The Electrode Process

Similarly to the response at hydrodynamic electrodes, linear and cyclic potential sweeps for simple electrode reactions will yield steady-state voltammograms with forward and reverse scans retracing one another, provided the scan rate is slow enough to maintain the steady state [28, 35, 36, 37 and 38]. The limiting current will be detemiined by the slowest step in the overall process, but if the kinetics are fast, then the current will be under diffusion control and hence obey the above equation for a disc. The slope of the wave in the absence of IR drop will, once again, depend on the degree of reversibility of the electrode process. 

In this scheme the reversible conversion of A to O is the reaction whose rate is to be studied, whereas the reduction of O to R is the electrode process. Scheme XIV can also represent a pseudo-first-order formation of O. A specific example is the acid-base equilibrium of pyruvic acid, shown in Scheme XV. 

The electrode process at -500 mV on this potential scale is correlated to the growth of 250 20 pm high islands. They grow immediately upon a potential step from the open circuit potential to -500 mV (arrow in Figure 6.2-13). 

Furthermore, the smaller the magnitude of /q the greater the magnitude of tja and the lower the rate of the electrode process at any given polarised... 

Although Table 2.16 shows which metal of a couple will be the anode and will thus corrode more rapidly, little information regarding the corrosion current, and hence the corrosion rate, can be obtained from the e.m.f. of the cell. The kinetics of the corrosion reaction will be determined by the rates of the electrode processes and the corrosion rates of the anode of the couple will depend on the rate of reduction of hydrogen ions or dissolved oxygen at the cathode metal (Section 1.4). 

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

Thus owing to the electrode process the potential will change and will become more negative in a cathodic process and more positive in an anodic process ... 

Activation Overpotential that part of an overpotential (polarisation) that exists across the electrical double layer at an electrode/solution interface and thus directly influences the rate of the electrode process by altering its activation energy. 

One possible reason for the reluctance of non-electrochemists to venture into this field is that in contrast to the electrochemists claim that controlled potential electrolysis offers a method for the selective introduction of energy into molecules, many electrode reactions carried out at a controlled potential have still been reported to give low yields and a diversity of products. The electrode potential is, however, only one of several variables and the lack of selectivity in the electrode process may be attributed to a failure to understand and to control all the parameters of the overall electrode reaction. 

The absence of any variation of these characteristics of the electrode reaction with pH indicates that it is the species which predominates in the bulk of the solution which undergoes the electron transfer at the electrode surface. Conversely, a variation shows that the electrode process is... 

At the extremes of pH it is common for the equilibrium to be driven completely to the left or right and then the electrode process becomes independent of pH since it is the bulk species which is electroactive. Thus the common shape of curves is shown in Fig. 7 (Zuman... 

It has been seen from the above simple examples that the concentration of the substrate has a profound effect on the rate of the electrode process. It must be remembered, however, that the process may show different reaction orders in the different potential regions of the i-E curve. Thus, electron transfer is commonly the slow step in the Tafel region and diffusion control in the plateau region and these processes may have different reaction orders. Even at one potential the reaction order may vary with the substrate concentration as, for example, in the case discussed above where the electrode reaction requires adsorption of the starting material. 

The major effect of an increase in temperature on the actual electron transfer process is to increase A , and hence to enhance the reversibility of the electrode process. The reversible potential is, however, itself temperature dependent, and... 

While it is widely realized that pressure is a useful variable for increasing the solubility of the electroactive species and hence the rate of the electrode process, it is mostly forgotten that it is also a variable which affects several of the steps in the overall process. In fact these more subtle effects of pressure on organic electrode reactions do not seem to have been investigated although it is possible to estimate their importance by considering the known effects of pressure on chemical systems (Hamann, 1957). 

Thus, the electrode processes occurring in zinc-carbon batteries with salt electrolytes are complicated, and their thermodynamic analysis is difficult. In a rough approximation disregarding secondary processes, the current-producing reaction can be described as... 

