Electrodes with soluble oxidants

If the reductant (Red) is soluble in water and none was originally present with the oxidant, it will diffuse from the surface of the electrode to the bulk of the solution. The concentration of [Red]s at the surface at any value of / will be proportional to the rate of diffusion of the reductant from the surface of the electrode to the solution under a concentration gradient [Red]s and hence also the current ... 

Anodic oxidation often involves the formation of films on the surface, i.e. of a solid phase formed of salts or complexes of the metals with solution components. They often appear in the potential region where the electrode, covered with the oxidation product, can function as an electrode of the second kind. Under these conditions the films are thermodynamically stable. On the other hand, films are sometimes formed which in view of their solubility product and the pH of the solution should not be stable. These films are stabilized by their structure or by the influence of surface forces at the interface. 

Electrodes may be classified into the following two categories as shown in Fig. 4-3 one is the electronic electrode at which the transfer of electrons takes place, and the other is the ionic electrode at which the transfer of ions takes place. The electronic electrode corresponds, for instance, to the case in which the transfer of redox electrons in reduction-oxidation reactions, such as Fe = Fe + e,occurs and the ionic electrode corresponds to the case in which the transfer of ions, such as Fe , , = Fe, occiirs across the electrode interface. Usually, the former is found with insoluble electrodes such as platinum electrodes in aqueous solution containing redox particles and the latter is found with soluble metal electrodes such as iron and nickel. In practice, both electron transfer and ion transfer can take place simultaneously across the electrode interface. 

Anion electrodes of the second kind have better properties in this respect. They respond to the corresponding anions over wide activity ranges and the only major interference with their function comes from anions forming salts less soluble than the salt covering the electrode. However, strong oxidants also interfere. 

Organic samples are usually mineralized to render them soluble in water and to eliminate possible interference from the matrix (for example through adsorption on the electrode surface). Mineralization can be carried out by heating with strong oxidizing acids, by fusion with alkalis or by combustion in an oxygen atmosphere [142]. Blood samples must be treated with heparin if they are not immediately analysed and the content of O2 and CO2 must be carefully maintained otherwise the dissociation equiUbrium of calcium is shifted [78,96]. 

A tetraruthenated porphyrin was electropolymerised onto glassy carbon and used to catalyse the oxidation of nitrite to nitrate, with the resultant current giving a selective measure of the concentration of nitrite ion [81]. As an alternative method, soluble poly(3-octyl thiophene) [82] was cast along with tridodecylmethylammonium chloride onto glassy carbon, to give electrodes with superior selectivity over PVC-based membranes to lipophilic ions such as bromide or nitrate. 

The accessible deepness of donor centers extraction remains to be relatively small (probably, no more than several oxide lattice constants) because of its limitation by the low diffusion of oxygen, which is necessary for the oxidation of donor centers. To explain the experimentally observed appearance of a rather small concentration of relatively big Ag particles on the Ti02 electrodes, account must be given to the possibility of the lateral electron transfer from the neighboring donor centers, that is the electrochemical mechanism being of widespread occurrence in the processes of the chemical deposition of metals. In any case, metal nanoparticles deposited via the interaction of semiconductor donor centers with soluble metal ions prove to be localized at the sites of the electrode surface exposure of donor centers including continuous donor clusters. 

Considering the charge transfer reaction (Ox + n e <=> Rd) with both oxidant (Ox) and reductant (Rd) being soluble, the electrode potential can be expressed as an equilibrium electrode potential plus a sinusoidal voltage perturbation ... 

Figure 3. The enantioselective bioelectrocatalyzed oxidation of glucose by glucose oxidase at an electrode modified by a chiral electron-transfer mediator. (A) Organization of the chiral ferrocene monolayer-modified Au electrode and its interaction with soluble GOx. EDC = l-(3-dimethylami-nopropyl)-3-ethylcarbodiimide hydrochloride. (B) Cyclic voltammograms of the ferrocene-modified electrode (curves a and b for (i )-Fc (2) and (5)-Fc (3), respectively) in the presence of 1 x 10 M GOx and 50 mM glucose 0.1 M phosphate buffer, pH 7.0 potential scan rate, 5 mV s electrode area, 0.26 cm. ...

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

This case arises, for example, when working with dropping or hanging mercury electrodes. Let us consider semi-infinite diffusion to a sphere of radius with both oxidized and reduced forms soluble in the solution. In this case Eq. (47) should be substituted by... 

In the redox electrode, an inert metal is in contact with a solution containing the soluble oxidized and reduced forms of the redox half-reaction. This type of electrode was mentioned in Chapter 12. 

The formal potential of this electrode, Ef" (Hg,HgO) is 0.9258 V [2], Because of its solubility properties, the use of the mercury/mercury oxide electrode is confined to strong alkaline solutions. According to Ives and Janz [2], the mercuric oxide is best prepared by gentle ignition of carefully crystallised mercuric nitrate. The construction is similar to the calomel electrode with an alkaline solution [e.g. saturated Ca(OH)2] instead of the potassium chloride as the electrolyte solution. 

The main soluble intermediates could be readsorbed and oxidized to form CO2 or extracted from the surface under configuration of continuous flow rate. The last situation represents a loss of energy due to an incomplete methanol oxidation. This is well elucidated in the experiments where the extraction of solution in front of the electrode results in lower current than in experiments without sample collection [9]. For supported platinum, Jusys et al. [10] observed that an increasing conversion to CO2 would be attained with increasing Pt load by the cost of faster consumption of formaldehyde facts that are attributed to an increased readsoption rate on electrodes with enlarged electrochemical surface area. 

The conventional type of CSV has been used to determine a variety of inorganic and organic compounds that form an insoluble film on the electrode material during the preconcentration step. The most commonly used working electrode is the HMDE. The preconcentration step involves the application of a positive (anodic) deposition potential to the working electrode, with the formation of a sparingly soluble compound with the mercury electrode. The application of the positive potential results in the oxidation of metallic mercury to mercury(I) ... 

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