Voltammetry voltammetric curves

From the foregoing treatment of voltammetry it is clear that for its application sophisticated apparatus is required, especially the electronics moreover, the construction of the electrodes and their mutual positions within the measuring vessel, of adapted size and without or with stirring, requires great care and experience. For this reason it is often advisable to purchase commercial apparatus, which has reached a high level of sophistication and reliability. Here the most desirable property is smooth recording of the voltammetric curve, which necessitates the kind of automation inherently required in the voltammetric method this is different from advanced automation, which is treated in Part C. 

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]... 

Assuming that solid-state electrochemical processes involved in our voltammetry of microparticles analysis satisfy Tafel dependence between current and potential at the rising portion of voltammetric curves, the current can be approached by the expression... 

Voltammetry experiments are not often performed in flow cells for analytical purposes. One reason for this is the special problem of ohmic potential losses (iR drops) at an electrode in a confined stream. Another reason is the problem of precisely pumping solution at a carefully controlled velocity. In general, rotating electrodes are more easily controlled and do not involve serious plumbing problems. On the other hand, flow cells operated at a fixed potential (i.e., at one point along the steady-state voltammetric curve) are eminently useful for electrosynthesis, chromatographic detection, and automated analysis systems. These features will be described in later chapters. 

Chemisorption of hydrogen on platinum metals is a reversible process as demonstrated by the symmetry of the cyclic voltammetric curves (- voltammetry). The voltammetric behavior of these systems depends on the... 

This is the well-known ECE case, in which a chemical reaction is interspersed between two electron transfers. The appearance of the voltammetric curves depends on whether Eg is negative or positive of E . Considering both possibilities for a reduction, the theory of cyclic voltammetry (CV) gives ... 

In the case of a charge transfer to a diffusing species, ac voltammetry or ac polarography is usually used and the impedance curves are determined from a series of ac voltammetric curves registered at different frequencies. The methods of analysis of such curves are described in the following sections. 

When microelectrodes are used instead of the normal-sized electrode in cyclic voltammetry experiments, voltammograms with the steady-state wave shape are obtained (Fig. 10.5.2). The half-wave potential, E1/2 is equal to the mid-peak potential of the cyclic voltammetric curve recorded using a large electrode. 

The redox pair of [Fe(CN)6] /[Fe(CN)6] is frequently used as a test system, e.g., in cyclic voltammetry (see Sect. II. 1.5.3) or rotating disk electrode studies, assuming an uncomplicated electrode reaction. However, as seen in Fig. II. 10.3, the characteristic cyclic voltammetric curves of [Fe(CN)6] ions are accompanied with a frequency decrease during reduction and an increase during oxidation. 

Fourteen years ago, the theory of elimination voltammetry with linear scan (EVLS) was published and experimentally verified for selected electrode systems [5, 6]. To this date, the method has been applied not only in electroanalytical chemistry, but also in the study of electrode processes of inorganic and organic electroactive substances at mercury, silver, or graphite electrodes [7-20]. EVLS can be considered as a mathematical model of the transformation of current-potential curves capable of eliminating certain selected current components while securing the conservation of others by means of elimination functions. For the calculation of the elimination functions, two or three voltammetric curves at different scan rates should be recorded under identical experimental conditions. It means that the linear sweep voltammetric (LSV) curves have to be recorded with the same potential step, so that the I-E data sets obtained for the same number of points on the potential axis, and... 

Figure 6.20 shows the repetitive cyclic voltammetry of the lithium deposition and stripping process on a nickel substrate from a cell using a (PE0)8LiCF3S03 polymer electrolyte, i.e. one of the first generation electrolytic membrane commonly used for the development of LPBs. The trend of the voltammetric curves indicates that the process at the lithium interface ... 

Cyclic voltammetry was first employed to define the potential region of the deposition process at changing Ag(I) concentratiOTi. The electrode potential was driven in the cathodic directiOTi from 0.7 to —0.7 V at a scan rate of 100 mV s. Figure 6.2 shows typical voltammetric curves at the glassy carbon electrode changing Ag(I) concentration in solution from 1 to 30 mM. 

The amount of stored hydrogen in AC is 1.49%wt H, whereas for nanotubes it is only 0.27%wtH because of mesoporosity character of multiwalled carbon nanotubes and lack of microporosity. On observing their electrochemical behavior by cyclic voltammetry, presented in Fig. 9.16, carbon nanotubes reveal extremely different shape of curves in cathodic area, without hysteresis loop between cathodic and anodic polarization. It suggests that hydrogen is evolved but is not being adsorbed on carbon nanotubes. The disturbances in voltammetric curves observed in cathodic area proved that recombination reactions are favored. 

Voltammetry. Cyclic voltammetric curves for the three surfaces investigated are shown in Figure 1. They are in a very good agreement with earlier data (4, 46-53). As already agreed upon (54-56), the voltammetric activity on the Pt(l 11) electrode in the potential ranges from -0.25 to 0.05V and from 0.05 to 0.25V corresponds to... 

Before the addition of cyt c, characteristic redox peaks at —0.45 and —0.34 V versus Ag/AgCl due to the immobilized CcR were observed on both the portable and commercial cychc voltammetry instruments. After the addition of 100 pM cyt c, the voltammetric curves obtained using the portable and commercial potentiostat systems are quite similar in shape and position, however, the cathodic peaks of the commercial and cost-efective potentiostat difier only by 7.2 pA. The above measurements confirmed that the developed electrochemical analyzer performs as good as a standard potentiostat for cyt c detection. 