Equation (6.13), in fact, reflects the physical nature of the electrode process, consisting of the anode (the first term) and cathode (the second term) reactions. At equilibrium potential, E = Eq, the rates of both reactions are equal and the net current is zero, although both anode and cathode currents are nonzero and are equal to the exchange current f. With the variation of the electrode potential, the rate of one of these reactions increases, whereas that of the other decreases. At sufficiently large electrode polarization (i.e., deviation of the electrode potential from Eg), one of these processes dominates (depending on the sign of E - Eg) and the dependence of the net current on the potential is approximately exponential (Tafel equation). 

Anodic stripping voltammetry (ASV) has been used extensively for the determination of heavy metals in samples of biological origin, such as lead in blood. ASV has the lowest detection limit of the commonly used electroanalytical techniques. Analyte concentrations as low as 10 M have been determined. Figure 16 illustrates ASV for the determination of Pb at a mercury electrode. The technique consists of two steps. The potential of the electrode is first held at a negative value for several minutes to concentrate some of the Pb " from the solution into the mercury electrode as Pb. The electrode process is... 

Metal insoluble-salt These consist of a metal in contact with one of its slightly soluble salts this salt in turn is in contact with a solution containing the anion of the salt. An example is represented as Ag AgCl Or (c). The electrode process at such an electrode as AgCl (s) Ag + Cl" Ag + e- —> Ag (s) or overall, AgCl (s) + e- Ag (s) + Cl". The electrode reaction involves only the concentration of Cl" as a variable, in contrast with the Ag Ag electrode, which has the Ag concentration as a variable. The most frequently electrode of this type is the calomel electrode (see text for description). 

The electrode processes that are reversible provide values for the equilibrium emfs of cells, which are related to the thermodynamic functions. The condition of reversibility is practically obtained by balancing cell emf against an external emf until only an unappreciable current passes through the cell, in order that the cell reactions proceed very slowly. It may, however, be pointed out that for many of the applications of electrometallurgy, it is clearly necessary to consider more rapid reaction rates. In that situation there is necessarily a departure from the equilibrium condition. Either the cell reactions occur spontaneously to produce electric energy, or an external source of electric energy is used to implement chemical reactions (electrolyses). 

The so-called indicator electrodes must be considered as microelectrodes, which means that the active surface area is very small compared with the volume of the analyte solution as a consequence, the electrode processes cannot perceptibly alter the analyte concentration during analysis in either non-faradaic potentiometry or faradaic voltammetry. 

By plotting E against log(icd - i)/i one can assess the reversibility of the electrode process, because a straight line with slope 2.303RTjnFshould then be obtained from the slope one can also find n many workers determine the slope by simply calculating Ei - Ei, which yields... 

In addition to its use in analysis, it can also be applied to elucidate the electrodic process. Most polarographic waves can be described by the equation (cf.. eqn. 3.43)... 

The flow of electric current through the electrolytic cell is connected with chemical, electrochemical and physical processes which, as a whole, are termed the electrode process. The main electrochemical step in the electrode process is the actual exchange of charged species between the electrode and the electrolyte, which will be termed the electrode reaction (charge transfer reaction). Substances participating directly in the charge transfer reaction are termed electroactive. These substances can be either soluble or insoluble in the electrolyte or electrode material. Common basic types of electrode reactions are as follows ... 

If the electrolyte components can react chemically, it often occurs that, in the absence of current flow, they are in chemical equilibrium, while their formation or consumption during the electrode process results in a chemical reaction leading to renewal of equilibrium. Electroactive substances mostly enter the charge transfer reaction when they approach the electrode to a distance roughly equal to that of the outer Helmholtz plane (Section 5.3.1). It is, however, sometimes necessary that they first be adsorbed. Similarly, adsorption of the products of the electrode reaction affects the electrode reaction and often retards it. Sometimes, the electroinactive components of the solution are also adsorbed, leading to a change in the structure of the electrical double layer which makes the approach of the electroactive substances to the electrode easier or more difficult. Electroactive substances can also be formed through surface reactions of the adsorbed substances. Crystallization processes can also play a role in processes connected with the formation of the solid phase, e.g. in the cathodic deposition of metals. 

In electrode processes, the overall ( brutto ) reaction must be distinguished from the actual mechanism of the electrode process. For example, by a cathodic reaction at a number of metal electrodes, molecular hydrogen is formed, leading to the overall reaction... 

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