Normalized potential sweep voltammetry

Normalized potential sweep voltammetry (NPSV) involves a three-dimensional analysis of the LSV wave where the normalized current (I/Ip) is taken as the Z axis, theoretical electrode potential data as the X axis, and experimental electrode potential data as the Y axis, with the potential axes defined relative to Ep/2. The method is illustrated by the voltammogram in Fig. 15. The projection of the wave on to the X—Y plane results in a straight line of unit slope and zero intercept if the theoretical and experimental data describe the same process. In practice, NPSV analysis simply involves the linear correlation of experimental vs. theoretical electrode potentials at particular values of the normalized current. 

A significant feature of NPSV analysis is that linear relationships were observed when theoretical data for Nernstian charge transfer were taken as the X axis and theoretical data for various electrode mechanisms were taken as the Y axis. The slopes of the resulting straight lines are indications of the mechanisms of the electrode processes. Some of the slopes are included in Table 25. 

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Linear sweep voltammetry Ep measurements have not been applied extensively for the study of heterogeneous charge transfer kinetics. A serious problem with the use of this method is that Ep in itself is not significant in this respect but rather Ep — Etev is the quantity of interest. While AEP in CV is readily measured, this cannot be said for Etev using only LSV as a measurement technique. Therefore, there does not appear to be any advantage in LSV for the study of electrode kinetics. A more detailed analysis of the LSV wave, by convolution potential sweep or normalized potential sweep voltammetry (both to be discussed later) can provide both a and k°. 

Aalstad and Parker, 1980, 1981 Linear current potential analysis df/df/Z/P) p Normalized potential sweep voltammetry Convolution potential sweep voltammetry... 

G. Theoretical normalized potential sweep voltammetry data... 

In this chapter, the voltammetric study of local anesthetics (procaine and related compounds) [14—16], antihistamines (doxylamine and related compounds) [17,22], and uncouplers (2,4-dinitrophenol and related compounds) [18] at nitrobenzene (NB]Uwater (W) and 1,2-dichloroethane (DCE)-water (W) interfaces is discussed. Potential step voltammetry (chronoamperometry) or normal pulse voltammetry (NPV) and potential sweep voltammetry or cyclic voltammetry (CV) have been employed. Theoretical equations of the half-wave potential vs. pH diagram are derived and applied to interpret the midpoint potential or half-wave potential vs. pH plots to evaluate physicochemical properties, including the partition coefficients and dissociation constants of the drugs. Voltammetric study of the kinetics of protonation of base (procaine) in aqueous solution is also discussed. Finally, application to structure-activity relationship and mode of action study will be discussed briefly. 

Most of the more advanced techniques have only rarely been used outside the laboratories where they have been developed, and for that reason it is not easy to give recommendations. Examples include normalized sweep voltammetry [34,35,157,158], linear current-potential analysis [33], and the so-called global analysis and related techniques [159-161]. However, one such technique, convolution potential sweep voltammetry, has gained some popularity, and is introduced briefly here. 

The two types of electrochemically formed chemisorbed oxygen on Pt films interfaced with YSZ are also clearly manifest via solid state linear potential sweep voltammetry (Fig. 9 bottom, Ref. [39]) The first oxygenX reduction peak corresponds to normally chemisorbed oxygen (y-state) and the second reduction peak which appears only after prolonged positive current application [39] corresponds to the 5-state of oxygen, i.e. backspillover oxidic oxygen, which is significantly less reactive than the y-state. 

Potential step voltammetry (chronoamperometry) or normal pulse voltammetry (NPV) and potential sweep or cyclic voltammetry (CV) were employed for investigating drugs at the NB/W or DCE/W interface. A thin O-layer cell [15,16,23] was used to realize the partition equilibrium of neutral species (that is, B) at the O/W interface initially at t = 0 within a reasonably short time. All measurements were carried out at 25°C. Experimental details should be consulted in the references cited. 

