Linear potential scan voltammetry

Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltannnetric scan. The pre-concentration step is carried out under fixed potential control for a predetennined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The detemiination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential wavefomis, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltaimnetries exist, all of which coimnonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17]. 

Sensitivity In many voltammetric experiments, sensitivity can be improved by adjusting the experimental conditions. For example, in stripping voltammetry, sensitivity is improved by increasing the deposition time, by increasing the rate of the linear potential scan, or by using a differential-pulse technique. One reason for the popularity of potential pulse techniques is an increase in current relative to that obtained with a linear potential scan. 

Selectivity Selectivity in voltammetry is determined by the difference between half-wave potentials or peak potentials, with minimum differences of+0.2-0.3 V required for a linear potential scan, and +0.04-0.05 V for differential pulse voltammetry. Selectivity can be improved by adjusting solution conditions. As we have seen, the presence of a complexing ligand can substantially shift the potential at which an analyte is oxidized or reduced. Other solution parameters, such as pH, also can be used to improve selectivity. 

Time, Cost, and Equipment Commercial instrumentation for voltammetry ranges from less than 1000 for simple instruments to as much as 20,000 for more sophisticated instruments. In general, less expensive instrumentation is limited to linear potential scans, and the more expensive instruments allow for more complex potential-excitation signals using potential pulses. Except for stripping voltammetry, which uses long deposition times, voltammetric analyses are relatively rapid. 

In hydrodynamic voltammetry the solution is stirred either by using a magnetic stir bar or by rotating the electrode. Because the solution is stirred, a dropping mercury electrode cannot be used and is replaced with a solid electrode. Both linear potential scans or potential pulses can be applied. 

The purity of a sample of K3Fe(CN)6 was determined using linear-potential scan hydrodynamic voltammetry at a glassy carbon electrode using the method of external standards. The following data were obtained for a set of calibration standards. 

For the individual types of transient measuring techniques, special names exist but their terminology lacks uniformity. The potentiostatic techniques where the time-dependent current variation is determined are often called chronoamperometric, and the galvanostatic techniques where the potential variation is determined are called chronopotentiometric. For the potentiodynamic method involving linear potential scans, the term voltammetry is used, but this term is often used for other transient methods as well. 

The transient method characterized by linearly changing potential with time is called potential-sweep (potential-scan) voltammetry (cf. Section 5.5.2). In this case the transport process is described by equations of linear diffusion with the potential function... 

Depending on the time variation of the applied potential, several types of voltammetry can be distinguished. Among them, the most widely used are linear and cyclic voltammetries. Here, the excitation signal is a linear potential scan that is swept between two extreme values, and in cyclic voltammetry the potential is swept up and down between the two values (or switching potentials) with the same absolute scan rate (v, usually expressed in mV/s), although it has the opposite sign [79]. 

In this equation, aua represents the product of the coefficient of electron transfer (a) by the number of electrons (na) involved in the rate-determining step, n the total number of electrons involved in the electrochemical reaction, k the heterogeneous electrochemical rate constant at the zero potential, D the coefficient of diffusion of the electroactive species, and c the concentration of the same in the bulk of the solution. The initial potential is E/ and G represents a numerical constant. This equation predicts a linear variation of the logarithm of the current. In/, on the applied potential, E, which can easily be compared with experimental current-potential curves in linear potential scan and cyclic voltammetries. This type of dependence between current and potential does not apply to electron transfer processes with coupled chemical reactions [186]. In several cases, however, linear In/ vs. E plots can be approached in the rising portion of voltammetric curves for the solid-state electron transfer processes involving species immobilized on the electrode surface [131, 187-191], reductive/oxidative dissolution of metallic deposits [79], and reductive/oxidative dissolution of insulating compounds [147,148]. Thus, linear potential scan voltammograms for surface-confined electroactive species verify [79]... 

Typical Tafel plots for different copper materials are shown in Fig. 3.11. In all cases, an excellent linearity was obtained for n i/ip) on E representations in terms of the correlation coefficient for linear fitting. Similar results were obtained for binary or ternary mixtures of such materials where highly overlapping peaks were recorded, both using linear potential scan and square-wave voltammetries. 

Kinetic parameters of dimerization can be determined by polarography, -> chronoamperometry, -> linear potential scan and -> convolution voltammetry, -> rotating disc voltammetry, and alternating current sinusoidal polarography. See also -> association. 

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. 

Rotating disk voltammetry is widely used for studying catalytic processes. For a reversible -electron transfer process controlled by mass transport in solution, the limiting current, Ziim (pA), recorded in a linear potential scan voltammogram varies with the rotation rate, co (sec" ), following the Levich equation (Bard and Faulkner, 2001) ... 

Excitation signals for potential scan voltammetries (a) linear sweep voltammetry (b-d) cyclic voltammetry (CV). 

