Single-Sweep Voltammetry

Now we apply a potential difference between the working and auxiliary electrode and take readings of the ammeter and the voltmeter. Note that the potent- 

We now repeat the measuring procedure in the same manner, but with an organic substrate added to the SSE. The polarogram in a standard case now looks as curve B, Fig. 2. It is evident that a new anodic process involving the substrate takes place in the system, since the current starts to rise at a much lower anode potential. If the concentration of substrate is not too high, we observe that the current reaches a plateau value, the diffusion-controlled current. At the plateau, 

In Fig. 3, the relation between the potential differences referred to above and cell dimensions is indicated. Note that the electrode potential lies across a very short distance, of the order of 10 A, thus creating a very strong electric field in the immediate vicinity of the electrode surface (around 107 V/cm). 

The polarogram of Fig. 2 (curve B) was recorded in a stirred solution, in which fresh electrolyte is brought into contact with the anode constantly. If one employs a stationary anode in an unstirred solution, the polarogram does not display a plateau but instead a peak, since now the solution near the electrode becomes depleted in substrate as the anode potential is increased (curve D, Fig. 2). Such a polarogram is called a peak polarogram. 

A polarogram of the type shown in Fig. 2 immediately tells us that the substrate is the electroactive species in a certain potential region below that of the anodic limit of the SSE. The potential at half the plateau value of the current is denoted the half-wave potential (E,, 2) of the substrate and is a measure of how easily the compound is oxidized. With a knowledge of the half-wave potential of the substrate it is now easy to link products isolated from macroelectrolyses with the electrode processes possible in the system. This is done by the technique of controlled potential electrolysis (Sect. 4.4). From a peak polarogram, the peak potential (IT ) may be used in the same way asEx, 2. 

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In linear sweep voltammetric techniques the applied electrode potential is varied from an initial value E to a final value f at a constant scan rate v (single sweep voltammetry). Once the value is reached the direction of the scan can be reversed, maintaining the same scan rate v, and the potential brought back to the initial value (cyclic voltammetry). In the two cases the form of the potential-time impulse can be represented as shown in Figure 1. 

The most direct way of determining E° and hence AG° for an organic redox couple is by voltammetric methods, such as single-sweep voltammetry... 

Note. Epj2, Half-wave potentials determined by single-sweep voltammetry using a Pt working electrode in Me4NC104/CH3CN. 

Non-Steady-State methods such as cyclic or single-sweep voltammetry are able to give both kinetic and (implicit) mechanistic information on organic electrode reactions in the following ways. 

Clavilier J. 1987. Pulsed linear sweep voltammetry with pulses of constant level in a potential scale, a polarization demanding condition in the study of platinum single crystal electrodes. J Electroanal Chem 236 87-94. 

Linear sweep voltammetry at the dme. In linear sweep voltammetry (LSV) at the dme a continuously changing rapid voltage sweep (single or multiple) of the entire potential range to be covered is applied in one Hg drop. Originally the rapidity of the sweep (about 100 mV s 1) required the use of an oscilloscope,... 

Fig. 5.18 Potentiostatic methods (A) single-pulse method, (B), (C) double-pulse methods (B for an electrocrystallization study and C for the study of products of electrolysis during the first pulse), (D) potential-sweep voltammetry, (E) triangular pulse voltammetry, (F) a series of pulses for electrode preparation, (G) cyclic voltammetry (the last pulse is recorded), (H) d.c. polarography (the electrode potential during the drop-time is considered constant this fact is expressed by the step function of time—actually the potential increases continuously), (I) a.c. polarography and (J) pulse polarography...

The underpotential deposition (UPD) of metals on foreign metal substrates is of importance in understanding the first phase of metal electrodeposition and also as a means for preparing electrode surfaces with interesting electronic and morphological properties for electrocatalytic studies. The UPD of metals on polycrystalline substrates exhibit quite complex behavior with multiple peaks in the linear sweep voltammetry curves. This behavior is at least partially due to the presence of various low and high index planes on the polycrystalline surface. The formation of various ordered overlayers on particular single crystal surface planes may also contribute to the complex peak structure in the voltammetry curves. 

