Potential scan rate, effect

Potential Scan Rate Effect on Specific Capacitance... 

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 )...

In other words, the response (which for fast kinetics is more anodic compared to E° ), due to the competitive effects of the potential scan rate, moves towards more cathodic values by 30/n (mV) for every ten-fold increase in the scan rate. However, as shown in Figure 14, it is noted that at the same time the reverse peak tends to disappear, in that on increasing the scan rate, the species Z does not have time to restore Red. This is demonstrated by the current ratio zpr/zpf which is about 1 at low scan rates, but it tends to zero at high scan rates. 

The major advantage of square-wave voltammetry is its speed. The effective scan rate is given by / AEs. The term / is the square-wave frequency (in Hz) and AEs is the step height. Frequencies of 1-100 cycles per second permit the use of extremely fast potential scan rates. For example, if AEs = 10 mV and /= 50 Hz, then the effective scan rate is 0.5 V/s. As a result, the analysis time is drastically reduced a complete voltammogram can be recorded within a few... 

There are those who feel that there are not two distinct potentials. These workers propose that, when measured correctly, Eb6 and Ew are one and the same. In standard testing, the nucleation of pits occurs at Ev, but owing to the time necessary for pits to become established, the probability that pits will repassivate, and the finite potential scan rate used, pits do not cause a dramatic increase in the current until EM. This explanation would rationalize the often-observed effect that increasing the scan rate increases Ebi but not Eip. If I i is properly measured, these workers feel that it can be used as a go-no go potential for applications, i.e., if the potential of the alloy is always below Eip, then pitting cannot occur. 

Equation (1.15) predicts that the capacitive plus resistive current is proportional to V. Since, in the case of diffusion-controlled processes, the peak current will vary with one can expect that the capacitive plus resistive effects will decrease on decreasing potential scan rate. This can be seen in Figure 1.4, where CVs recorded... 

To separate kinetic and resistive effects, one can perform experiments at variable scan rate and at different concentrations of electroactive species. As a result, the peak potential separation increases on increasing v and the concentration of the depolarizer, allowing for estimation of the uncompensated resistance from the slope of the peak potential separation versus peak current plot for different analyte concentrations at a given potential scan rate (DuVall and McGreery, 1999, 2000) using the relationship ... 

Fig.3. Effect of potential scan rate on cyclic voltammograms of 0.25 mM PrThz. 

The temperature dependence of the catalyst activity of an iron fluoro-porphyrin-coated graphite electrode was studied by RDE coupled with the surface cyclic voltammetry. The purpose was to investigate the surface adsorption and reaction, O2 reduction catalysis kinetics, and especially the temperature effect on the catalyst activity. Figure 7.11(A) shows the surface CVs of 5,10,15,20-Tetrakis(pentafluorophenyl)-21H,23H-porphine iron (III) chloride (abbreviated as Fe TPFPP)-coated graphite electrode, recorded in a pH 1.0 Ar-saturated solution at different potential scan rates. The 1-electron reversible redox peak of approximately 0.35 V can be seen, which has a peak current increased linearly with increasing the potential scan rate, indicating the electrochemical behavior of this peak follows the feature of a reversible redox reaction of an adsorbed species on the electrode surface. 

Chemical kinetics at tubular electrodes (systems of circular cross-section) have been considered recently in [77, 78]. Both for catalytic ErevCcat and for E vCirr (follow-up) mechanisms the theory of linear-sweep voltammetry and cyclic voltammetry was elaborated. The effects of the reaction rate constant, of the flow velocity, and of the potential scan rate on the shape of current curves are presented graphically. The deductions derived from both theories were tested on the reduction of Fe(III) in the presence of hydrogen peroxide (catalytic system) and on the oxidation of 1,4-phenylenediamine in alkaline medium (E vCin. mechanism). 

The first example [15] shows how cyclic voltammetry may be used to demonstrate the rapid isomerisation of the anion radical of diethylmaleate (DEM). Fig. 6.17 shows the voltammograms for DEM and diethylfumarate (DEF), the CIS and trans isomers respecively. The reduction of DEF shows a well formed reduction peak, Ep = —1.41 V vs SCE, and a coupled anodic peak above 0.1 Vs the curve has all the properties of a reversible le process. Below this scan rate, there is evidence for a slow chemical reaction, and more detailed investigations showed this to be dimerisation. The reduction peak for DEM, below 5 Vs is much broader, Ep = —1.61V, with no anodic peak corresponding to the reverse process. There is, however, an anodic peak at —1.35 V which is the same potential required for the oxidation of DEF. Also on the second and subsequent cycles a new reduction peak is seen to grow at —1.41V. On increasing the potential scan rate above 50Vs a small anodic peak is seen where the oxidation of DEM is expected and the anodic peak at —1.35 V becomes relatively smaller (i.e. when corrected for scan rate effects). Finally, the current functions, for the main reduction peaks for DEF and DEM are almost independent of v and correspond to reasonable values for a reversible and irreversible le reduction... 

Although the cyclic method is a reasonable method for screening variations in alloy composition and environments, the cycHc potentiodynamic polarization method has been found to have a number of shortcomings [80-85. One major problem concerns the effect of the potential scan rate. The values of both and pro, are a strong function of... 

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