Electrode processes, simultaneous

The basic relationships of electrochemical kinetics are identical with those of chemical kinetics. Electrochemical kinetics involves an additional parameter, the electrode potential, on which the rate of the electrode reaction depends. The rate of the electrode process is proportional to the current density at the studied electrode. As it is assumed that electrode reactions are, in general, reversible, i.e. that both the anodic and the opposite cathodic processes occur simultaneously at a given electrode, the current density depends on the rate of the oxidation (anodic) process, ua, and of the reduction (cathodic) process, vc, according to the relationship... 

In an analysis of an electrode process, it is useful to obtain the impedance spectrum —the dependence of the impedance on the frequency in the complex plane, or the dependence of Z" on Z, and to analyse it by using suitable equivalent circuits for the given electrode system and electrode process. Figure 5.21 depicts four basic types of impedance spectra and the corresponding equivalent circuits for the capacity of the electrical double layer alone (A), for the capacity of the electrical double layer when the electrolytic cell has an ohmic resistance RB (B), for an electrode with a double-layer capacity CD and simultaneous electrode reaction with polarization resistance Rp(C) and for the same case as C where the ohmic resistance of the cell RB is also included (D). It is obvious from the diagram that the impedance for case A is... 

In an individual molten carbamide, the electrode processes are feebly marked at melt decomposition potentials because of its low electrical conductivity. Both electrode processes are accompanied by gas evolution (NH3, CO, C02, N2) and NH2CN (approximately) is formed in melt. In eutectic carbamide-chloride melts electrode processes take place mainly independently of each other. The chlorine must evolve at the anode during the electrolysis of carbamide - alkali metal and ammonium chloride melts, which were revealed in the electrolysis of the carbamide-KCl melt. But in the case of simultaneous oxidation of carbamide and NH4CI, however, a new compound containing N-Cl bond has been found in anode gases instead of chlorine. It is difficult to fully identify this compound by the experimental methods employed in the present work, but it can be definitely stated that... 

In order to understand the methodology in some detail, we first consider homogeneous processes, where the electrochemical techniques used are well-established. Such processes are not central to this book, which is primarily concerned with electrode processes, but they do serve to illustrate the manner in which mechanisms can be explored. As indicated above, any step in the electrochemical mechanism must be either chemical (denoted by C) or electro-chemical (denoted by E) in nature. It is not normally the case that more than one electron is transferred simultaneously, so possible sequences may be written down straightforwardly. 

Having defined in situ and ex situ methodology, we have seen that in situ spectroelectrochemistry (simultaneous electrochemistry and spectroscopy) is a powerful technique for studying electrode processes. 

Quinolinic acid (133) was prepared by methods similar to those described for the monocarboxylic acids.182,183,189-191,196-202 In many cases the resulting diacid was decarboxylated to nicotinic acid (126). Quinoline (130) was simultaneously oxidized to the diacid (133) and reduced to tetrahy-droquinoline (134) in one of the rare reports of paired synthesis of pyridine compounds (Scheme 44).189 An attempt was made to delineate some of the electrode processes for the diacid (133).200... 

As written above, these processes seem rather numerous, but in all probability (6), (c), and (d) occur simultaneously and since the energy of adsorption of hydrogen molecules is so much less than that of hydrogen atoms (if indeed hydrogen can be adsorbed as molecules), (e) is probably a much slower process than (/). Also, at any rate with ordinary current densities, (a), the rate of movement of ions up to the surface, need not be smaller near the surface than anywhere else in the solution, so that it is simply part of the resistance of the whole solution to the passage of current and does not form part of the electrode processes. Therefore there are two possible stages at which the delay at the electrode may occur, namely... 

Since the early work of Kanazawa [1] and Bruckenstein in 1985 [2], quartz crystal resonators have been used for more than 12 years in contact with liquids to assess changes in mass during electrochemical surface processes. Extensive use of the electrochemical quartz crystal microbalance (EQCM) has been done in the study of electrode processes with change of mass simultaneous to charge transfer. 

Comninellis et al. (Comninellis 1994 Comninellis and De Battisti 1996 Simond et al. 1997 Foti et al. 1999) found that the nature of the electrode material strongly influences both the selectivity and the efficiency of the process and, in particular, several anodes favored the partial and selective oxidation of pollutants (i.e., conversion), while others favored complete combustion to C02. In order to interpret these observations, they proposed a comprehensive model for the oxidation of organics at metal oxide electrodes with simultaneous oxygen evolution. 

The anodic partial current may be a sum of several partial currents when two or more electrode processes take place simultaneously (see partial current) for instance, the evolution of chlorine and oxygen from aqueous hydrochloride acid solutions at high positive potentials. 

