High anodic potentials

A further effect which has been known for many years is that of anions, which are specifically adsorbed at high anodic potentials on platinum, on the products of the oxidation of carboxylate ions. For example, carbonium ion-derived products can be obtained in the presence of such specific adsorption and this demands a complete change in reaction route (Fioshin and Avrutskaya, 1967 Glasstone and Hickling, 1934). 

Two equal carboxylates can be coupled to symmetrical dimers (Eq. 4). In spite of the high anode potential, that is necessary for Kolbe electrolysis, a fair number of... 

At high anodic potentials when the electrode potential is more positive than both Eq and E, ineqnalities (la) and (2a) hold and step 1 is the RDS. The kinetic eqna-tion then is... 

Methods have been developed for perchloric acid synthesis which involve the electrolysis of solutions containing hydrogen chloride or molecular chlorine. These processes occur at high anode potentials (2.8 to 3.0 V vs. SHE), when oxygen is evolved at the anode in parallel with perchloric acid formation. The current yields of perchloric acid will increase considerably when the reaction is conducted at low temperatures (e.g., 20°C). 

Another group of reactions involving anodic oxidation is the anodic dimerization occurring at high anodic potentials. These reactions are considered in Section 15.6. 

FIGURE 15.9 Anodic polarization curves recorded at a platinum electrode in the region of high anodic potentials in the presence of acetate ions (1) total current (2) partial current of oxygen evolution (3) partial current of oxidation of adsorbed species. 

Many substances are oxidized only at rather high anodic potentials, which narrows the available selection of corrosion-resistant anode materials. An alternative to platinnm metals are certain metals with an oxide coating (Pb02, Mn02, Sn02) which... 

At high anodic potentials Prussian blue converts to its fully oxidized form as is clearly seen in cyclic voltammograms due to the presence of the corresponding set of peaks (Fig. 13.2). The fully oxidized redox state is denoted as Berlin green or in some cases as Prussian yellow . Since the presence of alkali metal ions is doubtful in the Prussian blue redox state, the most probable mechanism for charge compensation in Berlin green/Prussian blue redox activity is the entrapment of anions in the course of oxidative reaction. The complete equation is ... 

At sufficiently high anodic potentials, only the anodic reaction (1) wiU proceed at the experimental electrode. Then on the counter electrode the reactions (2) or (3) causing CMT measurements will proceed at the same electrical rate. These CMT measurements should coincide with the value of current measured electrically. The only restriction in this case (other than those discussed in Section II.2) is that dissolved metal ions must not be plated onto the counter electrode in a cathodic reaction in parallel with (2) or (3). 

A special problem can be the passivation of the electrode surface by insulating layers, for example, formation of oxides on metals at a too high anodic potential or precipitation of polymers in aprotic solvents from olefinic or aromatic compounds by anodic oxidation. As a result, the effective surface and the activity of the... 

Elemental sulfur (in Equation (5-15) is expected on Pt electrodes only at high anodic potentials, and at sufficiently high potentials, sulfur is oxidized to SO2. The extent of poisoning by H2S increases with increasing H2S concentration, electrode potential, and exposure time. H2S poisoning, however, decreases with increasing cell temperature. 

Whereas Cgg and C g are easy to reduce, their oxidation occius at comparatively high anodic potentials [1, 2]. Theoretical investigations predict the first oxidation potential ofCgg to be comparable to that of naphthalene [3]. Anodic electrochemistry with fuUerenes has been carried out with Cgg films [4] as well as in solution [5-7]. Cyclic voltammetry of Cgg in a 0.1 M solution of Bu4NPFg in trichloroethylene (TCE) at room temperatare exhibits a chemically reversible, one-electron oxidation wave at -tl.26 V vs Fc/Fc (Figiue 8.1) [7]. Under identical conditions, a one-electron, chemically reversible oxidation is also observed for Cyg. The oxidation of Cyg occurs 60 mV more negative than that of Cgg at -tl.20 V vs Fc/Fc. The energy difference between the first oxidation and the first reduction potential is a measiue of the... 

If these anions are oxidized at carbon anodes instead of Pt anodes, the main product is not the Kolbe dimer but an ester of the original carboxylic acid. This reaction (Hofer-Moest) is explained by the inherent instability of C radicals at highly anodic potentials, which are necessary for the anodic oxidation of carboxylate anions. At these potentials, the C radicals are oxidized to carbonium ions that react with carboxylate anions forming esters ... 

The primary adduct 53 (Eq. (117) ) of the anodically generated radical R undergoes a series of follow-up reactions a) hydrogen abstraction to 54, b) dimerization to additive dimers 55, c) coupling with R to 1,2-disubsti-tuted monomers56, d) le-oxidation to a carbonium ion that either solvolyzes to 57 or, when 1,3-dicarbonyl compounds are added cyclizes intramolecularly to tetra (58) - or dihydrofuran derivatives (59). Product control is possible in some cases by suitable choice of the anode potential. With a high anode potential,... 

Diamond electrodes thus open up new opportunities for work under extreme conditions, including very high anodic potentials, surfactant-rich... 

And yet, at high anodic potentials (+1.7 V vs. SHE, just prior to the onset of the O2 evolution) a minor current peak is observed at potentiodynamic curves taken with polycrystalline electrodes [38, 116]. As scans were repeated, the peak current increased until it reached a steady-state value (Fig. 27). The peak is found to be more pronounced for electrodes with higher sp2-carbon content. And, vice versa, it was not observed on single crystal electrodes. The peak current still is as low as 1 pA cm 2 therefore, it would be overlooked with Fig. 7 and similar graphs. The surface redox couple that manifests itself in this manner was ascribed to the oxidation of sp2-carbon of intercrystallite boundaries. The total charge transferred, if related to oxygen bound to surface carbon sites, indicated only 0.05% of the sites participated in... 

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