Rate-potential response

The scan rate, u = EIAt, plays a very important role in sweep voltannnetry as it defines the time scale of the experiment and is typically in the range 5 mV s to 100 V s for nonnal macroelectrodes, although sweep rates of 10 V s are possible with microelectrodes (see later). The short time scales in which the experiments are carried out are the cause for the prevalence of non-steady-state diflfiision and the peak-shaped response. Wlien the scan rate is slow enough to maintain steady-state diflfiision, the concentration profiles with time are linear within the Nemst diflfiision layer which is fixed by natural convection, and the current-potential response reaches a plateau steady-state current. On reducing the time scale, the diflfiision layer caimot relax to its equilibrium state, the diffusion layer is thiimer and hence the currents in the non-steady-state will be higher. 

Figure 1.3. Rate and catalyst potential response to step changes in applied current during C2H4 oxidation on Pt deposited on YSZ, an O2 conductor. T = 370°C, p02=4.6 kPa, Pc2H4=0.36 kPa. The catalytic rate increase, Ar, is 25 times larger than the rate before current application, r0, and 74000 times larger than the rate I/2F,16 of 02 supply to the catalyst. N0 is the Pt catalyst surface area, in mol Pt, and TOF is the catalytic turnover frequency (mol O reacting per surface Pt mol per s). Reprinted with permission from Academic Press.

Figure 4.15. Rate and catalyst potential response to application of negative currents (a,b), for the case of volcano-type behaviour, see text for discussion. Conditions pCo=2 kPa, p02=2 kPa, T=350°C. Catalyst Cl. 51 Reprinted with permission from Academic Press.

Figure 8.50. Rate and catalyst potential response to a step change in applied current during CH3OH dehydrogenation and decomposition on Ag. The experimental time constants x are compared with 2FNG/I T=660°C, Pch30h=5.2 kPa.56 Reprinted with permission from Academic Press.

Figure 9.9. Rate and catalyst potential response to application of negative currents (a,b), for the case of volcano-type" behaviour (a) and S-type behaviour (b) of the reaction rate, and to application of positive currents (c,d) see text for discussion. Conditions (a) pco 2 kPa, Po2=2 kPa, T=350°C, catalyst Cl (b) pCo=2 kPa, p02=4 kPa, T=350°C, catalyst Cl. (c,d) pCo =0.73 kPa, po2=0.86 kPa, T=402°C, catalyst C2. Reprinted with permission from Academic Press.1 ...

Temporal response to changes in abundance of MeHg A slow rate of response to altered MeHg loading could increase the potential for interference by confounding factors, whereas a rapid response, coupled with high intraannual variability would hinder identification of multiyear trends Hours to days Hours to days... 

Typical current-potential responses are shown in Figure 1.25b for several values of the standard potential separation. As in the case of fast and reversible one-electron transfers, the curves are proportional to the scan rate and are symmetrical around the potential axis. 

Deactivation of the Mediator Deactivation of the mediator is a commonly encountered event in the practice of homogeneous catalysis. Among the various ways of deactivating the mediator, the version sketched in Scheme 2.10 is particularly important in view of its application to the determination of the redox characteristics of transient free radicals (see Section 2.7.2).14 The current-potential responses are governed by three dimensionless parameters, 2ei = /F)(ke Cjl/v), which measures the effect of the rate-determining... 

The situation of interest in the above-mentioned applications is when intermediate values of pc can be explored, leading to its determination and hence of the ratio kin/ke2. In this connection, Figure 2.22 shows typical current-potential responses and the procedure by which the rate ratio k,/ke2 may be determined. 

A detailed description of the laser-excited vibrational fluorescence method and further results on relaxation processes in methane, including V - R transfer, have been given in reference In this paper, too, a comparison is made between the experimentally obtained F - F rates and calculations for the repulsive intermolecular potential responsible for these transitions. 

In derivative chronopotentiometry, the potential response signal of a normal chronopotentiometry experiment is electronically differentiated, and this rate of change of potential with time, dE/dt, is recorded as a function of time, as shown in Figure 4.9 [12]. The minimum in a derivative chronopotentiogram is quantitatively related to the transition time. Thus for a reversible couple,... 

Linear and cyclic sweep stationary electrode voltammetry (SEV) play preeminent diagnostic roles in molten salt electrochemistry as they do in conventional solvents. An introduction to the theory and the myriad applications of these techniques is given in Chapter 3 of this volume. Examples of the linear and cyclic sweep SEV current-potential responses expected for a reversible, uncomplicated electrode reaction are shown in Figures 3.19 and 3.22, respectively. The important equation of SEV, which relates the peak current, ip, to the potential sweep rate, v, is the Randles-Sevcik equation [67]. For a reversible system at some temperature, T, this equation is... 

In agreement with Eq. (1.189), the reversibility degree exhibited by the current-potential response will be determined not only by the value of the rate constants but also by the ratio Rt = k°/mi (with k° being the heterogeneous rate constant for the charge transfer reaction). Thus, for high values of/ Eq. (1.189) becomes... 

Therefore, the ratio / = kt]/ni allows us to define a reversibility criteria for a given current-potential response once the expression of the mass transport coefficient is obtained (see Sects. 3.2.1.4 and 5.3.2). Note that electrochemical reversibility thus considered is not only defined in terms of the intrinsic characteristic of the process (i.e., the particular value of the heterogeneous rate constant and other... 

Fig. 5.11 Current-potential response of CV and SCV (for A = 5mV) for a Nemstian charge transfer process taking place at a planar electrode for different values of the scan rate (shown in the figure). Dashed-dotted lines Pure faradaic component (SCV and CV) calculated by using the numerical procedure proposed in [21, 22]. Dashed lines Charging current calculated from Eqs. (5.77) (SCV) and (5.76) (CV). Solid lines total current calculated as indicated in Eq. (5.75). /JU = 0.1K 2, C,u 20pFcm 2, Area = 0.05 cm2, cj, = ImM, = 0, Do = Dr = 10 5cm2s 1...

Equation (6.41) for the current-potential response has been applied to the analysis of different experimental systems of interest. For example, the experimental SCV voltammograms of the two-electron reduction of anthraquinone-2-sulfonate (AQ) in different mixtures of alkylammonium salts obtained at a gold macroelectrode (radius = 0.9mm) with a scan rate v = lOOmVs 1 are shown in Fig. 6.3 when a staircase... 

Fig. 6.9 Cyclic voltagrams corresponding to an EC mechanism at a planar electrode calculated by following the numerical procedure given in [23, 24] (see also Appendix I). (a) Effect of the dimensionless rate constant = k /a on the current-potential response. K = 0. The values of appear in the figure, (b) Effect of the equilibrium constant /sfeq = 1 /K on the current-potential response.

In order to use SWV to obtain sufficiently precise kinetic data, it is essential to analyze how complications in a fast electron transfer affect the current-potential response. The usual way to do this for non-reversible electrochemical processes is by changing the frequency and, therefore, the dimensionless rate constant given by... 

Where exposures are short and infrequent, the recovery potential of affected populations and ecosystem functions is important in extrapolating potential responses. Rate of recovery is highly dependent on the life-cycle characteristics of the affected species. In the ecotoxicological literature, relatively little experimental information can be found on the recovery potential of species with a long and/or complex life cycle. In addition, for many aquatic species, basic information on life-cycle characteristics is not readily available. To further complicate matters, the number of generations per year of invertebrate species may vary with latitude. 

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