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Backward current component

Fig. 3.6 Cathodic SWV curves for three quinone dyes and pigments lawson (1, a quasireversible process), alizarin lake (2, a reversible process) and cochineal red (3, a quasireversible process). Scans from open-circuit potential toward negative potentials. Insets the net, forward and backward current components are shown for alizarin lake and cochineal red (reprinted from [186] with permission)... Fig. 3.6 Cathodic SWV curves for three quinone dyes and pigments lawson (1, a quasireversible process), alizarin lake (2, a reversible process) and cochineal red (3, a quasireversible process). Scans from open-circuit potential toward negative potentials. Insets the net, forward and backward current components are shown for alizarin lake and cochineal red (reprinted from [186] with permission)...
The voltammetric features of a reversible reaction are mainly controlled by the thickness parameter A = The dimensionless net peak current depends sigmoidally on log(A), within the interval —0.2 < log(A) <0.1 the dimensionless net peak current increases linearly with A. For log(A )< —0.5 the diSusion exhibits no effect to the response, and the behavior of the system is similar to the surface electrode reaction (Sect. 2.5.1), whereas for log(A) > 0.2, the thickness of the layer is larger than the diffusion layer and the reaction occurs under semi-infinite diffusion conditions. In Fig. 2.93 is shown the typical voltammetric response of a reversible reaction in a film having a thickness parameter A = 0.632, which corresponds to L = 2 pm, / = 100 Hz, and Z) = 1 x 10 cm s . Both the forward and backward components of the response are bell-shaped curves. On the contrary, for a reversible reaction imder semi-infinite diffusion condition, the current components have the common non-zero hmiting current (see Figs. 2.1 and 2.5). Furthermore, the peak potentials as well as the absolute values of peak currents of both the forward and backward components are virtually identical. The relationship between the real net peak current and the frequency depends on the thickness of the film. For Z, > 10 pm and D= x 10 cm s tlie real net peak current depends linearly on the square-root of the frequency, over the frequency interval from 10 to 1000 Hz, whereas for L <2 pm the dependence deviates from linearity. The peak current ratio of the forward and backward components is sensitive to the frequency. For instance, it varies from 1.19 to 1.45 over the frequency interval 10 < //Hz < 1000, which is valid for Z < 10 pm and Z) = 1 x 10 cm s It is important to emphasize that the frequency has no influence upon the peak potential of all components of the response and their values are virtually identical with the formal potential of the redox system. [Pg.133]

Ihe word net is used to emphasize that one is not talking of the component forward and backward current densities i and i, but of the resultant current density i = i — i. [Pg.496]

The analysis and methodology for the extraction of characteristic parameters obtained from cyclic voltammograms is shown in Fig. II. 1.9b. A zero current line for the forward scan data has to be chosen (dashed line) as baseline for the determination of the anodic peak current. For the reverse sweep data the extended forward scan (dashed line with Cottrell decay) is folded backwards (additionally accounting for capacitive current components) to serve as the baseline for the determination of the... [Pg.66]

The exchange current Iq is a measure of the rate of exchange of charge between oxidized and reduced species at any equilibrium potential without net overall change. The rate constant k, however, has been defined for a particular potential, the formal standard potential of the system. It is not in itself sufficient to characterize the system unless the transfer coefficient is also known. However, Eq. (2.21) can be used in the elucidation of the electrode reaction mechanism. The value of the transfer coefficient can be determined by measuring the exchange current density as a function of the concentration of the reduction or oxidation species at a constant concentration of the oxidation of reduction species, respectively. A schematic representation of the forward and backward currents as a function of overvoltage, 7] = E - E, is shown in Fig. 2.6, where the net current is the sum of the two components. [Pg.43]