When working with RDE, thickness of the diffusion layer is determined by the rotational angular velocity (Eq. (12)). For every given rotation rate there is an upper limit in sweep rate for which maxima, characteristic for voltammetry in quiescent solution, are not present. For example, in aqueous solutions at room temperature, for angular velocity of 5 rps (revolutions per second) shape of the j-E curve remains unchanged up to sweep rates of 50 mV s. Conversely, for angular velocity of 2 rps, sweep rate of 50 mV s is too excessive, and characteristic current maxima in voltammetric curve can occur. 

The key factor in voltammetry (and polarography) is that the applied potential is varied over the course of the measurement. The voltammogram, which is a current-applied potential curve, / = /( ), corresponds to a voltage scan over a range that induces oxidation or reduction of the analytes. This plot allows identification and measurement of the concentration of each species. Several metals can be determined. The limiting currents in the redox processes can be used for quantitative analysis this is the basis of voltammetric analysis [489]. The methods are based on the direct proportionality between the current and the concentration of the electroactive species, and exploit the ease and precision of measuring electric currents. Voltammetry is suitable for concentrations at or above ppm level. The sensitivity is often much higher than can be obtained with classical titrations. The sensitivity of voltammetric... 

In voltammetric titration the reaction is pursued by means of voltammetry interest is sometimes taken in the complete titration curve, but mostly in its part around the equivalence point in order to establish the titration end-point. 

The Metrohm 646 VA processor and 647 VA stand is based on a polaro-graphic/voltammetric analyser with method memory and automatic curve evaluation, combined with a multi-mode stand (see Fig. 5.9). The following four determination techniques for polarography, voltammetry and stripping... 

Cyclic voltammetry, square-wave voltammetry, and controlled potential electrolysis were used to study the electrochemical oxidation behavior of niclosamide at a glassy carbon electrode. The number of electrons transferred, the wave characteristics, the diffusion coefficient and reversibility of the reactions were investigated. Following optimization of voltammetric parameters, pH, and reproducibility, a linear calibration curve over the range 1 x 10 6 to 1 x 10 4 mol/dm3 niclosamide was achieved. The detection limit was found to be 8 x 10 7 mol/dm3. This voltammetric method was applied for the determination of niclosamide in tablets [33]. 

Alemu et al. [35] developed a very sensitive and selective procedure for the determination of niclosamide based on square-wave voltammetry at a glassy carbon electrode. Cyclic voltammetry was used to investigate the electrochemical reduction of niclosamide at a glassy carbon electrode. Niclosamide was first irreversibly reduced from N02 to NHOH at —0.659 V in aqueous buffer solution of pH 8.5. Following optimization of the voltammetric parameters, pH and reproducibility, a linear calibration curve over the range 5 x 10 x to 1 x 10-6 mol/dm3 was achieved, with a detection limit of 2.05 x 10-8 mol/dm3 niclosamide. The results of the analysis suggested that the proposed method has promise for the routine determination of niclosamide in the products examined [35]. 

FIGURE 2.6. EC reaction scheme in cyclic voltammetry. Mixed kinetic control by an electron transfer obeying a MHL kinetic law (Xt — 0.7 eV, koo — 4 x 103 cms-1, implying that kg = 0.69 cms-1) and an irreversible follow-up reaction (from bottom to top, k+ = 103, 105, 107, 109s 1). Temperature, 25°C. a Potential-dependent rate constant derived from convolutive manipulation of the cyclic voltammetric data (see the text), b Variation with potential of the apparent transfer coefficient (see the text) obtained from differentiation of the curves in part a. 

In cyclic voltammetry, the current-potential curves are completely irreversible whatever the scan rate, since the electron transfer/bond-breaking reaction is itself totally irreversible. In most cases, dissociative electron transfers are followed by immediate reduction of R, as discussed in Section 2.6, giving rise to a two-electron stoichiometry. The rate-determining step remains the first dissociative electron transfer, which allows one to derive its kinetic characteristics from the cyclic voltammetric response, ignoring the second transfer step aside from the doubling of the current. 

The use of hydrodynamic modulation voltammetry was reported [155] and modified [156]. A simple treatment of the data transforms the voltammetric current-voltage waves into peak shaped curves, which gave improved resolution in the analysis of mixtures. 

Figure 6.26. CO stripping voltammetry of UHV sputter-cleaned electrodes in 0.5 M H2S04 on (a) Pt and (b) Ru. Solid curves represent the stripping of CO in the first positive-going sweep dotted lines represent the voltammetric profiles in the absence of CO (adapted from Ref. [153]).

Voltammetry is a term used to include all the methods that measure current-potential curves (voltammograms) at small indicator electrodes other than the DME [6], There are various types of voltammetric indicator electrodes, but disk electrodes, as in Fig. 5.17, are popular. The materials used for disk electrodes are platinum, gold, graphite, glassy carbon (GC), boron-doped diamond8, carbon paste, etc. and they can be modified in various ways. For electrode materials other than mercury, the potential windows are much wider on the positive side than for mercury. However, electrodes of stationary mercury-drop, mercury-film, and mercury-pool are also... 

---