Scan rate, potential

Fig. 19.43 Effect of potential scan rate on the value of for Type 304 stainless steel in O-I mol dm NaCI (after Leckie )...

The selection of the pulse amplitude and potential scan rate usually requires a trade-off among sensitivity, resolution, and speed. For example, larger pulse amplitudes result in larger and broader peaks. Pulse amplitudes of 25-50 mV, coupled with a 5 mV s 1 scan rate, are commonly employed. Irreversible redox systems result in lower and broader current peaks (i.e., inferior sensitivity and resolution) compared with those predicted for reversible systems (6). In addition to improvements in sensitivity and resolution, the technique can provide information about the chemical form in which the analyte appears (oxidation states, complexa-tion, etc.). 

Figure 4. Intensity as a function of potential vs. SCE for two of the Raman bands (1346 cm and 699 cm ) of Fe-TsPc adsorbed on a silver electrode at different pH values. These measurements were obtained at a potential scan rate of 10 mV s. See caption Fig. 2.

The current is recorded as a function of time. Since the potential also varies with time, the results are usually reported as the potential dependence of current, or plots of i vs. E (Fig.12.7), hence the name voltammetry. Curve 1 in Fig. 12.7 shows schematically the polarization curve recorded for an electrochemical reaction under steady-state conditions, and curve 2 shows the corresponding kinetic current 4 (the current in the absence of concentration changes). Unless the potential scan rate v is very low, there is no time for attainment of the steady state, and the reactant surface concentration will be higher than it would be in the steady state. For this reason the... 

In a round-trip potential scan the values of corresponding to the anodic and cathodic direction are different. For reversible reactions the difference is minor, according to Eq. (12.9) (i.e., only 0.056/n V regardless of the component concentrations and of the potential scan rate v). It is typical for irreversible reactions that the difference between these potentials is much larger (Fig. 12.9) the gap between the maxima increases with decreasing value of the reaction rate constant and increasing scan rate v. 

The limits of transition region BC are not very distinct and depend on the experimental conditions. At high potential scan rates (short duration of the experiment), passivation will start later (i.e., potential will be somewhat more positive, and for a short time the currents may be higher than i ). 

The dissolution rate during potential cycling was reported to be around 3.0-5.5 ng/cm per cycle, with the upper potential limit between 1.2 and 1.5 V and various potential scanning rates [Johnson et al., 1970 Kinoshita et al., 1973 Rand and Woods, 1972 Wang XP et al., 2006]. The predominant forms of dissolved Pt were... 

Figure 13.1 Electrooxidation of COad and Ci adsorbate layers pie-adsorbed on a Pt/Vulcan thin-film electrode (7 JLgptCm , geometric area 0.28 cm ) in 0.5 M H2SO4 solution during a first positive-going potential scan, and subsequent response of the faradaic (a) and m/z = 44 ion current (b) to the electrode potential in the thin-layer DBMS flow cell. The potential scan rate was 10 mV s and the electrolyte flow rate was 5 p,L s at room temperature. The respective adsorbates were adsorbed at 0.11 V for 10 minutes from CO-saturated solution (solid line), 0.1 M HCHO solution (dashed line), 0.1 M HCOOH solution (dash-dotted line), and 0.1 M CH3OH solution (dash-double-dotted line).

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] ... 

Fig. 10. Logarithmic plot of apparent limiting current density as a function of potential scan rate at a rotating-disk electrode i = apparent limiting current (or peak current) density iL = true steady-state limiting current density d/dt = potential scan rate expressed in units RT/nF oj = rotation rate (rad sec"l). [From Selman and Tobias (S10).]... 

Figure 6. Current-potential curves at a /7-CdTe electrode in a DMF-0.1 M TBAP solution containing 5% water under irradiation with monochromatic light of 600 nm, compared with the current-potential curve in a C02 atmosphere at an In electrode.105 Electrode area 0.2 cm2 (p-CdTe) and 1 cm2 (In). Potential scan rate 0.1 V/s.

Figure 2.12 (Upper) piezoelectric signal curve and (lower) cyclic voltammogram of a gold electrode in pH 3,00,5 M NajSO /H SO electrolyte. The magnitude of the voltage modulation wasSmV at 200 Hz, with a potential scan rate of33.3mV s" From Seo, Jiang and Sato (1987).

