Reductants insoluble, electrode reaction

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. 

Electrochemical corrosion involves the simultaneous occurrence of (at least) two electrode reactions at the same interfacial potential between a metal and a solution. One of these is the reduction of some reducible species (e.g. 02 or H+) and the other is the anodic oxidation of the metal M to its ion Mz+, either to a soluble ionic species or to an insoluble compound (e.g. the metal oxide). In the stationary state, the reduction current Ic and the oxidation current /a compensate each other, i.e. — Jc = Ja, and the net current Ja + Jc is equal to zero at the so-called corrosion potential Ecori. 

A typical electrode of this kind consists of a silver wire covered with a thin coating of silver chloride, which is insoluble in water. The electrode reaction consists in the oxidation and reduction of the silver ... 

The electrode reaction is rarely as simple as described above. In many cases the product is either insoluble, or partly adsorbed at the electrode surface. Besides, the reactants of many reactions are also surface active. Furthermore, the electrode reaction can be either preceded or followed by chemical reactions. Hence, the choice of the working electrode also depends on the reaction mechanism. For instance, the reduction of lead ions on a platinum electrode is complicated by nucleation and growth of lead micro-crystals, while on a mercury electrode lead atoms are dissolved in mercury and the reduction is fast and reversible. Similarly, the well-known pigment alizarin red S and the product of its reduction are both strongly adsorbed on the surface of mercury and carbon electrodes [17]. In this case, the liquid mercury electrode is analytically much more useful because the adsorptive accumulation on the fresh electrode surface can be easily repeated by creating a new mercury drop. However, on the solid electrode, the film of irreversibly adsorbed substance is so stable that it can be formed in one solution and then transferred into another electrolyte for the measurement of the kinetics of the electrode reaction. After each experiment... 

Chronopotentiometry is an important molten salt technique because it can be used with electrodes of relatively large areas, such as simple flag electrodes without an insulating seal By using current-reversal chronopotentiometry, preliminary diagnostic work to determine whether the electrode reaction product is soluble or insoluble, and whether the electrode reaction is reversible or irreversible has proven to be convenient, especially for coitplex reactions such as the reduction of chromate (30). The important... 

Conducted in 10% CH2Cl2-90% acetonitrile for compounds [54] and [56] and in acetonitrile [55] upon addition of 2 equiv of the respective cation supporting electrolyte, 0.10 mol dm-3 TBABF4. The potential of the reduction current peak r, reversible q, quasi-reversible s, single reduction peak without corresponding reoxidation peak ec, electron transfer followed by a chemical reaction ec, ad, electron transfer followed by a chemical reaction with insoluble product which adsorbs on to the electrode surface. Prewaves are in parentheses. 

Disulfiram [bis(diethylthiocarbamoyl)disulfide] used in the treatment of alcoholism can be assayed directly by pulse polarography in an aliquot of a solution of a ground tablet dissolved in ethanol-acetate buffer (pH 4.5) [133]. A mechanism for the electrode process was proposed involving the reaction of the disulfiram with the mercury drop to form an insoluble mercuric salt, which then underwent reduction at the electrode surface. 

The products of reduction of salt anions are typically inorganic compounds like LiF, LiCl, Li20, which precipitate on the electrode surface. Reduction of solvents results, apart from the formation of volatile reaction products like ethylene, propylene, hydrogen, carbon dioxide, etc., in the formation of both insoluble (or partially soluble) components like Li2C03, semicarbonates, oligomers, and polymers.281 283 359 A combination of a variety of advanced surface (and bulk) analytical tools (both ex situ and in situ) is used286-321 332 344 352 353 360-377 to gain a comprehensive characterization... 

The electrodeposition of Al-Zr alloys was examined in the 66.7-33.3 mol% [EMIM]1 Cl /AlCli liquid [19]. The reduction of Zr(IV), which was introduced as ZrCLj in the liquid, produces a small ill-defined cathodic wave and a small negative shift to the Al deposition wave. Voltammetric data show that the small ill-defined cathodic wave corresponds to the Zr(IV)/Zr(III) reaction. It is noted that a surface passivating film is formed on the electrode surface after this reaction, indicating that the Zr(III) is insoluble in the liquid. 

Besides the effect of the electrode materials discussed above, each nonaqueous solution has its own inherent electrochemical stability which relates to the possible oxidation and reduction processes of the solvent,the salts, and contaminants that may be unavoidably present in polar aprotic solutions. These may include trace water, oxygen, CO, C02 protic precursor of the solvent, peroxides, etc. All of these substances, even in trace amounts, may influence the stability of these systems and, hence, their electrochemical windows. Possible electroreactions of a variety of solvents, salts, and additives are described and discussed in detail in Chapter 3. However, these reactions may depend very strongly on the cation of the electrolyte. The type of cation present determines both the thermodynamics and kinetics of the reduction processes in polar aprotic systems [59], In addition, the solubility product of solvent/salt anion/contaminant reduction products that are anions or anion radicals, with the cation, determine the possibility of surface film formation, electrode passivation, etc. For instance, as discussed in Chapter 4, the reduction of solvents such as ethers, esters, and alkyl carbonates differs considerably in Li or in tetraalkyl ammonium salt solutions [6], In the presence of the former cation, the above solvents are reduced to insoluble Li salts that passivate the electrodes due to the formation of stable surface layers. However, when the cation is TBA, all the reduction products of the above solvents are soluble. 

This low-temperature fuel cell uses H2 and O2 reactants and a highly alkaline aqueous KOH electrolyte. The advantages of this fuel cell are the faster oxygen reduction reaction in the alkaline electrolyte and the possibility of using low-cost, nonprecious metal electrode catalysts, such as Ag-loaded carbon powder. The greatest problem with alkaline fuel cells is that the electrolyte reacts with traces of CO2 to produce insoluble carbonates. 

It is generally desirable for bulk electrolytic processes to be carried out with high current efficiency. This requires that the working electrode potential and other conditions be chosen so that no side reactions occur (e.g., reduction or oxidation of solvent, supporting electrolyte, electrode material, or impurities). In electrogravimetric methods, 100% current efficiency is usually not necessary, as long as the side reactions do not produce insoluble products. In coulometric titrations at constant current, 100% titration efficiency (rather than current efficiency) is required the distinction is discussed in Section 11.4.2. 

---