions, the potential is estimated by the Nernst equation  

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Exchange current densities 4 for some electrode reactions... 

High product selectivity is one of the most important challenges in synthetic methods. Some electrode reactions of organic substrates show a surprisingly high chemoselectivity and regioselectivity Diastereoselectivity is occasionally observed,... 

The additional potential required to cause some electrode reactions to proceed at an appreciable rate. The result of an energy barrier to the electrode reaction concerned, it is substantial for gas evolution and for electrodes made of soft metals, e.g. Hg, Pb, Sn and Zn. It increases with current density and decreases with increasing temperature, but its magnitude is variable and indeterminate. It is negligible for the deposition of metals and for changes in oxidation state. 

In some electrode reactions, there are intermediates of a different kind, species more uncertain as to what they want to do. They do not always bond sufficiently to the substrate to remain there and undergo a consecutive surface reaction step, as does H that combines to H2. This latter type of intermediate comes loose" from the electrode surface. It may then contact the electrode again and react further, or quit the scene and diffuse off into the bulk of the solution, remaining lost to any continued reaction sequence that would be possible if the radical had stayed on the electrode surface for further consecutive reaction steps. 

Some redox processes consume or generate protons. Thus, some electrode reactions are influenced by the pH of the solution, while some conversely have an influence on the pH of the solution. In order to elucidate the electrode processes, it is desirable to measure both the pH of the solution and the electrode reaction. The relations between pH and the electrode processes have been well studied in aqueous solutions. However, in lion-aqueous solutions, such studies have been scarce, except a few cases in recent years. This problem is dealt with in Section 8.3.1. 

Cathodic limits on mercury. In aqueous or other protic solvents the reduction of hydronium ion or solvent generally will limit the negative potential range. The nature of some electrode reactions at highly negative potentials on mercury has been examined.63 For example, K(OH2) and Na(OH2)4 ions are reduced reversibly in aqueous solutions, but the process is accompanied by a parallel irreversible reaction due to an amalgam dissolution reaction of the alkali metal with water that produces hydrogen. 

Some electrode reactions involve a gaseous species such as H2, O2, or CI2. Such reactions must also be carried out on the surface of an electrochemically inert conductor such as platinum. A typical reaction of considerable commercial importance is... 

The limited reversibility of some electrode reactions might require consideration of consumable (cheap) ionic liquids in the anode compartment for technical applications and commercial electroplating. For example, the electrochemical oxidation of oxalate delivers carbon dioxide, hydride could be oxidized to hydrogen, halides to the halogen or trihalide salt in the case of iodide ionic liquids and so on. Since ionic liquids can readily form biphasic systems an alternative may be to have the anodic reaction in an immiscible solvent. In that case a common ion would be needed that can be transferred from one phase to the other. 

Fig. 13. Tafel relations for some electrode reactions on two electrocatalysts (land II). Though electrocatalyst II has higher exchange current density than I at most overpotentials, I is a better electrocatalyst than II at over potentials greater than jj. ...

The accumulation is a dynamic process that may turn into a steady state in stirred solutions. Besides, the activity of accumulated substance is not in a time-independent equilibrium with the activity of analyte in the bulk of the solution. All accumulation methods employ fast reactions, either reversible or irreversible. The fast and reversible processes include adsorption and surface complexation, the majority of ion transfers across liquid/liquid interfaces and some electrode reactions of metal ions on mercury. In the case of a reversible reaction, equilibrium between the activity of accumulated substance and the concentration of analyte at the electrode surface is established. It causes the development of a concentration... 

Some electrode reactions, such as the anodic dissolution of copper... 

In some electrode reactions it may be appropriate to consider that partial charge transfer occurs. In the case of the underpotential deposition of metals, the electrode process leads to the formation of a monolayer or sub-monolayer of metal atoms which interact with the substrate. In the one extreme, the metal-metal bond may have considerable ionic character, so that the reaction approaches ion adsorption, whereas at the other limit an essentially electroneutral surface atom is formed. These complex electrode processes have been discussed extensively in recent years, and the concept of the electrosorption valency [30] has emerged as a way of describing the nature of the under-potential deposit. 

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