Proof of highly unstable radical intermediates by electrochemiluminescence — A special mechanism of Electrochemiluminescence is observed when both reacting species of the electron transfer are generated simultaneously at the same potential at the same electrode (so-called DC-ECL or ECL with a coreactant). This is possible only when a chemical step is involved in the electrode process of the coreactant (CH). A typical example is the cleavage of its primarily formed ion radical into a stable ion and a strongly reducing or oxidizing... 

The kinetics is called irreversible in electrochemistry when the charge-transfer step is very sluggish, i.e., the standard rate constant (ks) and - exchange current density (j0) are very small. In this case the anodic and cathodic reactions are never simultaneously significant. In order to observe any current, the charge-transfer reaction has to be strongly activated either in cathodic or in anodic direction by application of -> overpotential. When the electrode process is neither very facile nor very sluggish we speak of quasireversible behavior. 

Ions in Two Valence Stages.—A special case of simultaneous electrode processes arises when a given ion can exist in two valence stages, e.g., mercuric (Hg++) and mercurous (HgJ ) the passage of one faraday then results in the discharge at the cathode or the formation at the anode of a total of one gram equivalent of the two ions. An equilibrium exists between a metal and the ions of lower and higher valence thus, for example,... 

The nucleus or development center in physical development can be described as a dual electrode on which the reduction of silver ion to silver and the oxidation of developing agent take place simultaneously. Electrochemical measurements of silver physical development in a hydroquinone/Phenidone physical developer with silver ion complexed with thiocyanate proceed as a catalytic electrode process [39]. 

If the second electrode process does not interfere with the first (e.g., it may be due to the background), the method may still be acceptable in the laboratory (e.g., anodic meth-oxylation or an electrolytic reduction with simultaneous hydrogen evolution). For industrial processes, however, a high current yield (or current efficiency) is generally desirable and waste of current on the background is less tolerable. The current yield is the theoretical amount of electricity divided by the amount actually employed for the production of a particular substance (usually expressed as a percentage). 

The second important quantity, the half-wave potential can be a measure of the standard free energy change (AG°) or free energy of activation AG ) associated with the electrolytic process. The value of the half-wave potential depends on the nature of the electroactive species, but also on the composition of the solution in which the electrolysis is carried out. If the composition of the solution electrolysed, consisting of the electroactive substance and a proper supporting electrolyte, often buffered, is kept constant, it is possible to compare the half-wave potentials of various substances. When the mechanism of the electrode process is similar for all compounds compared, the halfwave potential can be considered to be a measure of the reactivity of the compound towards the electrode. Hence the half-wave potentials are physical constants that characterize quantitatively the electrolysed compound, or the composition of the electrolyzed solution. In the application of polarography to reaction kinetics the half-wave potentials are of importance both for slow and fast reactions. For slow reactions a large difference in half-wave potentials makes a simultaneous determination of several components of the reaction mixture possible. In... 

The overall electrode process consists of carrier transport in the semiconductor, electrochemical reactions at the interface, and mass transport of the reactants and reaction products in the electrolyte. There are a number of physical phases associated in the current path and the change of potential in each phase has a specific effect in relation to surface geometry. Also, a number of different reactions can occur simultaneously on the surface and compete in surface coverage and in reaction rate. Particularly, the anodic reactions of silicon in HF solutions have two parallel paths silicon may react with fluoride species and dissolve directly or may react with water to form oxide. 

There is no doubt that the variants described above cannot comprehend aU the possible ways of the reaction zone extension, even for the relatively simple electrode system. It is possible that some of the electrode processes can take place simultaneously on the gas-electrolyte, gas-metal, and metal-electrolyte interfaces. The removal of oxygen in the second variant, for instance, can be represented by the following reactions diffusion of subions along the metal-electrolyte interface, and diffusion of oxygen atoms on the gas-metal interface. Prior to this, the oxidation reaction of subion to atom O should take place with the transfer of electrons into the metal. 

In practice, sensors with oxoanionic solid electrolytes are less successful till now, especially in tests of long-term stability. There are many reasons for this, a fundamental reason is given by the electrode processes taking place during unevitable current flows. Every direct current causes on one side a loss of solid electrolyte material in consequence of alkali ion migration and gas delivery. On the other side the discharge of alkali ions causes chemical reactions with gas components forming compounds like oxides, hydroxides, basic salts or hydrates which do not correspond to the solid electrolyte material. Every flow of direct current produces an asymmetry in the body of the oxoanionic solid electrolyte. At the cathode, besides the reactions (25-66) and (25-67), simultaneously electrode reactions are possible, for example. 

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