Potential Scales

The electrode process at -500 mV on this potential scale is correlated to the growth of 250 20 pm high islands. They grow immediately upon a potential step from the open circuit potential to -500 mV (arrow in Figure 6.2-13). 

Eq. (8)] represents by definition the zero point of the electrochemical potential scale (standard hydrogen electrode, often denoted SHE). 

Quantitatively, however, it is evident that directly measured A0 values are on average 0.2 to 0.3 eV higher than AX values. This shift in the potential scale has been discussed by Trasatti,31 34 who has attributed such a systematic difference to the different conditions of measurement (different temperatures, nonequivalence between thin water layers and bulk water, uncompensated partial charge transfer in UHV). For a more detailed discussion, the reader is referred to the original papers. 

D. Tsiplakides, and C.G. Vayenas, Electrode work function and absolute potential scale in solid state electrochemistry, J. Electrochem. Soc. 148(5), E189-E202 (2001). 

Work function, a quantity of great importance in surface science and catalysis, plays a key role in solid state electrochemistry and in electrochemical promotion. As will be shown in Chapter 7 the work function of the gas-exposed surface of an electrode in a solid electrolyte cell can be used to define an absolute potential scale in solid state electrochemistry. 

Equation (5.18) plays a key role in understanding and interpreting the NEMCA effect and it is therefore important to discuss it in some detail. Equation (5.19) is discussed in detail in Chapter 7 in connection with the absolute potential scale of solid state electrochemistry. 

The implications of Equations (7.11) and (7.12) are quite significant. They allow for the establishment and straightforward measurement of a unique absolute electrode potential scale in solid state electrochemistry. 

DEFINITION AND PROPERTIES OF THE ABSOLUTE POTENTIAL SCALE IN SOLID ELECTROCHEMISTRY... 

By comparing Figure 11.9 and the characteristic Po2(Uwr) rate breaks of the inset of Fig. 11.9 one can assign to each support an equivalent potential Uwr value (Fig. 11.10). These values are plotted in Figure 11.11 vs the actual work function G>° measured via the Kelvin probe technique for the supports at po2-l atm and T=400°C. The measuring principle utilizing a Kelvin probe and the pinning of the Fermi levels of the support and of metal electrodes in contact with it has been discussed already in Chapter 7 in conjunction with the absolute potential scale of solid state electrochemistry.37... 

In these reactions, (2) is the process taking place at the reference electrode which therefore determines the potential scale. In practice other reference electrodes, such as the saturated calomel electrode are frequently used but the data are normally expressed on the hydrogen scale. 

FIGURE 32.4 Potential dependence of the interfacial tension J and the capacity C for the interface between solutions of 5mM tetrabutylammonium tetraphenylborate in 1,2-dichloroethane and lOOmM LiCl in water. The potential scale E represents the Galvani potential difference relative to the standard ion transfer potential for tetraethylammonium ion, cP o EA+ = 0.02 V. 

The problem is basically similar to preventing scale inhibition in washing machines therefore similar chemicals are used to prevent scale deposition. Scale inhibition can be achieved either by adding substances that react with potential scale-forming substances so that from a thermodynamics standpoint the stable region is reached or by adding substances that suppress crystal growth. 

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