Rotating disk electrode following reaction

The absence of dimer radical cation formation by diphenyl selenide under the pulse radiolysis conditions is in contrast to bimolecular reactions believed to occur under electrochemical conditions/ In these experiments, a rotating disk electrode was used in combination with commutative voltammetry under anhydrous conditions. The results led to the conclusion that reversible one-electron oxidation is followed by disproportionation, then reaction of the resulting dication with diphenyl selenide or an external nucleophile, with the likely intermediacy of the dimer dication (Fig. 33). As expected, the dihydroxy selenane is formed when water is present. Based on the kinetics of the electrochemical reaction, the authors believe the diselenide dication, not the radical cation, to be the intermediate that reacts with the nucleophile. 

Tarasevich et al. [417, 420] employed the rotating disk electrode with an oxide disk electrode to study the electrochemical reactions of peroxide in conjunction with a gasometric method, by means of which the rate of the peroxide decomposition via a purely chemical pathway [eqn. (72)] could be followed independently. The authors compared the rate of gas evolution and peroxide electroreduction and oxidation, respectively, as a function of electrode potential and attributed the difference of these rates to chemical... 

Figure 1.5.1 Effect of an irreversible following homogeneous chemical reaction on nemstian i-E curves at a rotating disk electrode. (7) Unperturbed curve. (2) and (J) Curves with following reaction at two rotation rates, where the rotation rate for (3) is greater than for (2).

Diphenylanthracene (DPA) has also been an important substrate in pyridination reactions. Anodic reaction leads to a di-pyridinium ion [32]. Marcoux (1971) found from the use of working curves that although the pyridination data could be described by both ECE and disproportionation processes, the data fitted disproportionation better. In contrast, Blount (1973), with transparent electrodes, and Svanholm and Parker (1973), with rotating disk electrodes, find that pyridination of DPA is an ECE reaction, following the pattern of Sioda s (1968) hydroxylation reaction. Anodic oxidation of DPA in the presence of 2,5-, 2,6-, and 3,5-... 

Anions (e.g., halides) that form insoluble silver salts can be measured also at a rotating silver disk electrode. In this, the deposition and stripping steps involve the following reaction ... 

The anions were generated by exhaustive electrolysis of the dimers, and the electrophile (Me2CHBr) was subsequently added. The kinetics of reaction 83 was followed by monitoring the decrease with time in the current at a rotating Pt-disk electrode (RDE) held at the potential of the Ph2M" oxidation. The oxidation currents were in separate experiments shown to be directly proportional to the concentration of Ph2M in the range 5 X X 10 and the kinetic experiments were carried out under second-... 

Both p,p -DDT and methoxychlor are rapidly deprotonated by HO in aprotic solvents with subsequent elimination of Cl to form the dehydrochlorination products that is, DDT forms DDE (Scheme 7-6). The same products are formed in their reactions with 02 -. Because the reaction rates that are measured by the rotated ring-disk electrode method are fairly rapid, the primary step must be a direct reaction with 02 - and not with HO [from the reaction of 02 - with trace water in the solvent (2 02 - + H2O - 02 + HOO + HO )]. Hence, the initial reaction with 02 - is deprotonation followed by elimination of Cl to form DDE. 

The reduction of oxygen has been investigated over either unsubstituted or partially substituted perovskites (6, 234-247). Matsumoto et al. (240) used a rotating ring-disk electrode that consisted of a LaNi03 disc and a Pt-ring electrode. These authors proposed the following reactional... 

In the absence of the following reaction, we think of the concentration profile for R as decreasing linearly from a value Cr(x — 0) at the surface to the point where Cr = 0 at 5, the outer boundary of the Nemst diffusion layer. The coupled reaction adds a channel for disappearance of R, so the R profile in the presence of the reaction does not extend as far into the solution as 5. Thus, the added reaction steepens the profile and augments mass transfer away from the electrode surface. For steady-state behavior, such as at a rotating disk, we assume the rate at which R disappears from the surface to be the rate of diffusion in the absence of the reaction [(mRCR(x = 0) see (1.4.8)] plus an increment proportional to the rate of reaction [/jikCj (x = 0)]. Since the rate of formation of R, given by (1.4.6), equals its total rate of disappearance, we have... 

The analysis of the parallel-series reactions according to this scheme has been developed for the rotating disk-ring method by Bagotskii et al, The equations relating the ratio of currents at the disk Id) and the ring Ir) to the electrode rotation rate o> are as follows ... 

This question cannot be answered by measuring only the photocurrent because the latter depends only on the light intensity. More specific information can be obtained by chemical analysis or by application of a special rotating ring-disk electrode system. The latter consists of a semiconductor disk and a Pt ring as the collector electrode. Taking redox processes at n-type CdS electrodes as an example, one can discriminate between alternative courses of reactions at the semiconductor disk electrode as follows ... 

Prater KB, Bard AJ (1970) Rotating ring-disk electrodes. II Digital simulation of first and second-order following chemical reactions. J Electrochem Soc 117 335-340... 

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