The analysis and methodology for the extraction of characteristic parameters obtained from cyclic voltammograms is shown in Fig. II. 1.9b. A zero current line for the forward scan data has to be chosen (dashed line) as baseline for the determination of the anodic peak current. For the reverse sweep data the extended forward scan (dashed line with Cottrell decay) is folded backwards (additionally accounting for capacitive current components) to serve as the baseline for the determination of the cathodic peak current. This procedure can be difficult and an approximate expression for analysis based on the peak currents and the current at the switching potential has been proposed as an alternative [46]. If the blank current before the anodic peak starts cannot be neglected, this current has to be extrapolated into the range where the peak occurs, or, if possible, has to be subtracted from the sample voltammogram. Also, when the sample solution does not contain only the reduced form (as supposed... [Pg.61]

Fig. 10.6 Total currents with relevant cathodic and anodic components, i.e., forward and backward currents, respectively, as a function of the overvoltage for two different reversibility degrees, as simply computed by the Butler-Volmer expression in Eq. (10.27) in plot (b) the total current coincides with the anodic (red line) or cathodic (blue line) component, for positive and negative values of q, respectively... Fig. 10.6 Total currents with relevant cathodic and anodic components, i.e., forward and backward currents, respectively, as a function of the overvoltage for two different reversibility degrees, as simply computed by the Butler-Volmer expression in Eq. (10.27) in plot (b) the total current coincides with the anodic (red line) or cathodic (blue line) component, for positive and negative values of q, respectively...
Table 2.1 Square-wave voltammetry of fast and reversible electrode reaction (1.1). The dimensionless net peak current, the ratio of peak currents of the forward and backward components, the peak potentials of the components and the half-peak width as functions of SW amplitude ... Table 2.1 Square-wave voltammetry of fast and reversible electrode reaction (1.1). The dimensionless net peak current, the ratio of peak currents of the forward and backward components, the peak potentials of the components and the half-peak width as functions of SW amplitude ...
The application of (2.17)-(2.20) is shown in Fig. 2.18. The response depends on the sphericity parameter p = VD/rov7 [27]. Under the influence of increasing parameter p, the minimum of the forward component and the maximum of the backward component gradually vanish and both components acquire the form of a polarographic wave. At potentials much lower than the half-wave potential, both currents tend to the limiting value which is equal to —p. The net peak potential is equal to the reversible half-wave potential and independent of the sphericity parameter, but the dimensionless net peak current is a linear function of the parameter p. If n sw = 50 mV and uAE = -5 mV, this relationship is ... [Pg.28]

Figure 2.69 compares the theoretical responses of an adsorption coupled reaction with the simple reaction of a dissolved redox couple, for a reversible case. Obviously, the adsorption enhances considerably the response, making the oxidation process more difficult. The forward component of reaction (2.144) is a sharp peak, with a lower peak width compared to reaction (2.157). The relative position of the peak potentials of the forward and backward components of the adsorption comph-cated reaction is inverse compared to simple reaction of a dissolved redox couple. Finally, the peak current of the stripping (forward) component of adsorption coupled reaction is lower than the backward one, the ratio being 0.816. The corresponding value for reaction of a dissolved couple is 1.84. This anomaly is a consequence of the current sampling procedure and immobilization of the reactant, as explained in the Sect. 2.5.1. [Pg.99]

Figure 3.12 shows the forward and backward components of square-wave voltam-mograms of mercury(ll)-ferron complex adsorbed on the surface of static mercuiy drop electrode [208]. The ratio of the current and the corresponding SW frequency is reported. At pH 3.5 the electrode reaction involves the direct transfer of two electrons, whereas at pH 5.8 only one electron is exchanged. The simulated responses are presented by symbols. The best fit was achieved by using the following standard rate constants and the transfer coefficients k. = 1550 50 s and a = 0.5 (at pH 3.5), and = 1900 400s and a = 0.55 (at pH 5.8) [208]. [Pg.153]

Current of the forward SW component, A Current of the backward (reverse) SW component, A Crrrrent of the net SW component, A... [Pg.190]

