Linear sweep voltammetry limiting current

In linear-sweep voltammetry the current is recorded while the potential of the working electrode is swept from one selected limit to another at a rate between 1 mV/s and 1000 mV/s. In cyclic voltammetry, two linear sweeps are recorded, as shown in Table 8. The distinguishing feature of cyclic voltammetry is that electrogenerated species formed in the forward sweep are subject to the reverse electrochemical reaction in the return sweep [69]. Figure 17 shows a typical cyclic vol-... 

As with in vivo voltammetry, a variety of electrochemical techniques have been used for the stripping step. Because of its simplicity, linear sweep voltammetry has enjoyed widespread use however, the detection limit of this technique is limited by charging current. Differential pulse has become popular because it discriminates against the charging current to provide considerably lower detection limits. 

However, it is not ideal to use cyclic or linear-sweep voltammetry as a method for analytical purposes. A more suitable method is chronoamper-ometry, which in fact is the application of a constant potential located in the limiting-current plateau and measurement of the limiting-current as a function of time. With this method, it is possible to measure continuously, and the required equipment setup becomes much more simplified. 

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... 

Implicit in the above is the notation that current-voltage curves measured at macroelectrodes for all but fast voltage scan rates are characterized by a mass transport limited current plateau rather than a current peak as in linear sweep voltammetry at a planar electrode of larger than micro dimensions. 

To obtain reproducible limiting currents rapidly, either (1) the solution or the electrode must be in continuous and reproducible motion or (2) a dropping mercury electrode must be used. Linear-sweep voltammetry in which the solution is stirred or the electrode is rotated is called hydrodynamic voltammetry. Voltammetry with the dropping mercury electrode is called polarography. 

Linear sweep voltammetry is based on the potential being ramped up between the working and auxiliary electrodes as current is measured. The working electrode is usually a SMDE nowadays, in which case this technique would be called linear sweep polarography. In this set-up, the auxiliary electrode is a mercury pool electrode and may also serve as the reference electrode. The resultant current-potential recording (the polarogram) can yield much information which can be used to qualitatively identify the species and the medium in which it is determined as well as calculate concentrations. Analysis of mixtures is also possible. The detection limit is of the order of 10 M. 

Linear sweep voltammetry at ultramicroelectrode disks of radius r < 10 pm under mass transport control, usually achieved at scan rates <50 mV s , provides a limiting current /l that depends directly on D [25] ... 

Stationary electrode voltammetry was used in nitrate melts as early as 1948 by Lyalikov and Karmazin (50), using a platinum microelectrode in the form of a "dipping" electrode with bubbles of an inert gas to produce a periodically fluctuating current. A similar electrode was used by Flengas (51) and by Bockris, et al. (52) in 1956. In 1960, Hills, Inman and Oxley (53) described an improved version of the dipping electrode, which of course is limited by relatively ill-defined mass transport conditions. Later work has involved linear sweep voltammetry as described by Hills and Johnson (54) in 1961 or steady state voltammetry with stationary electrodes by Swofford and Laitinen (55) in 1963. 

The advantage of amperometric measurements is that the faradaic currents are observed, at fixed electrode potentials. In these circumstances, capacitative currents no longer contribute to the overall cell current, and much lower detection limits are obtainable compared to linear sweep voltammetry. However, some of the newer variants of amperometry do involve pulsing the electrode potential to the active region measurements in these cases need to be made carefully to produce optimum signal to noise ratios. 

The potential-time waveforms used for sweep measurements are shown in Fig. 6.1. The simplest of these techniques is linear sweep voltammetry (LSV), and this involves sweeping the electrode potential between limits Ei and E2 at a known sweep rate, v, before halting the potential sweep. A generally more useful (and consequently more widely applied) technique is cyclic voltammetry (CV). In this case the waveform is initially the same as in LSV, but on reaching the potential E2 the sweep is reversed (usually at the same scan rate) rather than terminated. On again reaching the initial potential, Ei, there are several possibilities. The potential sweep may be halted, again reversed, or alternatively continued further to a value 3. In both LSV and CV experiments the cell current is recorded as a function of the applied potential (it should be noted, however, that the potential axis is also a time axis). The sweep rates used in... 

Fig. 2.13 Current versus overpotential curves showing the effect of experimental parameters in the presence of forced convection, according to the relationship = /cL lnFc. (a) Electrode size (and shape). Ideally, in the presence of a uniform current-density distribution, Deviations may be due to edge effects, non-uniformity of flow (e.g. entrance length effects) or contributions from natural convection, (b) Concentration of electroactive species in the reactor. ii should be proportional to c. It is sometimes convenient to test this by incremental increases in c . The background curve is represented by = 0. (c) Relative velocity of the electrolyte or electrode, cc where x is a constant which depends upon the geometry and flow conditions, x may vary slightly over different ranges of Reynolds number. The limiting-current plateau may shorten and tilt as velocity increases, due to the increasing importance of electron transfer to the overall reaction kinetics. The maximum on the 1 curve may arise due to unsteady-state mass transport and is akin to a peak in linear sweep voltammetry, i.e. it may arise due to an excessive rate of potential change.

As an example. Figure 9.7 shows the linear sweep voltammetry (LSV) recorded under different backpressures from 0.05 to 0.2 MPa at 70 °C and 100% RH. The current density generated by electrochemical oxidation of the crossed over hydrogen at cathode (Ih2-x over, represented by the limiting... 

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