Reactions at High Anodic Potentials

The enhanced adsorption of anions and other substances that occurs at increasingly positive potentials causes a gradual displacement of water (or other solvent) molecules from the electrolyte layer next to the electrode. This leads to a markedly slower increase in the rate of oxygen evolution from water molecules and facilitates a further change of potential in the positive direction. As a result, conditions arise that are favorable for reactions involving the adsorbed species themselves (Fig. 15.9). In particular, adsorbed anions are discharged forming adsorbed radicals  

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. 

In surface layers still containing a certain amount of water, reaction (15.24) may occur in parallel, yielding groups, so that two different radical-type intermediates can react  

In concentrated sulfuric acid solutions at HAP, the adsorbed HS04 ions are converted, according to reaction (15.57), to HS 04 radicals which dimerize, forming peroxydisulfuric (persulfuric) acid H2S2O8. This acid is the intermediate for one of the commercialized methods of hydrogen peroxide production. The first efforts toward the electrosynthesis of peroxydisulfuric acid go back to 1878 commercial production started in 1908. The standard electrode potential of the overall reaction 

Many other peroxy compounds can analogously be produced in the region of HAP for instance, sodium perborate Na2(B03)2 (from sodium metaborate NaB02) and peroxycarbonates. These compounds are used as stable oxidizing and bleaching agents. 

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On the basis of recent findings by Tunon-Blanco and coworkers [139], one could also speculate that the adenine moiety of adsorbed NAD" " may undergo an oxidative reaction at high anodic potentials, forming a strongly mediating functionality on the electrode surface and thus facilitating the oxidation of NADH at potentials below the E° of the NADH /NADH redox couple, (Fig. 6). 

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). 

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. 

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 ... 

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 ... 

Reaction (5.12) results in Hz evolution which is of chemical nature and is responsible for the effective dissolution valence of 2. The Turner-Memming model explains the overall characteristics of the anodic reactions, that is, two different reaction paths in HF and in non-HF solutions passivation by an oxide film at high anodic potentials evolution of hydrogen to account for the effective dissolution valence being less than 4. However, it lacks the details to account for phenomena such as surface termination by hydrogen, current multiplication, and variation of effective dissolution valence. 

Reactions of Organic Substances at High Anodic Potentials on Thick Noble Meta Oxide Films... 

The IBM group led by Brusic et al. [57,58] also studied the use of polyaniline derivatives for corrosion protection of copper as well as silver. The unsubstituted polyaniline, in neutral base form, provided good corrosion protection both at open-circuit potential and at high anodic potentials. The dissolution of metal (both Cu and Ag) was decreased by a factor of 100 when the metal surface was completely covered by the neutral polyaniline. However, polyaniline doped with dodecylbenzene-sulfonic acid (the conductive form of the polymer) increased the corrosion rate of Cu and Ag in water. The doped polymer in contact with the metal is spontaneously reduced at a rate faster than the oxygen reduction rate. The faster cathodic process in turn increases the overall rate of the anodic reaction, which is the dissolution of Cu and Ag, as opposed to the formation of a passive oxide layer. 

Figure C2.8.4. The solid line shows a typical semilogaritlimic polarization curve (logy against U) for an active electrode. Different stages of reaction control are shown in tlie anodic and catliodic regimes tlie linear slope according to an exponential law indicates activation control at high anodic and catliodic potentials tlie current becomes independent of applied voltage, indicating diffusion control.

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