Electrodes with soluble reductants

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. 

The electrode material also may be manipulated to achieve more specific or more reliably controlled performance. Specific catalysts for desired reactions may be incorporated into the electrode material or bound to the surface of the electrode. A present example is the coating of a carbon or other inert electrode with a polymer film impregnated with a mercuric salt. The resulting electrode is catalytic for reduction of metals, such as Pb2+, that are soluble in mercury. This is an area of research that could pay off through qualitative improvements in accuracy, precision, and response time. 

The third approach is based on thermoinjection of electrochemically generated mercury [45]. Gold wire electrodes with diameter of 50 or 100 gm were used. Metallic mercury was formed on the gold wires by electrochemical reduction from water-soluble mercury salt. 

For instance, the reduction potential of many solvents depends on the salt used and, in particular, on the cation. The reduction potentials of alkyl carbonates and esters in the presence of tetraalkyl ammonium salts (TAA) are usually much lower than in the presence of alkaline ions (Li+, Na+, etc.). Similar effects were observed with the reduction potential of some common contaminants (e.g., H20, 02, C02). Moreover, the reduction products of many alkyl carbonates and esters are soluble in the presence of tetraalkyl ammonium salts, while in the presence of lithium ions, film formation occurs, leading to passivation of the electrode [3],... 

Many lithium salts, such as lithium perchlorate and the halides, are soluble in nonaqueous solvents. The reduction potential of Li depends on the electrode and the solvent. At a mercury cathode amalgam formation takes place, whereas formation of lithium metal occurs at platinum in aprotic media. Lithium metal is less reactive than sodium, and in some solvents sodium attacks the solvent whereas lithium is unreactive. A small water content in an aprotic solvent may react with lithium (or Li may react with hydroxyl ions formed at the cathode) to form lithium hydroxide, which may cover the electrode with an insoluble, insulating layer. 

In the presence of A and adenosine, the copper(II)/copper(Hg) couple split to the copper(II)/copper(I) and copper(I)/copper(Hg) couples [179, 180]. Sparingly soluble compounds of copper(I) with A and its ribonucleoside were accumulated on the electrode, either by reduction of the Cu(II) ions or by oxidation of the copper amalgam electrode. The copper(I) A deposit was stripped either cathodically or anodically. The stripping peaks obtained for copper complexes had higher detection limits, but appeared over a wider range of pH and at more negative potentials than the peaks related to mercury compounds [161]. It was shown that in addition to A, other purine bases, such as G, hypoxanthine, xanthine, and their nucleosides (guanosine and inosine) [181-183],... 

The layer formed instantaneously upon contact of the metal with the solution, consists of insoluble and partially soluble reduction products of electrolyte components. The thickness of the freshly formed layer is determined by the electron-tunneling range. The layer acts as an interphase between the metal and the solution and has the properties of a solid electrolyte with high electronic resistivity. For this reason it was called a solid-electrolyte interphase SEI. The batteries, consisting of SEI electrode, were called SEI batteries. ... 

Another key fact is that the electroreduction of CO2 may take place at the electrode surface or be mediated by a catalyst attached to the electrode or soluble in the solvent (Scheme 8.1). In case (A) the reaction occurs at the surface of the electrode which directly transfers electrons to CO2 this generally requires high overvoltage and may cause electrode deterioration or even consumption and scarce selectivity. In case (B) the electrode transfers electrons to the catalyst which then interacts with CO2 causing its reduction. In such a case, CO2 is not in contact with the electrode, and this results in better stability of the electrode and its longer life, which may be important in the overall costing determination of a process. 

Lead Telluride. Lead teUuride [1314-91 -6] PbTe, forms white cubic crystals, mol wt 334.79, sp gr 8.16, and has a hardness of 3 on the Mohs scale. It is very slightly soluble in water, melts at 917°C, and is prepared by melting lead and tellurium together. Lead teUuride has semiconductive and photoconductive properties. It is used in pyrometry, in heat-sensing instmments such as bolometers and infrared spectroscopes (see Infrared technology AND RAMAN SPECTROSCOPY), and in thermoelectric elements to convert heat directly to electricity (33,34,83). Lead teUuride is also used in catalysts for oxygen reduction in fuel ceUs (qv) (84), as cathodes in primary batteries with lithium anodes (85), in electrical contacts for vacuum switches (86), in lead-ion selective electrodes (87), in tunable lasers (qv) (88), and in thermistors (89). 

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 ... 

Occasionally the zinc electrode is wrapped in a polypropylene fleece filled with inorganic substances, such as potassium titanate, in order to reduce the solubility of zinc since the problem of dendrite growth is aggravated even by the metallization of the cellophane separator due to the aforesaid silver reduction and its promoting the generation of shorts. 

Ti/TiOa electrodes manufactured by impregnating a Ti surface with a soluble Tp compound and subsequent baking in air can be used for reduction processes with Ti " or Ti" species as proposed catalytic intermediates. The usefulness of such electrodes was demonstrated by the reduction of nitrobenzene in 1 M HjSO /CHjOH (1 1)... 

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