Differential Staircase Voltammetry

This chapter analyzes the subtractive techniques Differential Multipulse Voltammetry (DMPV), Differential Staircase Voltammetry (DSCVC), and Square Wave Voltammetry (SWV). Of these, the most employed SWV will be analyzed in more detail. Interesting alternatives to DSCVC and SWV are Differential Staircase Voltcoulometry (DSCVC) and Square Wave Voltcoulometry (SWVC), which are based on the analysis of the difference of converted faradaic charge signals obtained between two successive potential pulses when a staircase potential or a square wave potential is applied [4, 5], which is very useful for the study of surface-confined redox species. There exists, however, a book in this series devoted entirely to the theory and application of SWV [6], so in some of the reaction mechanisms analyzed, the reader will be directed to this title for a more thorough treatment of the SWV response. 

Scheme 7.1 Differential staircase voltammetry and cyclic differential staircase voltammetry, (a) Potentialtime waveform, (b) current-potential response with AI = /dscv- The black dots in (a) indicate the time at which the current is measured...

Fig. 7.29 Theoretical voltammograms in staircase cyclic voltammetry (a and b) and differential staircase voltammetry (c and d) at disc electrodes for the four-electron oxidation of bis (l,2-diferrocenyldithiolene)nickel in [NBu4][PF6]/CH2C12 solution (ETj — = 120mV,...

Differential staircase voltammetry — In this variation of staircase - voltammetry the current is sampled twice on each tread of the staircase potential-time waveform. The difference between the two currents sampled on the same step is amplified and recorded as a function of the... 

Peak height (in -> voltammetry) — It is the maximum current in - linear scan voltammetry, -> cyclic voltammetry, - staircase voltammetry, - differential staircase voltammetry, -> alternating current polarography... 

The dimensionless net peak current A p primarily depends on the product nEsw [31]. This is shown in Table II.3.1. With increasing nfsw the slope BA pIdnEsv, continuously decreases, while the half-peak width increases. The maximum ratio between Ahalf-peak width appears for nEv, = 50mV [6]. This is the optimum amplitude for analytical measurements. If sw = 0, the square-wave signal turns into the signal of differential staircase voltammetry, and A[Pg.124]

The Model 384B (see Fig. 5.10) offers nine voltammetric techniques square-wave voltammetry, differential-pulse polarography (DPP), normal-pulse polar-ography (NPP), sampled DC polarography, square-wave stripping voltammetry, differential pulse stripping, DC stripping, linear sweep voltammetry (LSV) and cyclic staircase voltammetry. 

The adsorption behavior of the psychotropic drug flunitrazepam (256) at the hanging mercury drop electrode was studied by staircase voltammetry and by adsorptive stripping differential pulse voltammetry. 256 can be determined down to nanomolar levels by using adsorptive preconcentration prior to the differential pulse voltammetry scan. The method was applied to determination of 256 in human urine530. 

Differential Pulse Voltammetry. As illustrated in Figure 38, in the Differential Pulse Voltammetry (DPV) the perturbation of the potential with time consists in superimposing small constant-amplitude potential pulses (10 < AEpUise <100 mV) upon a staircase waveform of steps of constant height but smaller than the previous pulses (1 < AEbase < 5 mV). 

Fig. 1.7 Potential-time waveforms obtained by superimposing the square-wave signal onto a staircase signal square-wave voltammetry (a), differential pulse voltammetry (6) and multiple square-wave voltammetry (c)...

Fig. 10.10. Differential pulse voltammetry, (a) Scheme of application of potentials (sometimes superimposed on a ramp rather than a staircase) (b) Schematic...

Other forms of voltammetry are as follows (1) fast-scan cyclic voltammetry useful in neuroelectrochemistry (2) nanosecond voltammetry for a 5-pm disk working microelectrode with RC < 1 gs, scan rates of 2.5 MV/s allow for fast kinetics measurements (3) differential-pulse voltammetry with staircase pulses, potential resolutions of 0.04 V and detection limits of 10 8M can be attained (4) anodic (cathodic) stripping voltammetry traces... 

Pulse voltammetry — A technique in which a sequence of potential pulses is superimposed to a linear or staircase voltage ramp. The current is usually measured at the end of the pulses to depress the - capacitive (charging) current. Depending on the way the pulses are applied and the current is sampled we talk about - normal pulse voltammetry, reverse pulse voltammetry and - differential pulse voltammetry. Several other, less popular pulse techniques are offered in commercial voltammetric instrumentation. Some people consider - square-wave voltammetry as a pulse technique. 

The use of a potential-step technique such as cyclic staircase voltammetry represents a simple alternative to Ichise s method (j0 of obtaining information on both adsorption and electron transfer kinetics. The current decay immediately after a step is primarily capacitive while current at later times is almost totally due to electron transfer reactions. Thus, by measuring the current at several times during each step and by changing the scan rate, information on both the kinetics of the electrode process and the differential capacity can be obtained with a single sweep. 

In linear potential scan (LSV) and cyclic (CV) voltammetries, a potential varying linearly with time is applied between an initial potential, usually at a value where no faradaic processes occur, and a final potential (LSV) or cycled between two extreme (or switching) potential values at a given potential scan rate v (usually expressed in mV/sec). In other techniques, such as normal and differential pulse voltammetries (NPV and DPV, respectively), or square-wave voltammetry (SQWV), the excitation signal incorporates potential pulses to a linear or staircase potential/time variation. 

Electrode potentials relative to the particular reference electrode are conveniently measured as the half-wave potential 1/2 in cyclic voltammetry (CV) [15,16, 20, 24, 36-48] or cyclic staircase voltammetry [49-51], or by differential pulse voltamme-... 

We will consider five subtopics tast polarography and staircase voltammetry, normal pulse voltammetry, reverse pulse voltammetry, differential pulse voltammetry, and square wave voltammetry. Tast polarography, normal pulse voltammetry, and differential pulse voltammetry form a sequence of development rooted historically in polarography at the DME. To illustrate the motivating concepts, we will introduce each of these methods within the polarographic context, but in a general way, applicable to both the DME and SMDE. Then we will turn to the broader uses of pulse methods at other electrodes. Reverse pulse voltammetry and square wave voltammetry were later innovations and will be discussed principally outside the polarographic context. 

Voltammetric techniques that can be applied in the stripping step are staircase, pulse, differential pulse, and square-wave voltammetry. Each of them has been described in detail in previous chapters. Their common characteristic is a bell-shaped form of the response caused by the definite amount of accumulated substance. Staircase voltammetry is provided by computer-controlled instruments as a substitution for the classical linear scan voltammetry [102]. Normal pulse stripping voltammetry is sometimes called reverse pulse voltammetry. Its favorable property is the re-plating of the electroactive substance in between the pulses [103]. Differential pulse voltammetry has the most rigorously discriminating capacitive current, whereas square-wave voltammetry is the fastest stripping technique. All four techniques are insensitive to fast and reversible surface reactions in which both the reactant and product are immobilized on the electrode surface [104,105]. In all techniques mentioned above, the maximum response, or the peak current, depends linearly on the surface, or volume, concentration of the accumulated substance. The factor of this linear proportionality is the amperometric constant of the voltammetric technique. It determines the sensitivity of the method. The lowest detectable concentration of the analyte depends on the smallest peak current that can be reliably measured and on the efficacy of accumulation. For instance, in linear scan voltammetry of the reversible surface reaction i ads + ne Pads, the peak current is [52]... 

Fig. 6 Differential pulse voltammetry (a) potential-time waveform sum of staircase and synchronized pulses and (b) schematic voltammogram.

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