Figure 2.110 (a) Simultaneous cyclic voltammetric and mass-potential curves at gold. Potential scan rate = 50mV/s,0.2 M perchloric acid, (b) Simultaneous charge-potential and mass-potential curves at gold. Potential scan rate = 50mV/s, 02M perchloric acid. From Bruckenstein and... 

FIGURE 6.1 CVs obtained at cysteine-modified (curves a and b) and bare Au (curve c) electrodes in 25 mM phosphate buffer in the presence (curves a and c) and absence (curve b) of 0.56 mM Cu, Zn-SOD. Potential scan rate, lOOmV s-1. (Reprinted from [98], with permission from Elsevier.)... 

FIGURE 6.5 CVs at MPA-modified Au electrode in 5mM phosphate buffer (pH 7.0) containing (1) Fe-SOD (0.32 mM), (2) Mn-SOD (0.40mM), and (3) Cu/Zn-SOD (0.20 mM). Inset CV of the MPA-modified Au electrode in pure phosphate buffer. Potential scan rate, lOOmV s-1. (Reprinted from [138], with permission from the American Chemical Society.)... 

Similar to those observed with the cysteine-modified electrode in Cu, Zn-SOD solution [98], CVs obtained at the MPA-modified Au electrode in phosphate buffer containing Fe-SOD or Mn-SOD at different potential scan rates (v) clearly show that the peak currents obtained for each SOD are linear with v (not v 1/2) over the potential scan range from 10 to 1000 mVs-1. This observation reveals that the electron transfer of the SODs is a surface-confined process and not a diffusion-controlled one. The previously observed cysteine-promoted surface-confined electron transfer process of Cu, Zn-SOD has been primarily elucidated based on the formation of a cysteine-bridged SOD-electrode complex oriented at an electrode-solution interface, which is expected to sufficiently facilitate a direct electron transfer between the metal active site in SOD and Au electrodes. Such a model appears to be also suitable for the SODs (i.e. Cu, Zn-SOD, Fe-SOD, and Mn-SOD) with MPA promoter. The so-called... 

FIGURE 6.7 CVs obtained at (a, b) Cu,Zn-SOD/cysteine-modified, (c) bare, and (d) cysteine-modified Au electrodes in 25 mM PBS (pH 7.4) saturated by N2 (a, c, d) or 02 (b). Solutions (b) and (c) contain 0.002U ml-1 XOD and 25mM xanthine. Potential scan rate, lOOmV s. (Reprinted from [150], with permission from the Royal Society of Chemistry.)... 

We consider again the redox reaction Ox + ze = Red with a solution initially containing only the oxidized form Ox. The electrode is initially subjected to an electrode potential Et where no reaction takes place. For the sake of simplicity, it is assumed that the diffusion coefficients of species Ox and Red are equal, i.e., D = D()s = DRcd. Now, the potential E is linearly increased or decreased with E(t) = Ei vt (v is a potential scan rate, and signs + and represent anodic scan and cathodic scan, respectively.) Under the assumption that the redox couple is reversible, the surface concentrations of Ox and Red, i.e., c()s... 

Figure 12. Dependence of the anodic peak current /peak on the potential scan rate v on a logarithmic scale obtained from the cyclic voltammograms for (a) Pt/polished AI2O3, (b) Pt/etched Ni, and (c) Pt/unpolished AI2O3 electrodes. The slope a means (d log /peak / d log v). Here, 14 and v0 means the upper and lower threshold scan rate, respectively. Reprinted from J. -Y. Go et al., A study on ionic diffusion towards self-affine fractal electrode by cyclic voltammetry and atomic force microscopy, J. Electroanal. Chem. 549, p. 49, Copyright 2003, with permission from Elsevier Science.

The CV shows an asymmetrical peak-shaped current wave or wave couple. The current output increases upon increasing the potential scan rate and it is the potential scan rate that contributes dominantly to the peak shape of the... 

A preliminary electrochemical overview of the redox aptitude of a species can easily be obtained by varying with time the potential applied to an electrode immersed in a solution of the species under study and recording the relevant current-potential curves. These curves first reveal the potential at which redox processes occur. In addition, the size of the currents generated by the relative faradaic processes is normally proportional to the concentration of the active species. Finally, the shape of the response as a function of the potential scan rate allows one to determine whether there are chemical complications (adsorption or homogeneous reactions) which accompany the electron transfer processes. 

Figure 1 Potential-time pulses in (a) linear sweep voltammetry (b) cyclic voltammetry. The slope of each line measures the potential scan rate...

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