Electrode Process Proper

The reactions taking place in the vicinity of the electrode surface can be classified as acid-base reactions, hydration-dehydration equilibria, interactions of negatively charged species with metal cations, and ringopening reactions. Some of these reactions precede the electrode process proper, some are interposed between two electrode processes and some are consecutive to the electrochemical step. 

For systems which involve rapidly established acid-base equilibria preceding the electrode process proper, the polarographic curves show a single wave the height of which is pH-independent. For the simple scheme (26) ... 

If the oxidized or reduced form of an electroactive substance undergoes a chemical reaction in solution, this chemical reaction competes with the transport of the species towards the surface of the electrode. The chemical reactions involving the electroactive substance can precede or occur in parallel with or be consecutive to the electrode process proper. General information can be found in monographs (Heyrovsky and Khta, 1966 Delahay, 1954 Mairanovskii, 1966) and reviews (Brdi5ka, 1954 BrdiCka et al., 1962 Niirnberg and v. Stackelberg, 1961). 

The principle how these chemical reactions affect polarographic curves will be demonstrated by reference to a system which involves a reaction preceding the electrode process proper. This is representative of the most commonly met type. In these reactions an electro-active species C is formed in a reversible reaction of the type (20) ... 

Electrode processes in which a chemical reaction precedes the electrode process proper belong to the most common type giving rise to a kinetic current. The chemical reaction involved is either an acid-base or another type of equilibrium. 

FAST REACTIONS ACCOMPANYING THE ELECTRODE PROCESS AND RATES OF ELECTRODE PROCESS PROPER From the measurements of polarographic limiting kinetic currents (and sometimes of their half-wave potentials), and their dependence on certain parameters (mainly pH, buffer composition, drop-time etc.), it is possible to compute rate constants for the fast chemical reactions, antecedent, parallel or consecutive to the electrode process proper. Rate constants of the second order reactions of the order 10 to 10 1. mol. sec have been determined in this way. The mathematical basis and the method of computation of the rate constants is beyond the scope of this text, and the reader is referred to other texts. 

From the slope of the polarographic wave much information can be obtained concerning the parameters of the electrode process proper (standard reaction rate k, and the transfer coefficient a. The reader is again referred to monographs< > and review articles. 

During electrochemical reduction of aldehydes and ketones other chemical reactions take place in addition to protonation in the region in the vicinity of the electrode. Thus, with aromatic aldehydes and ketones in sufficiently acidic solutions at the potentials of the first reduction wave the free radicals dimerize to pinacols immediately after the electron transfer, and under conditions where the electrode process proper is reversible the dimerization affects the... 

The low conductivity of high-purity water makes it difficult to study electrode processes potentiostatically, since too high an electrical resistance in the circuit can affect the proper functioning of a potentiostat, and it can also introduce large iR errors. The increase in conductivity of water with temperature has been measured and /7 -corrected polarisation data have been obtained in hot water that originally had very low conductivity at room temperature. Other results in high-temperature water are all for tests where the conductivity was deliberately increased through the addition of electrolytes. 

It is clear that the passage of charge across the electrode—solution interface may cause oxidation or reduction. Since the amount of chemical change is governed by Faraday s laws, such processes are termed faradaic. A link exists between the electrode potential, E, and the concentrations (properly activities) of compounds of the electrode process. In general... 

For this reason the consumable anodes must be replaced periodically. The cathode consists of a molten aluminum layer on the bottom of the cell, and the anode-cathode distance is 4-5 cm. Alumina is periodically added to the cell in the proportion that it is consumed by electrolysis. The electrode processes during aluminum electrolysis are very complex [141] and a proper understanding of these processes is important because of the economic implications energy and carbon consumption, cell control, pollution of the environment, etc. 

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... 

Generally, the rate of the electrode process is influenced by the mass transport of reactants to and products from the electrode surface, the proper electron transfer process, and the chemical reactions preceding or following the electron transfer. Accordingly, the value of 17 may involve a concentration (mass transport), activation (electron transfer), and ohmic (ohmic resistance) polarization contribution. 

If the electrode process is not complicated by adsorption or other phenomena, depending directly on the nature of the electrode, then the rate of the charge transfer stage proper should be independent of the electrode material. Indeed, this type of independence has been earlier ascertained experimentally for the electroreduction of anions and the electron photoemission into solution, fo such processes should belong also the cathodic generation of solvated electrons by the primary mechanism. 

If the surface process proper cannot be considered infinitely fast, the times r and can be limited by the duration of this process for example, by the duration of an elementary act of an electrode reaction. 

In many descriptions of electrochemical preparations of organic substances, only the overall current and voltage applied across the cell have been specified. It must be emphasized that this information is generally inadequate for a proper electrochemical specification of the experimental conditions and a characterization of the reaction mechanism. Under constant current conditions, as consumption of the reactant occurs, the potential normally becomes increased (greater polarization) until eventually some new electrode process becomes predominant (see Section 5.1). This may either be decomposition of the solvent or supporting electrolyte or, in some cases, a further reaction with the substrate involved in the electroorganic preparation. In the latter case, it is clear that the preparation will yield more than one principal product. A classical case, first investigated by Haber, is the electroreduction of nitrobenzene referred to above and also the Kolbe reaction. ... 

Let us consider the mechanism of the electrode process as a sequence of elementary reactions all of which, in a steady state, occur at the same velocity, equal to the overall velocity, when stoichiometry is properly taken into account. Let us designate by j the rate-determining step and let us assume that it occurs yj times during one occurrence of the overall process. The number yj is the stoichiometric number of step j. Let us consider the system as being in a state close to electrochemical equilibrium for which it can safely be assumed that the electrochemical affinities of all other... 

It is clear from this concluding discussion that the theory of simple electrode reactions must be modified considerably to deal with the majority of electrode processes. From the experimental point of view, it is important to measure the anodic and cathodic Tafel slopes as well as the reaction order with respect to all possible reactants (including the supporting electrolyte and the solvent). At the same time, non-electrochemical methods such as infrared and Raman spectroscopy [31,32] can provide direct molecular information which is essential for the proper understanding of electrode processes. 

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