I-E curves

The increase in the Cd content in the Sn + Cd alloy surface layer can also be deduced from the shift with time in the negative direction of polarization (log i,E) curves of SjOl" reduction. X-ray radiation investigations of Sn + Cd alloys show that the alloy consists of Sn crystals (of average size 2 to 3 fim) and substantially smaller Cd crystals arranged along the grain boundaries.824... 

The correct potential for a preparative electrolysis is normally chosen by inspection of a steady state current-potential (i-F) curve. Figure 1 shows a typical i-E curve for the reduction of anthracene at a mercury cathode in dimethylformamide (Peover et al., 1963) the curve shows two reduction waves. In the potential range where the current rises with variation of the potential, the rate of an electron transfer process is increasing while in the plateau regions the rate of the electron transfer... 

Fig. 1. i-E curve for the reduction of anthracene at a dropping mercury electrode in acetonitrile containing 01m tetraethylammonium perchlorate. 

The development and the very widespread use of the polarographic technique to record i-E curves and the more recent designing of electronic devices known as potentiostats which automatically control the potential of the working electrode at a pre-set value has led to many examples in the literature of organic electrode reactions whose products depend on the potential. Some examples are cited below ... 

The evidence for this type of mechanism is usually that the steady-state i-E curve for the electrolysis medium is unaltered by the addition of the substrate. 

More recently it has become apparent that proton equilibria and hence pH can be equally important in aprotic and other non-aqueous solvents. For example, the addition of a proton donor, such as phenol or water, to dimethylformamide has a marked effect on the i-E curve for the reduction of a polynuclear aromatic hydrocarbon (Peover, 1967). In the absence of a proton donor the curve shows two one-electron reduction waves. The first electron addition is reversible and leads to the formation of the anion radical while the second wave is irreversible owing to rapid abstraction of protons from the solvent by the dicarbanion. 

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 substrate concentrations are interesting variables when a mixture of two electroactive species is oxidized or reduced. If we take the example of two species and R2 which are oxidized at sufiBciently different potentials that two clear waves are obtained on an i-E curve, an electrolysis carried out at a potential on the plateau of the first wave must occur via the route... 

The surface concentration Cq Ajc in general depends on the electrode potential, and this can affect significantly the form of the i E) curves. In some situations this dependence can be eliminated and the potential dependence of the probability of the elementary reaction act can be studied (called corrected Tafel plots). This is, for example, in the presence of excess concentration of supporting electrolyte when the /i potential is very small and the surface concentration is practically independent of E. However, the current is then rather high and the measurements in a broad potential range are impossible due to diffusion limitations. One of the possibilities to overcome this difficulty consists of the attachment of the reactants to a spacer film adsorbed at the electrode surface. The measurements in a broad potential range give dependences of the type shown in Fig. 34.4. 

Fig. 1.4. Test for the survival of adsorbed CO on Pt after transfer in the UHV. i E curve during the oxidation of COad in 0.1 M H2S04 (a) immediately after elimination of CO in the bulk (b) after elimination of bulk CO and transfer in the UHV.

The disc is held at a potential where the reaction of interest takes place, and an I/E curve is then recorded at the ring. This allows the identification of any solution-free intermediates and/or products. 

An I/E curve is recorded at the disc while the ring potential is held at a constant value where the intermediates or products are reduced. This... 

Hence, the author was able to model the I/E curves in the hydrogen adsorption region of Pt. The calculations were compared with experiment (using highly purified electrolytes), at various temperatures, and the result obtained at 0°C is shown in Figure 3.4(a). 

The characteristic shape of i-E curve depends on the nature of the redox couple in the condensed phase, its thermodynamics, kinetics, mass transfer, and on the voltage-time profile (E—t). In this section we will discuss various voltammetric techniques and their applications in modern chemistry. 

Transfer of several electrons yields n > 1 from the above procedures, but CV additionally shows the relative thermodynamics and depending on the individual values, various shapes of i/E curves are obtained (Figure 8b). If the two E are sufficiently different AE > 100 mV), two separated peak couples occur (dash-dotted line). On the other hand, if decreases below F 100 mV, the voltammetric signals merge (dashed line). 

Wagner-Traud Diagram, According to the mixed-potential theory, the overall reaction of the electroless deposition, [Eq. (8.2)] can be described electrochemically in terms of three current-potential i-E) curves, as shown schematically in Eigure 8.2. First, there are two current-potential curves for the partial reactions (solid curves) (1) ic =f(E), the current-potential curve for the reduction of ions, recorded from the rest potential E eq M the absence of the reducing agent Red (when the activity of is equal to 1, eq,M E m) and (2) = f(E), the current-potential... 

Electroless Deposition in the Presence of Interfering Reactions. According to the mixed-potential theory, the total current density, is a result of simple addition of current densities of the two partial reactions, 4 and However, in the presence of interfering (or side) reactions, 4 and/or may be composed of two or more components themselves, and verification of the mixed-potential theory in this case would involve superposition of current-potential curves for the electroless process investigated with those of the interfering reactions in order to correctly interpret the total i-E curve. Two important examples are discussed here. 

Figure 10.8. Potentiostatic i-E curves with V = 800 V/s in HCIO4 + 1M CH3OH (curve B) and in 1N HCIO4 (curve A) starting from the open-circuit potential. (From Ref. 3, with permission from the Electrochemical Society.)...

Effects A and C have differently shaped dose-response curves, i.e., curve A is much steeper than curve C therefore, effect A could be interpreted as being more severe than effect C. Furthermore, effect A is observed at much lower dose levels than effect C, i.e., the... 

The number of bands present in a spectral window and their centers of symmetry are pre-requisites to other signal processing procedures i. e. curve fitting. For in situ spectroscopic reaction studies, a set of tracks can be assigned which specify the centers of symmetry for all the bands. Since the bands move as a function of composition, the tracks in a matrix or AF %xv drift. 

Fic. 7. Properties of the F, C, I, E curves for particular patterns of flow (dead-water regions and bjrpassing flow absent) in closed vessels (L13). 

UMEs of 10 pm in diameter and voltammetric instruments for use with such UMEs are commercially available. Electrodes of smaller dimensions can be prepared in the laboratory, although this requires considerable skill [74], In order to use UMEs successfully for high-speed voltammetry in highly resistive solutions, care must be taken concerning the effects of the ohmic drop and the capacitance of the cell system [65 b, 74, 75]. Moreover, two types of voltammograms, i.e. curves (a) and (b) in Fig. 5.23, should be used appropriately, according to the ob-... 

Wagner-Traud Diagram. According to the mixed-potential theory, the overall reaction of the electroless deposition, [Eq. (8.2)] can be described electrochemically in terms of three current-potential (i-E) curves, as shown schematically in Figure 8.2. 

The complete current-voltage characteristics of the sensor can be derived from the similar consideration that was used for derivation of the i-E curve for liquid electrolytes. Because the potentials at each electrode are reversible, their difference can be expressed by the Nernst equation for the concentration of oxygen at the anode Co(0) and at the cathode Co (A). The current flowing through the layer generates a voltage drop iRb, where Rb is the bulk resistance of the ZrC>2 layer. 

For either of these two methods, in which the measurement of the current is restricted to one selected potential, it is necessary to carry out a preliminary recording of the i — E curves in the reaction mixture at chosen time intervals. This makes it possible to choose the correct applied potential corresponding to the limiting current of the wave studiet. It is also advisable to record one complete i — E curve at the end of each kinetic run, in order to confirm that the applied voltage chosen really corresponded to the limiting current of interest. Such a current-voltage curve also allows unexpected reactions to be detected, if new waves appear. 

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