Potential differences at interfaces

Electrochemistry is the study of reactions in which charged Fig. 2 Electric double layer 

The interfacial potential differences which develop in electrode-solution systems are limited to only a few volts at most. This may not seem like very much until you consider that this potential difference spans a very small distance. In the case of an electrode immersed in a solution, this distance corresponds to the thin layer of water molecules and ions that attach themselves to the electrode surface— normally only a few atomic diameters. Thus a very small voltage can produce a very large potential gradient. For example, a potential difference of one volt across a typical 1CT8 cm interfacial boundary amounts to a potential gradient of 100 million volts per centimeter— a very significant value indeed  

Interfacial potential differences are not directly observable. The usual way of measuring a potential difference between two points is to bring the two leads of a voltmeter into contact with them. It s simple enough to touch one lead of the meter to a metallic electrode, but there is no way you can connect the other lead to the solution side of the interfacial region without introducing a second second electrode with its own interfacial potential, so you would be measuring the sum of two potential differences. Thus single electrode potentials, as they are commonly known, are not directly observable. 

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However, the electrode potentials measured for different types of electrolytes cannot quantitatively be compared with each other, even when the same reference electrode has been used throughout. This is due to the fact that the potential differences at interfaces between dissimilar electrolytes cannot be determined experimentally. For this reason the electrode potentials are measured separately for each type of electrolyte medium. 

Equation (3) suggests that the membrane potential in the presence of sufficient electrolytes in Wl, W2, and LM is primarily determined by the potential differences at two interfaces which depend on charge transfer reactions at the interfaces, though the potential differences at interfaces are not apparently taken into account in theoretical equations such as Nernst-Planck, Henderson, and Goldman-Hodgkin-Katz equations which have often been adopted in the discussion of the membrane potential. 

Jaegerman (1997) calls photoelectrochemical reactions that occur at high surface state configurations Bardeen model reactions because Bardeen was the first to discuss surface states. Here they are called Helmholtz reactions because the potential difference at interfaces at which they predominate is largely in the Helmholtz layer. 

Experimental Measurement of the Volta Potential Difference at Interfaces... 

What is the source of an open-circuit, zero-current cell potential When no electric current passes through the cell, the electric potential must be uniform within each bulk phase that is an electrical conductor, because otherwise there would be a spontaneous movement of charged particles (electrons or ions) through the phase. Electric potential differences in a cell without current therefore exist only at phase boundaries. The equilibrium cell potential is the cumulative result of these potential differences at interfaces between different conducting phases within the cell. 

We shall next look briefly at the origin and consequences of potential differences at interfaces between (1) two different metals, (2) a metal and an electrolyte solution, and (3) two different electrol54e solutions. Keep in mind that these potential differences are theoretical concepts whose values cannot be measured experimentally. 

Figure 3.1. Formation of potential difference at interface between metallic gate layer and isolating Si02 layer. Ha(ad) and Hb(ad) are adsorbed hydrogen — atoms, Hd hydrogen atoms mobile in Si02...

Fig. 5.70 Formation of electriceil and chemical potential differences at interfaces, as described in the text. (In diagram d the positive charge, on the right hand side, refers to the charge on the metal side (Pt) not to the charge of the adsorption layer (241).)...

Thus the potential difference at the interface between a metal and electrolyte solution is due to both the charges at the interface (electrostatic potential difference) and the surface dipole layers the latter is referred to as the surface or adsorption potential difference. On the basis of the above considerations it might appear that adsorption at a metal surface with an excess charge is solely due to electrostatic interaction with charged species in the solution, i.e. if the metal surface has an excess negative charge the cations... 

Fig. 20.4 Lippmann electrometer for studying the variation of the excess charge on mercury with variation in potential difference at the mercury solution interface...

THE POTENTIAL DIFFERENCE AT A METAL/SOLUTION INTERFACE The overpotential ij is defined as... 

The two perturbation terms are specific to the given interface and are experimentally inseparable. They measure the contact potential difference at the M/S contact. However, since no cpd is measured in this case <5/M + S%s are grouped into a single quantity denoted by X, called the interfacial... 

One of the features found at interfaces between two electrolytes (a) and ( 3) is the development of a Galvani potential, between the phases. This potential difference is a component of the total OCV of the galvanic cell [see Eq. (2.13)]. In the case of similar electrolytes, it is called the diffusion potential and can be determined, in contrast to potential differences across interfaces between dissimilar electrolytes. 

Equilibrium electrode potentials are readily established when metal electrodes are in contact with melts. However, two difficnlties arise in attempts to measnre them suitable, sufficiently corrosion-resistant reference electrodes must be selected, and marked diffusion potentials develop at interfaces between different melts. 

The Pt surface electro-oxidation process observed in the absence of dioxygen to form chemisorbed OH from water is driven by the potential difference at the Pt/ electrolyte interface, according to the reaction... 

The evolution of CO2 was also observed when controlled potential electrolysis was carried out by applying a definite potential difference at the W/NB interface in the cell of Eq. (6)... 

FIG. 6 Ratios of NADH reacted (curve 1) and CQ produced (curve 2) after the electrolysis for 4 h by applying a constant potential difference, at the interface between W containing 10... 

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