Irreversibility voltammetric

The voltammetric reduction of a series of dialkyl and arylalkyl disulfides has recently been studied in detail, in DMF/0.1 M TBAP at the glassy carbon electrode The ET kinetics was analyzed after addition of 1 equivalent of acetic acid to avoid father-son reactions, such as self-protonation or nucleophilic attack on the starting disulfide by the most reactive RS anion. Father-son reactions have the consequence of lowering the electron consumption from the expected two-electron stoichiometry. Addition of a suitable acid results in the protonation of active nucleophiles or bases. The peak potentials for the irreversible voltammetric reduction of disulfides are strongly dependent on the nature of the groups bonded to the sulfur atoms. Table 11 summarizes some relevant electrochemical data. These results indicate that the initial ET controls the electrode kinetics. In addition, the decrease of the normalized peak current and the corresponding increase of the peak width when v increases, point to a potential dependence of a, as discussed thoroughly in Section 2. 

When an unmodified electrode is brought into contact to cytochrome c strong adsorption to the surface is considered to block the DET to cytochrome c and the electrode often exhibits an irreversible voltammetric response. 

At Cu-MOF-modified electrodes, a reduction wave with a slightly larger intensity is recorded at -1.22 V. The cathodic peak, which is preceded by a weak shoulder at -0.90 V, looks like an irreversible voltammetric wave, because no anodic counterpart is detected. 

The interface between two immiscible electrolyte solutions offers the means to combine two-phase catalysis, colloid catalysts, and electrocatalysis. In the study of Lahtinen et al. [158] citrate-stabilized palladium and gold colloids were prepared by a traditional chemical reduction method. The voltammetric response of a system with an aqueous colloid and an electron donor in the organic phase revealed an irreversible voltammetric wave as the potential was swept positive. The response was detected only in the presence of both the colloid and the electron-donor DCMFc. The response was concluded to result from heterogeneous charging of the colloid with electrons from DCMFc. 

In liquid SO2 and CsAsFg as supporting electrolyte the oxidation of saturated hydrocarbons could be studied up to potentials of 6.0 V vs see by using platinum ultramicroelectrodes. Irreversible voltammetric waves were found for the compounds methane to n-octane that were studied The anodic peak currents suggest n-values around 2 a bulk electrolysis consumed 2 faradays per mole of alkane. The primary products in dilute solutions (1-10 mM) were shorter chain hydrocarbons. Electrolysis of more concentrated solutions led, via reaction of the oxidation products with starting material, to longer-chain hydrocarbons. 

Cytochrome c in the solution often exhibits an irreversible voltammetric response at gold electrodes, with the exception of a carefully prepared gold electrode [32]. A strong adsorption of cyt. c on an electrode surface is considered to block the ET reaction of cyt. c in the solution. Hin-nen and coworkers, and Hinnen and Nild measured the formal potential of horse heart cyt. c adsorbed on gold, ruthenium, and glassy carbon electrodes by... 

The simulated data shown are for a 1 mM solution of DPP+. It should be noted that the chemical step is bimolecular in nature. Hence, if the concentration of DPP+ were lowered, this would lead to a proportionally lower rate of formation of (DPP)2 and as such one would expect to observe a peak on the reverse scan at lower scan rates. Conversely, if the concentration of the species were increased, this would lead to an increase in the rate of formation of (DPP)2 and hence one would expect to see an irreversible voltammetric wave at higher scan rates. 

Figure 7.14 depicts the voltammetric response for the reduction of A to B both in the presence and absence of species X. In the absence of the species X, the voltammetric response is that for a reversible one-electron reduction (solid line). In the presence of an excess of species X a large, irreversible voltammetric wave is observed. 

Cytochrome c exhibits an irreversible voltammetric response in (0.03 M phosphate buffer + 0.12 mM cytochrome c) solution at pH 7.0 as shown in Figure 1. The reduction peak is observed at 0.3 V but the reoxidation peak is hardly observable. The silver electrode, on which cytochrome c is immobilized, exhibits a pseudo-capacitance peak at -0.3 V by phase sensitive ac polarography. Since this pseudo-capacitance peak is independent of the modulation frequency, it corresponds very likely to the redox reaction of adsorbed cytochrome c on the silver electrode. Similar redox behavior has been reported for cytochrome c adsorbed on a gold electrode . That is, the formal potential of the adsorbed cytochrome c on a silver electrode is -0.3 V, which is different from that of the native species in the bulk at +0.06 V. The formal potential of cytochrome c adsorbed on a... 

