Potential sweeps

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... 

Similarly to the response at hydrodynamic electrodes, linear and cyclic potential sweeps for simple electrode reactions will yield steady-state voltammograms with forward and reverse scans retracing one another, provided the scan rate is slow enough to maintain the steady state [28, 35, 36, 37 and 38]. The limiting current will be detemiined by the slowest step in the overall process, but if the kinetics are fast, then the current will be under diffusion control and hence obey the above equation for a disc. The slope of the wave in the absence of IR drop will, once again, depend on the degree of reversibility of the electrode process. 

The potentiostatic technique has a number of variations and the potential may be increased or decreased incrementally, changed continuously at a predetermined rate (potential sweep) or applied as pulses of very short duration. The applications of the potentiostatic technique are considered in detail in Sections 1.4, 1.5 and 19.2, and will not be considered here. 

Cyclic Voltammetry measurement of the current or current density as a function of the electrode potential by application of one or more potential sweep cycles. 

In this procedure, a constant sine wave a.c. potential of a few millivolts is superimposed upon the usual d.c. potential sweep. The applied d.c. potential is measured in the usual way and these results are coupled with measurements of the alternating current. 

Figure 20. Pit-dissolution current density pit radius and ion concentration buildup AC in the pit electrolyte corresponding to the critical condition for growing pits on 18Cr-8Ni stainless steel to passivate at different repassivation potentials, EK, in 0.5 kmol m 3 H2S04 + 0.5 kmol m-3 NaCl during cathodic potential sweep at different sweep rates.7 (From N. Sato, J. Electrochem. Soc. 129,261,1982, Fig. 1. Reproduced by permission of The Electrochemical Society, Inc.)...

As shown in Fig. 25, an example of the extrapolation of the current transient obtained from the potential sweep yields the critical potential after ascertaining that the data obtained are independent of the sweep rate. Figure 26 exhibits the results of the critical pitting potential measurement for the majority salt of NaCl and the minority ion of Ni2+when the concentration of NaCl is varied under the condition of constant Ni2+ionic concentration. From the plot in Fig. 26, it follows that... 

Figure 4. Log intensity vs. potential plots (Tafel plots) obtained from the voltammograms of a platinum electrode submitted to a 2 mV s l potential sweep polarized in a 0.1 M LiC104 acetonitrile solution having different thiophene concentrations. (Reprinted from T. F. Otero and J. Rodriguez, Parallel kinetic studies of the electrogeneration of conducting polymers mixed materials, composition, and kinetic control. Electrochim, Acta 39, 245, 1994, Figs. 2, 7. Copyright 1997. Reprinted with permission from Elsevier Science.)...

Angular Movement Described by the Free End of the Bilayer from the Initial Vertical Position when Submitted to a Potential Sweep from 400 mV to -170 mV at 1 mV s 1 in 0.1 M IJCIO4 Aqueous Solution at Ambient Temperature... 

After a critically sized nucleus is formed, it starts to grow. Under the same hypothesis stated above for chronoamperometric growth, and taking into account that the radial growth of each cylinder occurs at the same time that the overpotential (tj) rises owing to the potential sweep (o) ... 

Figure 56. Plot of (a) 1 -1) N/tf% vs. t)c and (b) tjN vs. tjc for polypyrrole films submitted to potential sweeps, from which the nucleation parameters (z and / ) can be obtained. (Reprinted from T. F. Otero, H.-J. Grande, andJ. Rodriguez, J. Phys. Chem. 101,8525,1997, Figs. 3-11, 13. Copyright 1997. Reproduced with permission from the American Chemical Society.)...

Equations (57) and (58) describe the electrochemical oxidation of conducting polymers during the anodic potential sweep voltammograms (/f vs. q) or coulovoltagrams (Qr vs. tj) under conformational relaxation control of the polymeric entanglement initiated by nucleation in the reduced film. They include electrochemical variables and structural and geometric magnitudes related to the polymer. 

Taking into account the variation in the oxidized area as a function of the overpotential, and the counter-ion flows, the charge consumed during the potential sweep in those regions where the structure was previously opened under conformational relaxation control, is given by... 

Figure 11. Dynamic microwave conductivity-potential curves taken with a ZnO single crystal and shown for two potential sweep velocities (a) and (b) and a corresponding dynamic (photo)current-potential curve (bottom). The dark effects and photoeffects are indicated for the two cases. Curves 1 and 2 correspond to (a) and (b) respectively.

Figure 25. Effect of corrosion and prepolarization on (a) PMC voltage and (b) photocurrent voltage dependence. Left n-Si (covered with Pt particles) in contact with a 5 M HBr/0.05 M Br2 aqueous solution. A comparison is made of the PMC peak during the first and the third potential sweeps. Right n-WSe2 in contact with an aqueous 0.05 M Fe2+/3+ solution. The effect of cathodic prepolarization on position and height of the PMC peak is shown.

Figure 35. Dynamic change of lifetime in an n-type silicon/polymer (poly(epichlorhydrine-co-elhylenoxide-co-allyl-glycylether plus iodide) junction during a potential sweep. The arrows show the direction of sweep (0.25 V s" ). A shoulder in the accumulation region and a peak in the depletion region of silicon are clearly seen.

Part of the problem is the variety of solvents, electrolytes, concentrations, and electrochemical techniques (potentiostatic, galvanostatic, potential sweep, etc.) that have been employed.20 However, even when stringent efforts are made to keep all parameters and conditions constant, there are still problems with reproducibility.83... 

Potential sweep kinetics, with polymer formation, 416... 

Potential sweep measurements, with microwave frequency effects, 455 Pourbaix diagrams, applied to adlayers on copper, 93... 

Hamilton IC, Woods R (1981) An investigation of surface oxidation of pyrite and pyrrhotite by linear potential sweep voltammetry. J Electroanal Chem 118 327-343... 

Figure 3. Frequency shift of the Raman band at 612 cm for Fe-TsPc adsorbed on a sliver electrode as a function of the applied potential vs. SCE In 0.05 M H2S0. Laser excitation line 514.5 nm potential sweep rate 10 mV s electrode area 0.27 cm. See caption Fig. 2.

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