However, in order to achieve these objectives a good interpretation of corporate business needs into achievable R D objectives is essential. In some cases a supplier of a raw material, intermediate or component may choose to forward-integrate their business into production of the finished product. In other cases a manufacturer may wish to backwards-integrate into the production of the raw material(s) required for its current product(s). In either case it is obvious that in order to achieve success a company must make products available to its customers with useful functional properties and at a price that allows cost-effective use of that product in the customer s own products. That is, the product must give the customer s products a competitive advantage. [Pg.468]

At electrochemical dynamic equilibrium, the net current is zero, i.e., the electron transfer process occurs at equal rates, both in the forward and in the reverse direction because of the formation of excess charges on both sides of electrode and electrolyte. The exchange current density is large when the chemical components of Gibbs free energies for forward and backward reactions are small [Eq. (38)]. Eq. (37),... [Pg.2510]

Separation of the cathodic and anodic components of the net current (measured at the end of forward and backward pulses) in square-wave voltammetries (SQWVs) provided only anodic components for PTA Y electrodes immersed into BU4NPI ),/ MeCN, as depicted in Figure 8.15. In contrast, SQWVs display well-developed anodic and cathodic components for PTA Y electrodes in contact with LiClO4/ MeCN. This feature, indicative of reversible electron transfer processes, was found to be more pronounced on decreasing square-wave frequency. [Pg.182]

The very displacement in potential that activates kf also suppresses k hence the backward component of the electrode reaction becomes progressively less important at potentials further to the negative side of. If is very small, a sizable activation of kf is required for all points where appreciable current flows, and is suppressed consistently to a negligible level. The irreversible regime is defined by the condition that k y/kf 0 (i.e., 0 0) over the whole of the voltammetric wave. Then (5.5.11) becomes... [Pg.195]

In Fig. II. 1.12, cyclic voltammograms incorporating both IR drop and capacitance effects are shown. Effects for the ideal case of a potential independent working electrode capacitance give rise to an additional non-Faradaic current (Fig. II. 1.12b) that has the effect of adding a current, /capacitance = Cw x v, to both the forward and backward Faradaic current responses. Tbe capacitance, Cw, is composed of several components, e.g. double layer, diffuse layer, and stray capacitance, with the latter becoming relatively more important for small electrodes [61]. On the other hand, the presence of uncompensated resistance causes a deviation of the applied potential from the ideal value by the term R x /, where R denotes the uncompensated resistance and I the current. In Fig. II. 1.12, the shift of the peak potential, and indeed the entire curve due to the resistance, can clearly be seen. If the value of Ru is known (or can be estimated from the shape of the electrochemically reversible... [Pg.72]

Esw, is one-half of the peak-to-peak amplitude, and the potential increment AE is the step height of the staircase waveform. The scan rate is defined as AE/r. Relative to the scan direction, AE, forward and backward pulses can be distinguished. The currents are measured at the end of each pulse and the difference between the currents measured on two successive pulses is recorded as a net response. Additionally, the two components of the net response, i.e., the currents of the forward and backward series of pulses, respectively, can be displayed as well [6, 27-30]. The currents are plotted as a function of the corresponding potential of the staircase waveform. [Pg.122]

The peak currents and potentials of the forward and backward components are listed in Table II.3.2. If the square-wave amplitude is not too small nEsw > 10 mV), the backward component indicates the reversibility of the electrode reaction. In the... [Pg.124]


See other pages where Backward current component is mentioned: [Pg.108]    [Pg.108]    [Pg.13]    [Pg.584]    [Pg.104]    [Pg.183]    [Pg.13]    [Pg.584]    [Pg.195]    [Pg.7]    [Pg.13]    [Pg.21]    [Pg.22]    [Pg.35]    [Pg.30]    [Pg.127]    [Pg.385]    [Pg.177]    [Pg.178]    [Pg.380]    [Pg.584]    [Pg.136]    [Pg.115]    [Pg.4]    [Pg.90]    [Pg.123]   
See also in sourсe #XX -- [ Pg.7 ]

See also in sourсe #XX -- [ Pg.7 ]




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