Absolute potentials

C is a constant which is characteristic of the metal of which the electrode is composed, and is sometimes called the electrolytic solution pressure. Its numerical value is equal to the ionic concentration of a solution against which the metal would have no difference of potential. This quantity is of the greatest importance for the electrochemical behaviour of the metal. It cannot be determined, however, by measurement with concentration cells, for the solution pressure C disappears from the sum of the various potential differences in equation (5). The calculation of C from the total e.m.f. would be possible if we could choose the ionic concentration of one solution, say Cg, so that the potential difference 3 would be zero. Numerous experiments have actually been carried out with the object of constructing an electrode which would have the absolute potential zero against the solution. These experiments, although in themselves interesting and important, are based on special electrochemical hypotheses and not on purely thermodynamical principles. They are therefore beyond the scope of this book.  

Although it is not yet possible to determine with certainty the absolute value of the potential difference between a metal and a solution, we can determine its value relative to an arbitrarily fixed standard of potential difference by determining the E.M.F. of the cell 

The standard electrodes most commonly used are the hydrogen electrode (platinum immersed in n H3SO4 solution under hydrogen at atmospheric pressure) and the calomel electrode (mercury against n KCl saturated with HgCl). The potential differences at the surface of contact between the two liquids may either be eliminated by the addition of suitable intermediate solutions f or estimated by calculation from the known values of the concentrations and mobilities of the various ions present. J In this 

Joule s law states that an electric current produces in a circuit an amount of heat which is proportional to the square of the current intensity, to the resistance in the circuit, and to the duration of the current. The heat produced in the circuit is 

If Q is measured in calories, 1 in amperes, a in ohms, and t in seconds, the constant k is called the electrical equivalent of heat. According to the latest and most accurate measurements the value of this constant is 0-2392, so that a current of 1 ampere produces in a resistance of 1 ohm approximately 0-24 calorie per second. This production of heat is entirely irreversible. No current is produced when the circuit is heated from the outside. 

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Atp absolute potential of A[Pg.41]

Fig. 5. Tentative mixed potential model for the sodium-potassium pump in biological membranes the vertical lines symbolyze the surface of the ATP-ase and at the same time the ordinate of the virtual current-voltage curves on either side resulting in different Evans-diagrams. The scale of the absolute potential difference between the ATP-ase and the solution phase is indicated in the upper left comer of the figure. On each side of the enzyme a mixed potential (= circle) between Na+, K+ and also other ions (i.e. Ca2+ ) is established, resulting in a transmembrane potential of around — 60 mV. This number is not essential it is also possible that this value is established by a passive diffusion of mainly K+-ions out of the cell at a different location. This would mean that the electric field across the cell-membranes is not uniformly distributed.

In galvanic cells it is only possible to determine the potential difference as a voltage between two half-cells, but not the absolute potential of the single electrode. To measure the potential difference it has to be ensured that an electrochemical equilibrium exists at the phase boundaries, e.g., at the electrode/electrolyte interface. At the least it is required that there is no flux of current in the external and internal circuits. 

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. 

It will also be shown that the absolute electrode potential is not a property of the electrode but is a property of the electrolyte, aqueous or solid, and of the gaseous composition. It expresses the energy of solvation of an electron at the Fermi level of the electrolyte. As such it is a very important property of the electrolyte or mixed conductor. Since several solid electrolytes or mixed conductors based on ZrC>2, CeC>2 or TiC>2 are used as conventional catalyst supports in commercial dispersed catalysts, it follows that the concept of absolute potential is a very important one not only for further enhancing and quantifying our understanding of electrochemical promotion (NEMCA) but also for understanding the effect of metal-support interaction on commercial supported catalysts. 

One obvious but important aspect of the absolute potential defined by Eq. (7.7) is that its value does not depend on the material of the electrode. Thus, although different metals (e.g. Pt, Ag, Hg) have significantly different 3>w,o values, the change AOw induced by the presence of the aqueous overlayer is such that Ow.o+AOw (=eUw(abs)) does not depend on the metal. 

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

Consequently the absolute potential is a material property which can be used to characterize solid electrolyte materials, several of which, as discussed in Chapter 11, are used increasingly in recent years as high surface area catalyst supports. This in turn implies that the Fermi level of dispersed metal catalysts supported on such carriers will be pinned to the Fermi level (or absolute potential) of the carrier (support). As discussed in Chapter 11 this is intimately related to the effect of metal-support interactions, which is of central importance in heterogeneous catalysis. 

H. Reiss, and A. Heller, The absolute potential ofthe standard hydrogen electrode A new estimate, J. Phys. Chem. 89, 4207 (1985). 

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