In the linear sweep technique, a recording of the current during the potential sweep (say, from 0.0 V on the normal hydrogen scale to 1.2 V positive to it in a 1 M H2 S04 solution) completes one act of the basic experiment. However, and hence the title of this part of the chapter, the electronics can be programmed so that when the electrode potential reaches 1.20 V, it begins a return sweep, going from 1.2 to 0.00 V, NHS. Completion of the two sweeps and back to the starting point is one act in what is called cyclic voltammetry.16 The current is displayed on a cathode ray oscilloscope screen on an X Y recorder, and it is normal to cany out not one but several and often many cycles. Much information is sometimes contained in the difference between the second and other sweeps in comparison with the first (Fig. 8.10). 

Stripping program Select linear sweep voltammetry (Normal) in Autolab software. The parameters are conditioning potential (used for accumulation) 0.0 V conditioning time 10 min stripping range 0.0-1.2V at 100 mV/s. 

The kind of voltammetry described in Sect. 4.2. is of the single-sweep type, ie., only one current-potential sweep is recorded, normally at a fairly low scan rate (0.1-0.5 V/min), or by taking points manually. Cyclic voltammetry is a very useful extension of the voltammetric technique. In this method, the potential is varied in a cyclic fashion, in most cases by a linear increase in electrode potential with time in either direction, followed by a reversal of the scan direction and a linear decrease of potential with time at the same scan rate (triangular wave voltammetry). The resulting current-voltage curve is recorded on an XY-recorder,... 

After a description of how to control the sweep experiment and its two forms, linear sweep voltammetry (LSV) and cyclic voltammetry (CV) (where the sweep direction is inverted at a certain, chosen potential), the voltammetric waveshape obtained for slow and fast electrode reactions is analysed. Recent advances in these topics are considered. Finally, the type of curve obtained from linear sweep in a thin-layer cell is presented thin-layer cells are important because they permit almost 100 per cent conversion of the electroactive species, and show differences in relation to electrochemical behaviour in a normal-sized cell. 

The scan rate, u = IdE/dtl, plays a very important role in sweep voltammetry as it defines the time scale of the experiment and is typically in the range 5 mV s to 100 V s for normal macroelectrodes, although sweep rates of 10 V s are possible with microelectrodes (see later). The short time scales in which the experiments are carried out are the cause for the prevalence of non-steady-state diffusion and the peak-shaped response. When the scan rate is slow enough to maintain steady-state diffusion, the concentration profiles with time are linear within the Nemst diffusion layer which is fixed by natural convection, and the current-potential response reaches a plateau steady-state current. On reducing the time scale, the diffusion layer cannot relax to its equilibrium state, the diffusion layer is thiimer and hence the currents in the non-steady-state will be higher. 

In stripping voltammetry, the stripping step comprises the application of a potential sweep and the measurement of current versus potential. In chronopotentiometric stripping, the analyte is stripped by the application of a constant oxidative or reductive flux while the change in the potential of the working electrode versus time is measured. This is normally performed in an unstirred solution to minimize mass transport and consequently maximize the stripping time. 

The analytical technique described here is called linear sweep voltammetry or cyclic voltammetry (effecting only one half the cycle that normally returns to the initial potential) (Bard and Faulkner 2001). 

In potential sweep methods, the current is recorded while the electrode potential is changed linearly with time between two values chosen as for potential step methods. The initial potential, E, is normally the one where there is no electrochemical activity and the final potential, 2, is the one where the reaction is mass transport controlled. In linear sweep voltammetry, the scan stops at E2, whereas in cyclic voltammetry, the sweep direction is reversed when the potential reaches 2 and the potential remmed to j. This constitutes one cycle of the cyclic voltammogram. Multiple cycles may be recorded, for example, to study film formation. Other waveforms are used to study the formation and kinetics of intermediates when studying coupled chemical reactions (Figure 11.4c). 

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