Potential Steps

The principle of this method is quite simple The electrode is kept at the equilibrium potential at times t 0 at t = 0 a potential step of magnitude r) is applied with the aid of a potentiostat (a device that keeps the potential constant at a preset value), and the current transient is recorded. Since the surface concentrations of the reactants change as the reaction proceeds, the current varies with time, and will generally decrease. Transport to and from the electrode is by diffusion. In the case of a simple redox reaction obeying the Butler-Volmer law, the diffusion equation can be solved explicitly, and the transient of the current density j(t) is (see Fig. 13.1)  

Under these conditions a plot of j versus t1/2 gives a straight line, and the kinetic current can be obtained from the intercept. If the reaction is fast the straight portion can be too short for a reliable determination of jk in this case one should obtain estimates for y and A from this plot, and use them in fitting the whole curve to Eq. (13.3). 

There are two difficulties with this method. The first one is due to the fact that in reality the potentiostat keeps the potential between the working and the reference electrode constant there is an ohmic resistance Rq between the tip of the Luggin capillary (see Chapter 2) and the working electrode, giving rise to a potential drop I R-n (7 is the current). Since I varies in time, so does the potential drop by which ry is in error. However, modern potentiostats can correct for this to some extent. The second difficulty is more serious. Immediately after the 

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Pick s second law of difflision enables predictions of concentration changes of electroactive material close to the electrode surface and solutions, with initial and boundary conditions appropriate to a particular experiment, provide the basis of the theory of instrumental methods such as, for example, potential-step and cyclic voltanunetry. 

The measurement of the current for a redox process as a fiinction of an applied potential yields a voltaimnogram characteristic of the analyte of interest. The particular features, such as peak potentials, halfwave potentials, relative peak/wave height of a voltaimnogram give qualitative infonnation about the analyte electrochemistry within the sample being studied, whilst quantitative data can also be detennined. There is a wealth of voltaimnetric teclmiques, which are linked to the fonn of potential program and mode of current measurement adopted. Potential-step and potential-sweep... 

Potential-step teclmiques can be used to study a variety of types of coupled chemical reactions. In these cases the experiment is perfomied under diffrision control, and each system is solved with the appropriate initial and boundary conditions. 

Double potential steps are usefiil to investigate the kinetics of homogeneous chemical reactions following electron transfer. In this case, after the first step—raising to a potential where the reduction of O to occurs under diffrision control—the potential is stepped back after a period i, to a value where tlie reduction of O is mass-transport controlled. The two transients can then be compared and tlie kinetic infomiation obtained by lookmg at the ratio of... 

The great advantage of the RDE over other teclmiques, such as cyclic voltannnetry or potential-step, is the possibility of varying the rate of mass transport to the electrode surface over a large range and in a controlled way, without the need for rapid changes in electrode potential, which lead to double-layer charging current contributions. 

This expression is the sum of a transient tenu and a steady-state tenu, where r is the radius of the sphere. At short times after the application of the potential step, the transient tenu dominates over the steady-state tenu, and the electrode is analogous to a plane, as the depletion layer is thin compared with the disc radius, and the current varies widi time according to the Cottrell equation. At long times, the transient cunent will decrease to a negligible value, the depletion layer is comparable to the electrode radius, spherical difhision controls the transport of reactant, and the cunent density reaches a steady-state value. At times intenuediate to the limiting conditions of Cottrell behaviour or diffusion control, both transient and steady-state tenus need to be considered and thus the fiill expression must be used. Flowever, many experiments involving microelectrodes are designed such that one of the simpler cunent expressions is valid. 

If sufficient experience does not exist, you should consider whether the consequence potential (Step 4) or the expected frequency of accidents (Step 5) is great. Consideration of consequence potential should include personnel exposure, public demographics, equipment density, and so forth in relation to the intrinsic hazard posed by the material of concern. In Step 5 you may perceive that the expected frequency of accidents alone is important enough to justify a QRA. However, even though your company may not have much relevant experience with the activity of interest, if the consequence potential of these accidents is not great, you may conclude that the expected frequency of the potential accidents is low enough for you to make your decisions comfortably using qualitative information alone. 

