Resistive and Capacitive Effects

It is well known that experimental CVs for species in solution phase frequently diverge from theoretical ones for -electron reversible couples. The divergence can be caused by a variety of factors deviations from reversibility, occurrence of coupled chemical reactions and/or surface effects, and resistive and capacitive effects (Nicholson and Shain, 1964 Nicholson, 1965a). These last effects will be briefly treated here because of their potential significance when microheterogenous deposits or more or less homogeneous coatings of microporous materials cover the electrode surface. 

In general, for a potential scan experiment initiated at a potential E tart and conducted with a potential scan rate v, the applied potential E satisfies the relationship (see, e.g., Bard and Faulkner, 2001)  

the background current-potential curve will be given by  

In short, the capacitive plus resistive effects mainly result in an enhancement of the background currents in both the positive- and negative-directed scans. Apart from this, resistive and capacitance effects also influence the peak profile so that the peak is flattened and decreased and shifted toward more negative (cathodic peak) or more positive (anodic peak) potentials. 

FIGURE 1.4 CVs for a 2.5-mM solution of Klc,( l in water (0.15 M NaClO4) at a zeolite Y-modified glassy carbon electrode. Potential scan rates of 10, 100, and 1000 mV/sec. 

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A first-order system coupled with dead time (transportation lag) is a good model for many process systems. The dead time (L or td) is the time that has to elapse before the output first starts to respond to a change in the input. The effect of a change in steam rate on the water temperature at the end of the pipe will depend not only on the resistance and capacitance effects in the tank but will also be influenced by the length of time necessary for the water to be transported through the pipe. The effect of dead... 

It should be noted, however, that cathodic-to-anodic peak potential separation can also be increased as a result of coupled chemical reactions. Frequently, resistive and capacitive effects are superimposed to more or less complicated reaction pathways. This can be seen in Figure 1.7, where a CV for a deposit of NiO in contact with 1.0 M KOH is depicted. Here, ill-defined cathodic and anodic signals appear over a large background current. The oxidation process can be described as (Srinivasan and Weinder, 2000 Xing et al., 2004) ... 

This method allows us to distinguish and evalnate the equivalent resistive and capacitive effects attributed to the various constituents of the material. 

Simulations must thus handle the nonlinear boundary conditions. Some have taken the easy way out and used explicit methods [15-18], others used hopscotch [12, 19], ADI (for a two-dimensional problem) [20, 21] and other methods [4, 5, 22-26]. Bieniasz [27] used the Rosenbrock method (see Chap. 9), which makes sense because it effectively deals with nonlinearities without iterations at a given time step. Some have simulated both resistance and capacitive effects [12, 15, 16, 20-22, 25]. 

According to Ohm s law, the potential difference arising from driving a current I through a medium with resistance is IR. Hence the overpotential associated with overpotential increases with driven current, and so with scan rate. At megavolt-per-second scan rates, specialised equipment is required to disentangle useful information given by Faradaic currents from current distortions due to resistive and capacitive effects. 

The answer is b. (tlardman, p 229. Katzung, p 168.) Terazosin blocks a receptors in arterioles and venules It is 0 -selective. Perhaps this selectivity permits NE to exert unopposed negative feedback on its own release because of little or no effect on presynaptic a.2 receptors. Alpha blockers reduce arterial pressure in both resistance and capacitance vessels andT thereforet are quite effective in reducing blood pressure when a patient is in the upright position... 

An EG G PARC 273 Potentiostat/Galvanostat was used in both the electrolysis and the CV experiments, coupled with an HP 7044B X/Y recorder. A Solartron 1255 HF Frequency Response Analyzer and a Solartron 1286 Electrochemical Interface were employed for the a.c. impedance measurements, using frequencies from 0.1 to 65 kHz and a 10 mV a.c. amplitude (effective) at either the open circuit potential (OCP) or at various applied potentials. As the RE can introduce a time delay at high frequencies, observed as a phase shift owing to its resistance and capacitance characteristics, an additional Pt wire electrode was placed in the cell and was connected via a 6.8 pF capacitor to the RE lead [32-34]. 

In case of a homogeneous temperature distribution in the heated area, h corresponds to the temperature coefficient of the heater material, otherwise h includes the effects of temperature gradients on the hotplate. As a consequence of the aheady mentioned self-heating, the applied power is not constant over time, and the hotplate cannot be simply modelled using a thermal resistance and capacitance. Replacing the right-hand term in Eq. (3.28) by Eq. (3.35) leads to a new dynamic equation ... 

Equations 2.37-2.40 result in the commonly used presentation of the impedance, e.g. the Nyquist and the Bode plots. The first one shows the total impedance vector point for different values of co. The plane of this figure is a complex plane, as shown in the previous section. Electrochemical-related processes and effects result in resistive and capacitive behaviour, so it is common to present the impedance as ... 

Alpha blockers reduce arterial pressure by dilating both resistance and capacitance vessels. As expected, blood pressure is reduced more in the upright than in the supine position. Retention of salt and water occurs when these drugs are administered without a diuretic. The drugs are more effective when used in combination with other agents, such as a 13-blocker and a diuretic, than when used alone. 

Resistance and capacitance in combination are the most common in industrial processes involving heat transfer, mass transfer, and fluid flow operations. The combined effect of supplying a capacity through a resistance is time retardation (time constant). Combining a capacitance-type process element (tank) with a resistance-type process component (valve) results in a single-time-constant (t) process. If the tank was initially empty and then an inflow was started at a constant rate of m, the level in the tank would rise as shown in Figure 2.23 and would eventually rise to the steady-state height of c = Rm in the tank. 

Diffusion resistances can occur for Li in the electrode, but also for the salt in the electrolyte (if anion conductivity in the electrolyte is significant). Further effects are due to depletion of carriers at a phase boundary. In such cases, time dependencies of the electrical properties occur (in addition to Rs, effective capacitances Cs also appear). The same is true for impeded nucleation processes. Since any potential step of the electrochemical potential can be connected with current-dependent effective resistances and capacitances, the kinetic description is typically very specific and complex. As the storage processes in Li-based batteries are solid-state processes, the... 

The simulated Nyquist plot of resistance and capacitance in series is a vertical line in the complex-plane impedance diagram, as shown in Figure 4.2(b). The effect of the parameter R on the position of the line is presented in Appendix D (Model Dl). 

Now, for the electrode to change its potential the electrical double layer (Section 8-6) must be charged or discharged by the passage of current, which must flow through an effective resistance equal to the polarization resistance. The time constant for the charging process is the product of resistance and capacitance, or... 

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