Influence of the Kinetics of Electron Transfer on the Faradaic Current The rate of mass transport is one factor influencing the current in a voltammetric experiment. The ease with which electrons are transferred between the electrode and the reactants and products in solution also affects the current. When electron transfer kinetics are fast, the redox reaction is at equilibrium, and the concentrations of reactants and products at the electrode are those specified by the Nernst equation. Such systems are considered electrochemically reversible. In other systems, when electron transfer kinetics are sufficiently slow, the concentration of reactants and products at the electrode surface, and thus the current, differ from that predicted by the Nernst equation. In this case the system is electrochemically irreversible. 

The presence of redox catalysts in the electrode coatings is not essential in the c s cited alx)ve because the entrapped redox species are of sufficient quantity to provide redox conductivity. However, the presence of an additional redox catalyst may be useful to support redox conductivity or when specific chemical redox catalysis is used. An excellent example of the latter is an analytical electrode for the low level detection of alkylating agents using a vitamin 8,2 epoxy polymer on basal plane pyrolytic graphite The preconcentration step involves irreversible oxidative addition of R-X to the Co complex (see Scheme 8, Sect. 4.4). The detection by reductive voltammetry, in a two electron step, releases R that can be protonated in the medium. Simultaneously the original Co complex is restored and the electrode can be re-used. Reproducible relations between preconcentration times as well as R-X concentrations in the test solutions and voltammetric peak currents were established. The detection limit for methyl iodide is in the submicromolar range. 

Many handbooks like the CRC Handbook of Chemistry and Physics provide, on behalf of electrochemistry investigation, values of standard reduction potentials, listed either in alphabetical order and/or in potential order. These must be considered as potentials of completely reversible redox systems. In current analytical practice one is interested in half-wave potentials of voltammetric, mostly polarographic analysis in various specific media, also in the case of irreversible systems. Apart from data such as those recently provided by Rach and Seiler (Spurenanalyse mit Polarographischen und Voltammetrischen Methoden, Hiithig, Heidelberg, 1984), these half-wave potentials are given in the following table (Application Note N-l, EG G Princeton Applied Research, Princeton, NJ, 1980). 

Reaction offac(S)-[ v(iicl)n] with VC13 gave [V Ir(aet)3 2]3+ according to Reaction Scheme 21 389 The structure of (218)(C104)3 confirms the linear-type structure. Cyclic voltammetric studies of (218) show irreversible oxidation and reduction processes. 

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]. 

Wangfuengkanagul and Chailapakul [9] described the electroanalysis of ( -penicillamine at a boron-doped diamond thin film (BDD) electrode using cyclic voltammetry. The BDD electrode exhibited a well-resolved and irreversible oxidation voltammogram, and provided a linear dynamic range from 0.5 to 10 mM with a detection limit of 25 pM in voltammetric measurement. In addition, penicillamine has been studied by hydrodynamic voltammetry and flow injection analysis with amperometric detection using the BDD electrode. 

As the kinetic parameter Ahset decreases, either because the standard rate constant decreases or because the scan rate is increased, the cyclic voltammetric response passes rapidly from the symmetrical reversible Nernstian pattern described in Section 1.2.1 to an asymmetrical irreversible curve, while the cathodic peak shifts in the cathodic direction and the anodic peak shifts in the anodic direction. 

The normalized current-potential curves are thus a function of the two parameters A and oc. An example corresponding to a = 0.5 is shown in Figure 1.19. Decreasing the parameter A as a result of a decrease in the rate constant and/or an increase in scan rate triggers a shift of the cathodic potential toward negative values and of the anodic potential in the reverse direction, thus increasing the irreversibility of the cyclic voltammetric response. When complete irreversibility is reached (i.e., when there is no anodic current underneath the cathodic current, and vice versa), a limiting situation is reached, characterized by... 

As transpires from equation (2.2), a steady state is established by mutual compensation of diffusion and chemical reaction. The concentration profile is indeed the product of a time-dependent function, i, by a space-dependent function in the exponential. The conditions required for the system to be in zone KP, K small and A large, will often be termed pure kinetic conditions in following analyses. Besides its irreversibility, the main characteristics of the cyclic voltammetric wave in this zone can be derived from its dimensionless representation in Figure 2.2b and its equation (see Section 6.2.1),4 where... 

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. 

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