Electrode anodic potential range

The limited anodic potential range of mercury electrodes has precluded their utility for monitoring oxidizable compounds. Accordingly, solid electrodes with extended anodic potential windows have attracted considerable analytical interest. Of the many different solid materials that can be used as working electrodes, the most often used are carbon, platinum, and gold. Silver, nickel, and copper can also be used for specific applications. A monograph by Adams (17) is highly recommended for a detailed description of solid-electrode electrochemistry. 

TABLE 5.6 Anodic Potential Range of Electrode and Peak Potential for Oxidation of Fen(CN>6... 

Electrode Material Anodic Potential Range in Aqueous Solution (V vs. SCE) Peak Potential for Oxidation of 10 3 M Fen(CN)4- ... 

Lindquist, J- (1973) Carbon paste electrode with a wide anodic potential range. Anal Chem., 45, 1006—1008. 

Among the various tungsten carbide phases, WC has been known for the most stable phase. Chen and co-workers reported that WC was more stable than W2C under anodic potential range (6). In this report, W2C led to the anodic current peak at lower anodic potential (between 0.4 and 0.6 V vs Normal Hydrogen Electrode (NHE)) than WC (above 0.6 V vs NHE) in 0.5 M H2SO4 electrol3dn. This anodic current at lower potential means that W2C is more easily oxidized than WC and could be more easily lost in electrochemical applications. In addition, W2C showed higher corrosion currents than WC at the same potential (147). 

In general platinum and gold are the most widely used metallic electrodes. These electrodes present favorable electron transfer kinetics and a large anodic potential range. However, their use on cathodic potential (-0.2 V imtil -0.5 V) is limited by the low hydrogen overvoltage and the formation of oxide on the surface of the electrode. The oxide film can be eliminated with a cleaning-reactivation cycle [27]. The problem is less severe in nonaqueous electrolyte, but in nonaqueous media platinum can present catalytic characteristics [16, 21]. 

The relationship of anode current density with electrode potential for mild steel in dilute aqueous soil electrolytes has been studied by Hoar and Farrer. The study shows that in conditions simulating the corrosion of mild steel buried in soil the logarithm of the anode current density is related approximately rectilinearly to anode potential, and the increase of potential for a ten-fold increase of current density in the range 10 to 10 A/cm is between 40 and 65 mV in most conditions. Thus a positive potential change of 20 mV produces a two- to three-fold increase in corrosion rate in the various electrolyte and soil solutions used for the experiments. 

Figure 60. Experimental responses to anodic potential sweeps carried out on a polypyrrole electrode in a 0.1 M UC104-propylene carbonate solution from -2500 to 300 mV, at 30 mV s-1 and different temperatures ranging between -10 and 40°C. Cathodic prepolarization was always performed at 25°C and maintained for 2 min, avoiding any difference in the degree of closure of the polymeric entanglement at the beginning ofthe potential sweep. (Reprinted from T. F. Otero, H.-J. Grande, and J. Rodriguez, J. Phys. Chem. 101, 8525, 1997, Figs. 3-11, 13. Copyright 1997. Reproduced with permission from the American Chemical Society.)...

The equilibrium (1) at the electrode surface will lie to the right, i.e. the reduction of O will occur if the electrode potential is set at a value more cathodic than E. Conversely, the oxidation of R would require the potential to be more anodic than F/ . Since the potential range in certain solvents can extend from — 3-0 V to + 3-5 V, the driving force for an oxidation or a reduction is of the order of 3 eV or 260 kJ moR and experience shows that this is sufficient for the oxidation and reduction of most organic compounds, including many which are resistant to chemical redox reagents. For example, the electrochemical oxidation of alkanes and alkenes to carbonium ions is possible in several systems... 

Finally, the electrode potential may affect the overall process by determining the state of oxidation of the electrode surface. It is well known that m aqueous solution a platinum electrode has a bare surface only over the narrow potential range from approximately -t-0-4 V to -tO-8 V versus N.H.E. at more cathodic potentials it is covered by adsorbed hydrogen atoms while at more anodic potentials it is covered by... 

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