Oxidation peak

Tire deprotonation of thiazolium salts (see Section II) under argon at room temperature allowed the characterization of nonfused DTDAF of types 52 and 53 by cyclic voltammetry. Their very good donor properties were confirmed by two quasi-reversible peaks of equal intensity (93CC601). It is noteworthy that upon a second scan the first oxidation peak was shifted from -0.03 to -0.04 V. Upon further scans the voltam-mogram remains unchanged. Tliis interesting feature has been observed previously with TTF analogs. It was demonstrated that the neutral form... 

In the second step, which correlates with the second oxidation peak in the CV diagram, four protons and two electrons are transferred. 

Thermoanalytical techniques such as differential scanning calorimetry (DSC) and thermogravi-metric analysis (TGA) have also been widely used to study rubber oxidation [24—27]. The oxidative stability of mbbers and the effectiveness of various antioxidants can be evaluated with DSC based on the heat change (oxidation exotherm) during oxidation, the activation energy of oxidation, the isothermal induction time, the onset temperamre of oxidation, and the oxidation peak temperature. 

Monolayers of l-tert-bntyl-l,9-dihydrofullerene-60 on hydrophobized ITO glass exhibited three well-defined rednction waves at -0.55 V, -0.94 V, and -1.37 V (vs. satn-rated calomel electrode, SCE), with the first two stable to cycling [283]. Improved transfer ratios near nnity were reported. The peak splitting for the first two waves was 65-70 mV, mnch less than reported for the pnre C60-modified electrodes. The rednction and oxidation peak cnrrents were equal however, the peak currents were observed to be proportional to the sqnare root of the scan rate instead of being linear with the scan rate as normally expected for snrface-confined redox species. 

In the cyclic voltammetry, the oxidation peaks of PH were clearly observed in positive scans for all the modified electrodes. In contrast, reduction peaks of Cgo were clearly observed in the absence of magnetic processing but not in the presence of magnetic processing. 

At near-saturation coverages, the oxidation peak potential is displaced to approximately 1.03 V. There is a second oxidation process at 1.1 V that involves desorption of Te adatoms, which has been ascribed to the displacement of the Te(IV) oxide by surface Pt oxide. 

A second oxidation peak at 0.96 V can be observed, ascribable to the oxidative desorption of Bi(II) adsorbed on defects, to form Bi(III) species in solution. 

At low CO coverages, the adatom oxidation peak can be distinguished from the CO oxidation peak. Lateral interactions between CO and the adatom stabilize the elemental Bi state, increasing the potential of the adatom redox peak. For As, a displacement of the redox peak to lower potentials is observed, indicating an stabilization of the As(III) state on the CO-As mixed adlayer. 

On the basis of theoretical calculations Chance et al. [203] have interpreted electrochemical measurements using a scheme similar to that of MacDiarmid et al. [181] and Wnek [169] in which the first oxidation peak seen in cyclic voltammetry (at approx. + 0.2 V vs. SCE) represents the oxidation of the leucoemeraldine (1 A)x form of the polymer to produce an increasing number of quinoid repeat units, with the eventual formation of the (1 A-2S")x/2 polyemeraldine form by the end of the first cyclic voltammetric peak. The second peak (attributed by Kobayashi to degradation of the material) is attributed to the conversion of the (1 A-2S")x/2 form to the pernigraniline form (2A)X and the cathodic peaks to the reverse processes. The first process involves only electron transfer, whereas the second also involves the loss of protons and thus might be expected to show pH dependence (whereas the first should not), and this is apparently the case. Thus the second peak would represent the production of the diprotonated (2S )X form at low pH and the (2A)X form at higher pH with these two forms effectively in equilibrium mediated by the H+ concentration. This model is in conflict with the results of Kobayashi et al. [196] who found pH dependence of the position of the first peak. 

Franke [47] undertook a comprehensive electroanalytical study of K2S207 mixtures with K2S04, which is formed by Eqs. (47) and (48) and V2Os, a widely-used oxidation catalyst for S02. Pure pyrosulfate under N2 or air (Fig. 38a,b) shows only the reduction to S02 and sulfate, Eq. (48) (all potentials are vs. Ag/Ag+). When S02 is added, a new reduction and oxidation peak appear (Fig. 38c,d). When the electrolyte was pre-saturated with K2S04 (ca. 4 wt.%) (Fig. 39) the gas composition had no direct effect on the voltammetry. Although the equilibrium for Eq. (49) lies well to the right at this temperature, 400 °C, the kinetics are quite slow in the absence of a catalyst. The equilibrium between pyrosulfate and sulfate, Eq. (47), lies well to the left (K = 2 x 10-6), but will proceed to the right in the absence of S03. Thus, the new peaks are sulfate oxidation, Eq. (43), and S03 reduction to sulfite ... 

Fichter and Kern O first reported that uric acid could be electrochemically oxidized. The reaction was studied at a lead oxide electrode but without control of the anode potential. Under such uncontrolled conditions these workers found that in lithium carbonate solution at 40-60 °C a yield of approximately 70% of allantoin was obtained. In sulfuric acid solution a 63% yield of urea was obtained. A complete material balance was not obtained nor were any mechanistic details developed. In 1962 Smith and Elving 2) reported that uric acid gave a voltammetric oxidation peak at a wax-impregnated spectroscopic graphite electrode. Subsequently, Struck and Elving 3> examined the products of this oxidation and reported that in 1 M HOAc complete electrochemical oxidation required about 2.2 electrons per molecule of uric acid. The products formed were 0.25 mole C02,0.25 mole of allantoin or an allantoin precursor, 0.75 mole of urea, 0.3 mole of parabanic acid and 0.30 mole of alloxan per mole of uric acid oxidized. On the basis of these products a scheme was developed whereby uric acid (I, Fig. 1) is oxidized in a primary 2e process to a shortlived dicarbonium ion (Ha, lib, Fig. 1) which, being unstable, under-... 

Studies of the linear sweep and cyclic voltammetric behavior of N-methyl-ated xanthines 35 -37> reveals that they undergo electrochemical oxidation over a fairly wide pH range at the PGE (Table 1). All but three of the xanthines studied show just a single voltammetric oxidation peak, although it is prob-... 

Thiopurine (6-mercaptopurine) gives rise to three pH-dependent voltam-metric oxidation peaks at the PGE (Table 1) 6S>. The first, least positive peak is an adsorption pre-peak due to the one-electron oxidation of 6-thiopurine (I, Fig. 18) to an adsorbed layer of product, bis (6-purinyl) disulfide (III,... 

Fig. 3a-c. Differential pulse voltammetric patterns for oxidation of decanuclear compounds in acetonitrile solution a 20 b 19 c 18. Fc indicates the oxidation peak of ferrocene, used as an internal standard... 

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