A single sweep into the oxide formation with the Pt(lll) surface causes a considerable change of voltammetry curve for a well-ordered surface, as can be seen in Fig. 4a. Two new small peaks are seen on the curve of the original Pt(lll). These peaks can be seen on the curves for surfaces by believed to be... 

In contrast to linear single sweep or cyclic voltammetry, which give non-symmetric peaks, DPV affords symmetric peaks which start from zero-current values and finish at zero-current values, see Figure 39. 

Of hundreds of theoretically possible pathways, the list can be trimmed to four using linear sweep voltammetry (LSV) and chemical arguments [22]. The LSV method is an exceptionally powerful one for analyzing electrochemical processes [24-27]. From LSV studies, it was concluded that a single heterogeneous electron transfer precedes the rate-determining step, cyclization is first order in substrate, and that proton transfer occurs before or in the rate-determining step. The candidates include (a) e-c-P-d-p (radical anion closure). 

Potential Sweep Method, In the transient techniques described above, a set of measurements of the potential for a given current or the current for a given potential is measured in order to construct the current-potential function, i = f(E). For example, the Tafel lines shown in Figure 6.20 were constructed from a set of galvanostatic transients of the type shown in Figure 6.18. In the potential sweep technique, i = f(E), curves are recorded directly in a single experiment. This is achieved by sweeping the potential with time. In linear sweep voltammetry, the potential of the test electrode is varied linearly with time (Fig. 6.23a). If the sweep rate is... 

Staircase Voltammetry and Linear Sweep Voltammetry in Single and Cyclic Modes... 

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,... 

Both single-sweep and cyclic voltammetry can provide information about the approximate number of electrons transferred in each wave or peak. This is done by comparing the plateau or peak height with that of a known one- or two-electron transfer process under identical conditions (as an example, the oxidation of 9,10-diphenylanthracene to the cation radical is a commonly used reference reaction). 

The electrochemical behavior of nimodipine was studied in ammonia buffer containing 10% (v/v) ethanol [8]. A single-sweep oscillopolaro-graphic method was then developed for nimodipine in tablets. The calibration graph (peak current at —0.73 V vs. concentration) was linear from 0.2 to 70 pM, and the detection limit was 10 pM. The same authors applied linear sweep voltammetry for the determination of nimodipine in tablets [9]. A reduction peak at —0.62V vs. the Ag/ACl reference... 

Osmium tetroxide is an electroactive marker of the polynucleotide chain and is a good probe of the DNA structure since dsDNA is modified by osmium to a much lesser extent than single-stranded polynucleotides. The limit of detection of osmium-labelled DNA was below 5 ng cm-3 after 2 min accumulation time [66]. Adsorptive stripping linear sweep voltammetry of osmium tetroxide labelled DNA at a mercury electrode [66, 67] was shown to be a good sensor for hybridization of DNA. 

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 sweep voltammetry (LSV), the potential of the working electrode is changed linearly from an initial to a final potential in a single scan. If the potential... 

Cyclic Voltammetry. In this method, the potential of the working electrode is varied linearly with time from an initial potential of E to a final value of f, and then reversed from Ef to E at the same rate, as shown in Fig. 4.3.15. The resulting current is measured as a function of time or potential. This scheme can be used as a single sweep or a multisweep, as used in cyclic voltaimnetry. The potential range is generally selected to suit the reaction under study. 

Polarography, invented in 1922 by Czech electrochemist J. Heyrovsky, was in the period 1930 up to the end of the 1950s the worldwide commonly used electroanalytical method. In its classic form it represents a special case of linear sweep voltammetry characterized by the use of the dropping mercury electrode (DME). The linear voltage scan applied to the electrolytic cell is slow (typically 0.1 V up to 0.4 V min ). With regard to the usual mercury drop-life (1 -4 s) the fundamental assumption can be accepted that each single drop is polarized at nearby constant potential. 

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