Potential Ramps

Differential-pulse voltammetry is an extremely useful technique for measuring trace levels of organic and inorganic species, hi differential-pulse voltammetry, fixed-magnitude pulses—superimposed on a linear potential ramp—are applied to the working electrode at a time just before the end of the drop (Figure 3-5). The current... 

Williams, B. A. and Vigh, G., Effect of the initial potential ramp on the accuracy of electrophoretic mobilities in capillary electrophoresis, Anal. Chem., 67, 3079, 1995. 

In connection with the square-wave technique, mention can be made of high-frequency polarography, also called radiofrequency polarography and developed by Barker53, in which a sinusoidal radio-frequency cu, (100kHz to 6.4 MHz) square-wave modulated at co2 (225 Hz) is superimposed on to the dc potential ramp as the wave form includes (apart from additional higher... 

The current peaks observed when fast potential ramps are applied appear similar to the one shown in Fig. 8. The peak currents (triangles in Fig. 10) can be satisfactorily interpreted in terms of a pure-diffusion model (S10) in equimolar ferri-ferrocyanide solution,... 

As shown above, potentiodynamic generation of limiting currents is more rapid and, therefore, preferable in principle to the galvanodynamic technique. However, during a linear decrease of potential to the limiting-current condition, the current density initially rises more rapidly than when a current ramp is used. Therefore, in the case of copper deposition at the cathode, a linear potential ramp tends to yield a rougher deposit and a less well-defined plateau than a linear current ramp (see Section III,F). 

Unsteady-state mass transfer caused by excessively fast current or potential ramps. This is especially likely to occur in measurements involving laminar flow past elongated surfaces and in free-convection studies, in which the establishment of secondary flow patterns may require long times. A compromise between the time sufficient to reach steady-state transport and the time necessary to avoid bulk depletion and surface roughening (in metal deposition) is required, and is found most reliably by preliminary experimentation. 

As was discussed above, it is essential to determine the effect, if any, that the emersion process has on the double layer. To do this, Wilhelm and colleagues have performed the definitive type of blank experiment. CO was adsorbed onto the Pt working electrode from sulphuric acid electrolyte. After adsorption, the CO-saturated solution was replaced with pure electrolyte. The potential of the electrode was then ramped in order to oxidise off the adsorbate, as C02, and the voltammogram so obtained is shown in Figure 2.118(a). The experiment was then repeated CO was adsorbed as before, but the electrode was emersed and transferred into the UHV chamber, before being re-immersed and the potential ramp applied. The voltammogram so... 

Figure 6.12 Linear-sweep voltammogram for the reduction reaction, O - - ne" —> R, at a solid electrode, shown as a function of the scan rate u. The solution was under diffusion control, which was achieved by adding inert electrolyte and maintaining a still solution during potential ramping. Note that the x-axis has been normalized to , that is, thex-axis represents an overpotential. Reproduced from Greef, R., Peat, R., Peter, L.M., Pletcher, D. and Robinson, J., Instrumental Methods in Electrochemistry, Ellis Horwood, Chichester, 1990, with permission of Profes.sor D. Pletcher, Department of Chemistry, University of Southampton, Southampton, UK.

A greater improvement in sensitivity is obtained if the drop remains unpolarized during the majority of its lifetime. In practice, instead of a simple potential ramp... 

In many respects, differential pulse voltammetry is more similar to classical polarography than to the normal pulse methods (see above). A linear potential ramp of dE/dt is applied to the working electrode (see Figure 6.24). However, in common with normal pulse voltammetry, a succession of pulses are also applied to the working electrode. (The WE is often a DME, and then we refer to differential pulse polarography .)... 

Differential pulse voltammetry A form of voltatiunetry in which a linear potential ramp of dE/dt is applied to the working electrode, superimposed on which is a succession of pulses. 

The principles of thin-layer anodic stripping voltammetry were discussed, and a model for the stripping stage was developed for anodization by a linear potential ramp [35]. The experimental test of the theories was carried out for cadmium and lead amalgams. 

Many of the experimental parameters for normal-pulse polarography are the same as with differential-pulse polarography. Differential-pulse polarography is a technique that uses a series of discrete potential steps rather than a linear potential ramp to optimize specific applications (130). Unlike normal-pulse polarography, each potential step has the same amplitude, whereas the return potential after each pulse is slightly negative of the potential prior to the step. In this manner, the total waveform applied to the dropping mercury electrode is very much like a combination of a linear ramp with a superimposed square wave. 

As an alternative to a stepwise variation with time, a continuously changing potential may be imposed. Though other possibilities have been used [42, 43], a linearly changing potential—time waveform, known as a potential ramp [Fig. 17(a)], is the most common. The technique has many names, including linear sweep voltammetry [44]. If the direction of the ramp is reversed [Fig. 17(b)], the technique is often termed cyclic voltammetry (see Chap. 3), though this name is more appropriately applied after sufficient ramp reversals [Fig. 17(c)] have caused the experiment to become periodic. 

Fig. 17. Potential sweep waveforms, (a) Linear potential ramp (b) isosceles triangular potential ramp (c) periodic triangular potential ramp.

In cyclic voltammetry, the potential applied to the working electrode is varied linearly (Fig. 2.1) between potentials Ex and E2, E2 being a potential more positive (for oxidation) or negative (for reduction) than the peak maximum observed for the oxidation/reduction reaction concerned. At E2, the voltage scan is reversed back to E3 or to another end potential value, E3. The application of this type of potential ramp can be done in a number of ways, varying the starting potential Eu the reverse potential E2, the end potential E3 and the scan rate. The latter is the rate that is applied to vary the potential as a function of time, commonly represented in Vs 1 or mVs"1. 

Variation of the applied potential in cyclic voltammetry. Typical triangle wave between two potentials followed by an opposite triangle (.) or by another type of potential ramp ( ). 

Note that digital instrumentation approximates the linear potential ramp as a staircase waveform [3, 6, 7]. There is a good agreement between the linear and staircase currents for ls/r = 0.25 — 0.30 for reversible processes (with ts being the time between the application of the potential pulse and the current sampling), if the potential step AE is less than 8 mV. 

Another more sophisticated approach is to make a Fourier Transform analysis of the response in the way proposed by Bond et al. [84, 85]. In this case, the perturbation is a continuous function of time (a ramped square wave waveform) which combines a dc potential ramp with a square wave of potential that can be described as a combination of sinusoidal functions. Under these conditions, the faradaic contribution to the response generates even harmonics only (i.e., the non-faradaic current goes exclusively through odd harmonics). Thus, the analysis of the even harmonics will provide excellent faradaic-to-non-faradaic current ratios. 

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