Differential pulse voltammetry (DPV) is essentially an instrumental manipulation of chronoamperometry. It provides very high sensitivity because charging current is almost wholly eliminated. More important for CNS applications, it often helps to resolve oxidations which overlap in potential. The method combines linear potential sweep and square-wave techniques. The applied signal is shown in Fig. 16A and consists of short-duration square-wave pulses (<100 msec) with constant amplitude (typically 20 or 50 mV) and fixed repetition interval, superimposed on a slow linear potential scan. The Fapp waveform can be generated with a laboratory-built potentiostat, but most DPV work is done with a commercial pulse polarograph (see Appendix). The inset of Fig. 16A shows an enlargement of one pulse. The current is measured just before the pulse... 

In ac cyclic voltammetry, a small amplitude sine wave perturbation is superimposed upon the linear potential scan, and a phase sensitive detector is used to extract and display the in-phase current response as a function of potential. This technique [40-42] is discussed in more detail in Chapter 8, but it is worth noting here that it may have some advantage for the study of fast coupled chemical reactions. At the present stage it is, however, a technique for obtaining numerical parameters when the mechanism is known and there is little in the literature concerning its use as a tool to investigate complex mechanisms. 

Similar to EIS, SWV (square-wave voltammetry) is another frequency-dependent electrochemical technique that could also be used in label-free Faradaic immunosensing [167]. In this case, a train of potential pulses is superimposed on a staircase potential signal with the latter centered between a cathodic pulse and an anodic pulse of the same amplitude. During each cathodic pulse, the analyte diffuses to the electrode surface and it is immediately reduced. During the anodic pulse, analyte that was just reduced is reoxidized. The current is sampled just before and at the end of each pulse and the current difference between these two points is then plotted against the staircase potential in a SW voltammogram. A linear potential scan in SWV is faster than EIS record and a familiar peak-shaped signal is more easily interpreted. 

EPS voltammetry employing a linear potential scan j = vt is a widely applied technique. The theoretical basis of this method was elaborated for different... 

Different transient techniques have also been suggested for the measurement of corrosion rate. Pulse techniques can be used to eliminate from the polarization data the effects of uncompensated solution resistance and mass transport, or to minimize the effect of time-dependent phenomena. However, these techniques must be used with caution because the classical electrode kinetic theory can be used in the data evaluation only if /corrA/<0.9. The square-wave techniqueand ac impedance techniquehave also been used to measure the polarization resistance. The linear potential scan (potentiodynamic) technique has been used to obtain the polarization curve or the polarization resistance (small-amplitude cyclic voltammetry and exponential scan techniques were also proposed to determine the polarization curve. 

As mentioned in the introduction of the amperometry techniques, the voltammetry with periodical renewal of the diffusion layer is particularly effective in monitoring a process differently involving an electroactive species, e.g., in the already mentioned amperometric titrations, in the determination of the stability of a species, etc. In particular cases, also simple chronoamperometry, i.e., at a fixed, suitably chosen potential, may be effective to this purpose. Noteworthy, it will be clear in the following that the much more widely diffused linear potential scan and cyclic voltammetric techniques are not always suitable to substitute for voltammetry with periodical renewal of the diffusion layer to the purpose of monitoring electroactive species during their transformation. Voltammetry with periodical renewal of the diffusion layer, as well as the voltammetry at rotating disk electrode, only allows the estimation of the concentrations of both partners of a redox couple, on the basis of the ratio between the anodic and cathodic limiting currents. 

During cyclic voltammetry, the potential is similarly ramped from an initial potential E but, at the end of its linear sweep, the direction of the potential scan is reversed, usually stopping at the initial potential E (or it may commence an additional cycle). The potential at which the reverse occurs is known as the switch potential ( >.) Almost universally, the scan rate between E and Ex is the same as that between Ex and E. Values of the scan rates Vforwani and Ubackward are always written as positive numbers. 

Another evolution from the linear sweep mode is cyclic voltammetry, namely, a sequential combination of two (or more) linear sweep potential scans in the opposite direction for this reason, the current versus potential response supplies information about the reversibility of redox systems. 

Equation (25) is general in that it does not depend on the electrochemical method employed to obtain the i-E data. Moreover, unlike conventional electrochemical methods such as cyclic or linear scan voltammetry, all of the experimental i-E data are used in kinetic analysis (as opposed to using limited information such as the peak potentials and half-widths when using cyclic voltammetry). Finally, and of particular importance, the convolution analysis has the great advantage that the heterogeneous ET kinetics can be analyzed without the need of defining a priori the ET rate law. By contrast, in conventional voltammetric analyses, a specific ET rate law (as a rule, the Butler-Volmer rate law) must be used to extract the relevant kinetic information. 

The convolution analysis is based on the use of convolution data and further manipulation to obtain information on the ET mechanism, standard potentials, intrinsic barriers, and also to detect mechanism transitions. It is worth noting that the general outlines of the methodology were first introduced in the study of the kinetics of reduction of terf-nitrobutane in dipolar aprotic solvents, under conditions of chemical stability of the generated anion radical. For the study of concerted dissociative ET processes, linear scan voltammetry is the most useful electrochemical technique. 

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