Voltammetric linear-scan

Let us see now what happens in a similar linear scan voltammetric experiment, but utilizing a stirred solution. Under these conditions, the bulk concentration (C0(b, t)) is maintained at a distance S by the stilling. It is not influenced by the surface electron transfer reaction (as long as the ratio of electrode area to solution volume is small). The slope of the concentration-distance profile [(CQ(b, t) — Co(0, /))/r)] is thus determined solely by the change in the surface concentration (Co(0, /)). Hence, the decrease in Co(0, t) duiing the potential scan (around E°) results in a sharp rise in the current. When a potential more negative than E by 118 mV is reached, Co(0, t) approaches zero, and a limiting current (if) is achieved ... 

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. 

Various voltammetric waveforms can be employed during the stripping step, including linear scan, differential pulse, square-wave, staircase, or alternating-current operations. The differential pulse and square-wave modes are usually performed at the hanging mercury drop electrode, while linear scan stripping is usually performed in connection with the mercury film electrode. 

After several voltammetric scans in the range 0.25-2.10 V versus Li[C/ R] at 333 K, the negative limit (Emw) was extended beyond the potential associated with Li bulk deposition, first to -0.1 V (dotted line, Figure 41) and then to -0.2 V (solid line in Figure 37). A well-defined peak centered at 0.5 V could be identified in the scan in the negative direction without a clear counterpart for EK, = -0.1 V (dotted line). In the case of Emy = -0.2 V, however, the subsequent linear scans in the positive direction displayed three peaks (A, B and C ) not... 

Randles-5>evcik equation — An equation introduced by - Randles [i] and - Sevcik [ii] describing the magnitude of the voltammetric peak current /p (in - linear scan voltammetry or in - cyclic voltammetry) for a reversible electron transfer ( rev mechanism -> Erev diagnostics in cyclic voltammetry). 

Adsorption stripping methods are quite similar to the anodic and cathodic stripping methods we have just considered. Here, a small electrode, most commonly a hanging mercury drop electrode, is immersed in a stirred solution of the analyte for several minutes. Deposition of the analyte then occurs by physical adsorption on the electrode surface rather than by electrolytic deposition. After sufficient analyte has accumulated, the stirring is discontinued, and the deposited material is determined by linear-scan or pulsed voltammetric measurements. Quantitative information is based on calibration with standard solutions that are treated in the same way as samples. 

Single-fiber sensors and catheter-protected sensors can operate in an amperometric or voltammetric mode. In both methods a current proportional to NO concentration is measured. Of the several voltammetric methods available, differential pulse voltammetry (DPV) is most suitable for the measurement of NO. In DPV, a potential modulated with rectangular pulses is linearly scanned from 0.4 to 0.8 V. The resulting voltammogram (alternating current versus voltage plot) contains a peak due to NO oxidation. The peak current should be observed at a potential of 0.63-0.67 V which depends on the pulse amplitude. This potential is the characteristic potential for NO oxidation on Nafion coated porphyrinic sensor. 

Finally, the stripping procedure can also be applied to the interface between two immiscible electrolyte solutions [96]. By a proper polarization of the interface, a certain ion can be transferred from the sample solution into a small volume of the second solution. After this accumulation, the ion can be stripped off by linear scan voltammetry, or some other voltammetric technique. The shipping peak current is linearly proportional to the concentration of ions in the second solution and indirectly to the concentration of ions in the sample solution. The method is used for the determination of electroinactive ions, such as perchlorate anion [97]. The principles of the procedure are the same as in the case of faradaic reactions, and the differences arise from the particular properties of phenomena on the interface that are beyond the scope of this chapter. 

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]... 

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 5.9 shows the voltammetric behavior of Pt/C (a) and Pd-Co-Pt/C (b) towards ORR in the presence and absence of methanol. As can be also observed, the presence of 0.5 mol methanol causes a negative shift of 50 mV in the halfwave potential, in contrast to Pt/C for which there is a severe loss of activity. The linear scan voltammograms of the methanol oxidation on all the investigated materials in 0.5 mol H2SO4 + 0.5 mol L CH3OH solution, showed that the current densities of the methanol oxidation reaction on Pd-Co-X alloy catalysts (X = Au, Ag, Pt) diminish to values much lower than for Pt/C catalyst, and the onset of methanol oxidation occurs at more positive potentials, demonstrating the lowered MOR activity of the Pd-Co-Pt alloy catalysts. 

Similar results were reported by Fan et al. in their voltammetric studies of nitrite reduction by Hb contained in DNA film electrodes at pH 4. These films showed well-defined Fe V couple at —0.36 V SCE, Eq. (4.26), and upon addition of nitrite two new cathodic peaks were observed. Also, when the lower scan potential was limited to —0.5 V, the Fe / peak current decreased linearly due to formation of stable Hb-Fe NO complex, as previously seen for Mb. Pre-electrolysis followed by linear scan voltammetric sweeps showed that at H-0.3 V the predominant species is met-Hb, at —0.5 V the Hb-NO complex is the predominates species, and at —0.9 V the Hb-Fe predominates, apparently due to NO depletion in the electroactive layer. 

Figure 25-2 is a schematic showing the components of a modern operational amplifier potentiostat (see Section 24C-1) for carrying out linear-scan voltammetric measurements. The cell is made up of three electrodes immersed in a solution containing the analyte and also an excess of a nonreactive electrolyte called a supporting electrolyte. One of the three electrodes is the work-... 

Linear-scan voltammograms generally have a sigmoid shape and are called voltammetric waves. The constant current beyond the steep rise is called the diffusion-limited current, or simply the limiting current ii because the rate at which the reactant can be brought to the surface of the electrode by mass-transport processes limits the current. Limiting currents are usually directly proportional to reactant concentration. Thus, we may write... 

Many of the limitations of traditional linear-scan voltammetry were overcome by the development of pulse methods. We will discuss the two most important pulse techniques, dijferential-pulse voltammetry and square-wave voltammetry. The idea behind all pulse-voltammetric methods is to measure the current at a time when the difference between the desired faradaic curve and the interfering charging current is large. These methods are used with many different types of solid electrodes, the HMDE, and rotating electrodes (Section 25C-4). 

Caster, D., Toman, J. Brown, S. (1983). Curve fitting of semiderivative linear scan voltammetric responses - Effect of reaction reversibility. Analytical Chemistry 55(13) 2143-2147. 

The voltammetric response of the surface-confined analyte is directly related to its surface concentration. Using linear scan measurements, under ideal Nernstian behavior, the faradaic peak current arising from the presence of r mol/cm2 of an attached analyte is given by... 

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