Oxidants soluble, electrode reaction

Assume that both the initial substances and the products of the electrode reaction are soluble either in the solution or in the electrode. The system will be restricted to two substances whose electrode reaction is described by Eq. (5.2.1). The solution will contain a sufficient concentration of indifferent electrolyte so that migration can be neglected. The surface of the electrode is identified with the reference plane, defined in Section 2.5.1. In this plane a definite amount of the oxidized component, corresponding to the material flux J0x and equivalent to the current density j, is formed or... 

Let us consider a chemical species which possesses two different oxidation states, oxidized (Ox) and reduced (Red), both stable and soluble in the electrolytic medium (solvent + inert electrolyte). The simplest formulation of the electrode reaction which converts Ox to Red ... 

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. 

There have been numerous applications of controlled-potential coulometry to analysis. Many electrodeposition reactions that are the basis of electrogravimetric determinations can be employed in coulometry as well. However, some electrogravimetric determinations can be used when the electrode reactions occur with less than 100% current efficiency, for example, the plating of tin on a solid electrode. Coulometric determinations can, of course, also be based on electrode reactions in which soluble products or gases are formed (e.g., reduction of Fe(III) to Fe(II), oxidation of 1 to I2, oxidation of N2H4 to N2, reduction of aromatic nitro compounds). Many reviews concerned with controlled-potential coulometric analysis have appeared (1, 20-22) some typical applications are given in Table 11.3.2. 

At open circuit, electrode reactions that charge the electrodes lead to a slow oxidation of the electrolyte with H2 evolution at the anode and O2 evolution at the cathode. These reactions represent an irreversible self-discharge. Once the electrolyte is introduced, the battery has a poor shelf life. Under development are acidic aqueous electrolytes in which Pb(II) is soluble rather than condensing into the solid PbS04. This development of the lead-acid cell promises a flow battery not requiring a separation membrane. The separation membrane of redox-flow batteries (see last section) remains a challenging problem for the aqueous redox-flow technology. 

Electrochemical cells can also be used to attempt to obtain data on the mechanisms of the salt-induced corrosion processes. Cyclic voltammetry has been used [78-22] to obtain information on the oxidation and reduction reactions that may occur during molten salt corrosion. Chronopotentio-metric investigations with platinum as the working electrode in cells can also be used to determine and control the compositions of molten salts, as well as to measure the solubilities of various oxidation products in melts [25-28,30]. 

The potential advantages associated with an electrosynthesis include high material utilization and significantly less energy requirement, ease of control of the reaction, less hazardous process, and the ability to perform wide range of oxidation and reduction reactions. Therefore, many electrosynthetic reactions have been reported so far [1]. However, only a few have been employed industrially. The conunercializa-tion of electrosynthetic processes has been restricted by the limited solubility of substrates and products in conventimial electrolytic solutions, the poor interphase mass transport characteristics associated with two phase system in which the reaction occurs at solid (electrode)-liquid (electrolyte) interfaces, the low selectivity for desired reaction products, and the complex processing schemes often used to recover products. 

For diffusion controlled reactions, in which both the oxidized and reduced forms are soluble, and in which the electrode reactions are mobile (i.e. for so-called reversible systems) the wave is S-shaped and the dependence of log (ium — i) i on potential is linear. The linear plot has a slope oi RT. 2-303InF (here n is the... 

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