Randles—Sevcik equation

Reversible Systems The peak current for a reversible couple (at 25°C), is given by the Randles-Sevcik equation ... 

Radical ions, 33, 44 Raman spectroelectrochemistry, 45 Randles-Sevcik equation, 31 Rate constant, 12, 18 Rate determining step, 4, 14 Reaction mechanism, 33, 36, 113 Reaction pathway, 4, 33 Reaction rate, 12 Receptor-based sensors, 186 Redox recycling, 135... 

According to Eq. (1) the steady-state current across a micro-ITIES is proportional to the bulk concentration of the transferred species. Thus, the micro-ITIES can function as an amperometric ion-selective sensor. Similarly, the peak current in a linear sweep voltam-mogram of ion egress from the micropipette obeys the Randles-Sevcik equation. Both types of measurements can be useful for analysis of small samples [18a]. 

As reversible ion transfer reactions are diffusion controlled, the mass transport to the interface is given by Fick s second law, which may be directly integrated with the Nernst equation as a boundary condition (see, for instance. Ref. 230 232). A solution for the interfacial concentrations may be obtained, and the maximum forward peak may then be expressed as a function of the interfacial area A, of the potential scan rate v, of the bulk concentration of the ion under study Cj and of its diffusion coefficient D". This leads to the Randles Sevcik equation [233] ... 

It is interesting to note that the conventional Randles-Sevcik equation, relating 7peak to v1 2, does not hold for the fractal electrode. Instead it is replaced with the following generalized... 

Such an expression is called the Randles-Sevcik equation. 

Finally, if Red) is more easily reduced than Ox (i.e. Ef > Ef), once again one has a single catho-anodic peak-system, Figure 3Id. However, in this case the peaks are sharper. In fact, A.Ep is equal to 28.5 mV and the forward peak current is 2.83-fold larger than that of a one-electron process (i.e. 23/2, according to the Randles-Sevcik equation, Section 1.1). The average potential measured between Ep( and. Epr is, in this case, intermediate between E and Ef. 

As mentioned in the introduction to controlled potential electrolysis (Section 2.3), there are various indirect methods to calculate the number of electrons transferred in a redox process. One method which can be rapidly carried out, but can only be used for electrochemically reversible processes (or for processes not complicated by chemical reactions), compares the cyclic voltammetric response exhibited by a species with its chronoamperometric response obtained under the same experimental conditions.23 This is based on the fact that in cyclic voltammetry the peak current is given by the Randles-Sevcik equation ... 

Figure 6.15 Plot of peak current (/p) in a voltammogram (either linear-sweep or cyclic) against analyte concentration. The linear portion obeys the Randles-SevCik equation, while the horizontal plateau at low Ca aiyie values is usually caused by non-faradaic components of Ip, such as double-layer charging.

Why does the plot in Figure 6.17 deviate from the Randles-SevCik equation ... 

Figure 6.17 broadly follows the Randles-Sevcik equation insofar as most of the plot of p against v is linear. The figure does, however, have a non-zero intercept, and is not linear at the highest scan rates. 

The Randles-SevCik equation usually gives a more accurate value since all measurements are performed under pseudo-steady-state conditions. In practice, a Randles-SevCik plot of Ip against is drawn for a redox couple of known... 

Two important parameters in SEV are the peak potential Ep and the peak current ip, which are the potential and current at the characteristic voltammogram peak as shown in Figure 3.18. For a reversible system the peak current is defined by the Randles-Sevcik equation,... 

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... 

This is the well-known Randles-Sevcik equation, which establishes a linear dependence between the peak current and the square root of the scan rate for a reversible process under linear diffusion conditions [23, 24]. For a typical value... 

Peak currents were measured and the linear plot of ip versus v1/2 (R = 0.997) showed that the oxidation process was diffusion-controlled (inset in Figure 9.6). On the basis of the Randles-Sevcik equation [112]. the number of ferrocene molecules was calculated to be around 240 ferrocene moieties per CPMV particle. The appearance of a unique reversible process indicates that the multiple ferrocenyl centers behave as independent, electronically isolated units therefore the CPMV-Fc conjugates are similar to metallodendrimers and could find applications as... 

Sevcik, Augustin — (1926-2006) Research student of -> Heyrovsky, /. In his dissertation he derived an equa-tionfor diffusion-controlledvoltammetric curves [i],independently of - Randles. See also - Randles-Sevcik equation. 

For a reversible system, the Randles-Sevcik equation gives the peak current (ip) for a reversible couple ... 

Quantitative information is obtained from the Randles-Sevcik equation, which at 25 °C is... 

Figure 3.19 Plots of normalized peak current (n = 1) for dioxygen reduction, using the Randles-Sevcik equation, [39] (open circles) and corrected normalized current (solid circles) versus Fe(lll) TMPyP concentration in solutions containing 02 at 0.24 mM for v = 0.05Vs-1.

---