Electroactive Form dissociation

In case (i) the current is expected to depend on pH in the shape of a dissociation curve in case (ii) the current would increase linearly with hydrogen ion concentration finally, for (iii), steadily increasing current, dependent on the buffer composition, would be observed. In practice either several dissociations or combinations of these factors are involved. The observed pH-dependence may show a U-shape or bell-shape, or a maximum. Sometimes the above-mentioned reactions are combined with acid-base reactions involving the electroactive form, of the type mentioned in the preceding paragraph. 

Many organic electrode processes require the adsorption of the electroactive species at the electrode surface before the electron transfer can occur. This adsorption may take the form of physical or reversible chemical adsorption, as has been commonly observed at a mercury/water interface, or it may take the form of irreversible, dissociative chemical adsorption where bond fracture occurs during the adsorption process and often leads to the complete destruction of the molecule. This latter t q)e of adsorption is particularly prevalent at metals in the platinum group and accounts for their activity as heterogeneous catalysts and as... 

Chemical reaction steps Even if the overall electrochemical reaction involves a molecular species (O2). it must first be converted to some electroactive intermediate form via one or more processes. Although these processes are ultimately driven by depletion or surplus of intermediates relative to equilibrium, the rate at which these processes occur is independent of the current except in the limit of steady state. We therefore label these processes as chemical processes in the sense that they are driven by chemical potential driving forces. In the case of Pt, these steps include dissociative adsorption of O2 onto the gas-exposed Pt surface and surface diffusion of the resulting adsorbates to the Pt/YSZ interface (where formal reduction occurs via electrochemical-kinetic processes occurring at a rate proportional to the current). 

Charge transfer from the catalytic surface to the electroactive species Electroactive species are formed in the first step by dissociation of the complex... 

Typical examples are the dissociation of weak acids either in reaction of hydrogen evolution or regarding their oxidation when only the protonated forms are electroactive (e.g., oxalic acid) as well as the reduction of formaldehyde whose hydrate existing in aqueous solutions is electroinactive. 

Apoferritin is a spherical protein shell composed of 24 protein subunits, forming an outer diameter of 12.5 nm and an aqueous interior about 8 nm in diameter [65], This protein cage is capable of holding about 4500 iron atoms and can be reversibly dissociated into its 24 subunits at low pH (2.0), and reassembled at high pH (8.5). Modulation of pH can thus serve as a method to load and release electroactive markers allowing apoferritin to be employed as an electroactive label. Such an approach avoids the use of harsher acid dissolution of quantum dot NP labels and complicated semiconductor NP... 

Less frequently it is found that the electroactive species 0 is not the major species in bulk solution but is only formed by a chemical process, i.e. the electrode reaction is disturbing an equilibrium in homogeneous solution. An example is the reduction of acetic acid to hydrogen which proceeds via dissociation prior to electron transfer. 

The formation of complex ions in molten salts is considered elsewhere. However, in certain cases, where the complex ions themselves are electroinac-tive, they can dissociate to form electroactive species. The kinetic effects which are then manifested may be detected and quantified by electroanalytical... 

Recent interest has focused especially on the role of the coupled chemical steps mentioned above in determining the overall kinetics and mechanisms of electrode reactions. Although this leads to an increase in the amount of chemical information that can be obtained, it may also lead to various complications and ambiguities. Schematically, a complex ion which is normally electroinactive itself dissociates to form an electroactive species O, viz.. 

In the 1950s and 1960s, many papers dealing with the estimation of kinetics of dissociation and recombination of electroactive weak acids appeared. The classic example is the polarography of phenylglyoxylic acid [41]. At low pH-values, the non-dissociated form HA is reduced in a single, diffusion-controlled wave (see Fig. 5). At pH 5.5 a new wave (at more negative potentials) appears. In alkaline buffer solutions only... 

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