Volta potential difference metal solution interface

This is not the only possibility for obtaining an estimate of the absolute SHE potential from electrochemical experiment. As shown in [7], the absolute SHE potential can also be determined from measurements of the potential of zero charge and contact potential (Volta potential difference) of the interface of an inert metal electrode, such as Hg with an electrolytic solution. Surveying the experimental data available at the time Trasatti decided that (abs) = 4.44 V is the best estimate. This value was accepted by lUPAC as their recommended value. Fawcett has recently reviewed the procedure, also taking into account experimental data from mass spectroscopy experiments." His conclusion was that only a small adjustment was necessary (from 4.44 to 4.42 V). 

At this stage, let the two conceptually separated pails of the double layer be brought together again. The interface has been reassembled. One can now refer to the outer potential difference, sometimes called the Volta potential difference, between the metal and solution. This outer potential difference is written... 

On the basis of fig. 8.5, the relationship between the Galvani potential difference between the metal and the solution, and the Volta potential difference just outside of these phases where they form an interface with the gas phase is... 

Typical values of <.Ajj,vl/o the metal solution interface at the PZC are given in table 8.8. The Volta potential difference is negative, indicating that the metal air interface is negative with respect to the solution air interface. Combining equations (8.8.3) and (8.8.4), one obtains... 

Thus the Volta potential difference at the PZC is related to the dipole potential differences at the metal solution, metal air, and solution air interfaces. 

From Volta potential difference measurements at the metal solution interface discussed in section 8.8, the value of sAHg r at the potential of zero charge (PZC) of mercury is —0.248 V. However, the PZC is 0.192 V negative of the standard potential of the SHE. Thus, the value of the Volta potential difference at the standard potential of the hydrogen electrode is —0.056 V. PFsing the work function for an electron in Hg (4.50 eV), the estimate of E °, is... 

Table 1 summarizes the basic relationships that link energy characteristics of excess electrons with the values measured by the aforementioned methods (see also Fig. 1). In the equations given therein, i.e. in Eqs. (5) and (6) w , w , and w denote respectively metal-to-vacuum, metal-to-solution, and solution-to-vacuum photoemission work functions AT is the Volta potential difference for a metal-solution system Eg is the equilibrium potential of the electrode in solvated electron solution and il(RE) is the Fermi level of the reference electrode. Equation (6) is approximate (see above) because the solvated electron entropy has not been taken into consideration. The main error in equating the heat of electron solvation and the activation energy of the thermoemission current for the solvated electron solution is caused by the variation in the solution s surface potential with temperature apparently, here specific adsorption of solvated electrons (or of an alkali metal) on the solution/vapour interface makes major contribution to the surface potential . This error can be probably neglected if measurements are taken in very dilute solutions (<10 mol/1, see ) of the alkali metal. This follows from the dependence measured in between thermoemission current and the concentration of sodium in hexamethylphosphotriamide. 

Volta problem of the nature of the emf of an electrochemical circuit. The Volta potential difference for the liquid metal solution interface can be determined by using the following cell reference electrode solution inert gasjHgjsolutionjreference electrode, when the solution flows to the system through the internal walls of a vertical tube, where metal (e.g., mercury) flows out via a capillary placed axially in a vertical tube and is dispersed into drops. These metal drops carry away the free charges, thus, eliminating the potential difference in inert gas between the metal and the solution. A similar technique can be used for the solution j solution interface. There are also other techniques to determine the Volta potential difference [ii-vj. 

The basic principle of every measurement of the Volta potential and generally of the investigations of voltaic cells too, in contrast to galvanic cells, may thus be presented for systems containing metal/solution (Fig. 2) and liquid/liquid interfaces (Fig. 3), respectively. This interface is created at the contact of aqueous and organic solutions (w and s, respectively) of electrolyte MX in the partition equilibrium. Of course, electrolyte MX, shown in Fig. 2 and other figures of this chapter, may be different in organic (s) and aqueous (w) phases. 

The effects of the crystallographic face and the difference between metals are evidence of the incorrectness of the classical representations of the interface with all the potential decay within the solution (Fig. 3.13a). In fact a discontinuity is physically improbable and experimental evidence mentioned above confirms that it is incorrect, the schematic representation of Fig. 3.136 being more correct. This corresponds to the chemical models (Section 3.3) and reflects the fact that the electrons from the solid penetrate a tiny distance into the solution (due to wave properties of the electron). In this treatment the Galvani (or inner electric) potential, (p, (associated with EF) and the Volta (or outer electric) potential, ip, that is the potential outside the electrode s electronic distribution (approximately at the IHP, 10 5cm from the surface) are distinguished from each other. The difference between these potentials is the surface potential x (see Fig. 3.14 and Section 4.6). 

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