Au(m) nanoclusters with heights of up to nm form at-FlOO mV vs. AI/AICI3 (a) a typical height profile is shown in (b). Upon a potential step to -fITOO mV vs. AI/AICI3 the clusters dissolve immediately and leave holes in the surfaces as well as small Au islands (c) alloying between Al and Au is very likely. 

Au(in) a potential step from -f500 mV vs. Cu/Cu (a) to -f450 mV results in the growth of small Cu islands at the steps of the gold terraces (b) (picture from [66] - with permission of the Peep owner societes). 

The electrode process at -500 mV on this potential scale is correlated to the growth of 250 20 pm high islands. They grow immediately upon a potential step from the open circuit potential to -500 mV (arrow in Figure 6.2-13). 

If the germanium layers are partly oxidized by a short potential step to -1500 mV, random worm-like nanostructures form, healing in a complex process if the electrode potential is set back to more negative values (Figure 6.2-15). 

Na+/Ca2+ Exchangers. Figure 3 Potential steps in the pharmacological regulation of NCX isoform expression and activity. This scheme reproduces the potential levels at which drugs can interfere with the transduction, transcription, translation, and activity of NCX. 

FIGURE 1-2 Concentration profiles for different times t after the start of a potential-step experiment. 

Alternately, for potential-step experiments (e.g., chronoamperometry, see Section 3-1), the charging current is die same as that obtained when a potential step is applied to a series RC circuit ... 

Example 2-4 A potential-step spectroelectrochemistry experiment using a reactant concentration of 2 mM generated a product with an absorbance (sampled after 25 s) of 0.8. Calculate the reactant concentration that yielded an absorbance of 0.4 upon sampling at 16 s. 

The basis of all control Icd-potcntial techniques is the measurement of the current response to an applied potential. There exist a multihide of potential excitations, including a ramp, potential steps, pulse trains, a sine wave, and various combinations thereof. The present chapter reviews those techniques that are widely used. 

The potential-step experiment can also be used to record the charge versus tune dependence. This is accomplished by integrating the current resulting from the... 

Derive the Cottrell equation by combining Fick s first law of diffusion with the tune-dependent change of the concentration gradient during a potential-step experiment. 

Potential of zero charge, 20, 23, 25, 66 Potential scanning detector, 92 Potential step, 7, 42, 60 Potential window, 107, 108 Potentiometry, 2, 140 Potentiometric stripping analysis, 79 Potentiostat, 104, 105 Preconcentrating surfaces, 121 Preconcentration step, 121 Pretreatment, 110, 116 Pulsed amperometric detection, 92 Pulse voltammetry, 67... 

For pitting dissolution after applying a constant potential step, however, as shown in Fig. 32, the sign of L remains negative, so that the above condition is not fulfilled. Therefore the initial fluctuations do not grow, but decrease as the dissolution proceeds. 

After a constant potential step beyond the pitting potential is applied to a nickel electrode in NaCl solution, the current transient shown in Fig. 39 is observed. The J vs. 1/VT plot according to Eq. (104) is shown in Fig. 40. From the linear portion corresponding to Eq. (104), the slope of the plot can be described as a function of the surface coverage 6 of the passive film in the following... 

Figure 52. Nickel surface computed at / = 0.5 s after imposing the potential step beyond the critical potential.99 Other data for calculation are the same as in Fig. 51. (Reprinted from M. Asanuma and R. Aogaki, Morphological pattern formation in pitting corrosion, J. Electrocuted. Chem. 396, 241, 1995, Fig. 9. Copyright 1995, reproduced with permission from Elsevier Science.)...

Electrochemical Structure closure under cathodic prepolarizatiuon Anodic potential step or scan Oxidation current... 

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