Potential difference metal/solution interface

The difference in the nature of the metal manifests itself in that, at the same electrode potential, the Galvani potential drop at the metal-solution interface is different, the Galvani potential difference being equal to the difference of fXg s. Since, however, the absolute value of an individual potential drop cannot be measured, but only the overall energetic effect, these differences are impossible to observe. 

Similarly, all points within a metal, which consists of an ordered rigid lattice of metal cations surrounded by a cloud of free electrons, are electrically neutral. Transport of charge through a metal under the influence of a potential difference is due to the flow of free electrons, i.e. to electronic conduction. The simultaneous transport of electrons through a metal, transport of ions through a solution and the transfer of electrons at the metal/solution interfaces constitute an electrochemical reaction, in which the electrode at which positive current flows from the solution to the electrode is the cathode (e.g. M (aq.) + ze M) and the electrode at which positive flows from it to the solution (e.g. M - M (aq.) -)- ze) is the anode. 

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

Principles and Characteristics Voltammetric methods are electrochemical methods which comprise several current-measuring techniques involving reduction or oxidation at a metal-solution interface. Voltammetry consists of applying a variable potential difference between a reference electrode (e.g. Ag/AgCl) and a working electrode at which an electrochemical reaction is induced (Ox + ne ----> Red). Actually, the exper-... 

We should like to define a work function of an electrochemical reaction which enables us to calculate outer potential differences in the same way for a metal-solution interface, and this work function should also refer to the vacuum. For this purpose we consider a solution containing equal amounts of Fe3+ and Fe2+ ions in contact with a metal M, and suppose that the reaction is at equilibrium. We now transfer an electron from the solution via the vacuum to the metal in the following way ... 

When a semiconducting electrode is brought into contact with an electrolyte solution, a potential difference is established at the interface. The conductivity even of doped semiconductors is usually well below that of an electrolyte solution so practically all of the potential drop occurs in the boundary layer of the electrode, and very little on the solution side of the interface (see Fig. 7.3). The situation is opposite to that on metal electrodes, but very similar to that at the interface between a semiconductor and a metal. 

These several techniques for the solid-solution interface give different kinds of information. However, the one which gives most information about the nature of entities on the surface, and potentially near the surface, is fourier transform IR spectroscopy, which is not restricted to a particular metal, or, indeed, to the type of substrate (except that this must be reflecting). 

According to Bockris and Habib, the potential difference at the metal/solution interface at pzc is a result of the contribution of two components the surface potential (electron overlap) of the metal go and solvent dipoles oriented at the electrode surface, go- The value of go cannot be experimentally measured because the absolute value of the electrode potential is not known. However, the value of go can be estimated from the relation... 

In electrochemical conditions, the electrons are transferred from the metal to the solution rather than to a vacuum. Moreover, the metal/solution interface is charged and the potential difference between the metal and the solution should be taken into account. The situation is simplified when the work function and uncharged interface are considered. The relationship between the work function and potential of zero charge was propos nearly 30 years ago by Bockris and Argade and by Frumkin (see e.g., Ref. 66) and later intensively discussed by Trasatti (e.g., Refs. 5, 21, 67). The relationship is given by the equation... 

Four types of fundamental subjects are involved in the process represented by Eq. (1.1) (1) metal-solution interface as the locus of the deposition process, (2) kinetics and mechanism of the deposition process, (3) nucleation and growth processes of the metal lattice (Mi ttice), and (4) structure and properties of the deposits. The material in this book is arranged according to these four fundamental issues. We start by considering in the first three chapters the basic components of an electrochemical cell for deposition. Chapter 2 treats water and ionic solutions Chapter 3, metal and metal surfaces and Chapter 4, the metal-solution interface. In Chapter 5 we discuss the potential difference across an interface, and in Chapter 6,... 

In general, when a metal is immersed in a solution of (i.e., contairung) its own ions, some surface atoms in the metal lattice do become hydrated and dissolve into the solution. At the same time, ions from the solution are deposited on the electrode. The rate of these two opposing processes is controlled by the potential differences at the metal-solution interface. The specific potentials at which these two reaction rates are equal, called standard potentials, are usually given in the literature for solutions at 25°C (room temperature) and at an activity value of unity. 

Consider the operations necessary to measure the potential difference across a metal/solution interface. Various potential-measuring instruments can be used potentiometers, electrometers, etc. All these instruments have two metallic terminals that must he connected to the two points between which the potential difference is to he measured. 

Fig. 6.29. If electrode M, and the connecting wires M2 are dissimilar metals, a contact potential difference PC /M, at the metal Mumetal M2 interface is generated in the measurement process in addition to the extra metal-solution potential difference PDm2/S-...

Does the impossibility of measurement of a quantity preclude further thought about it Discussion of a concept, even if it cannot be measured, often leads to better understanding of it. With this view, attempts will be made to probe further into the question of the absolute potential difference across an individual metal/solution interface. 

Hence, the Y / potentials of the metal and solution phases correspond to the charges that these phases actually have in the presence of the double layer at a metal/solution interface. The outer potential difference MAS / is, therefore, the contribution to the potential difference across an electrified interface arising from the charges on the two phases. 

As explained in Section 6.3.11, the inner potential difference—A( )—seems to encompass all the sources of potential differences across an electrified interface—Ax and A jf—and therefore it can be considered as a total (or absolute ) potential across the electrode/electrolyte interface. However, is the inner potential apractical potential First, the inner potential cannot be experimentally measured (Section 6.3.11). Second, its zero point or reference state is an electron at rest at infinite separation from all charges (Sections 6.3.6 and 6.3.8), a reference state impossible to reach experimentally. Third, it involves the electrostatic potential within the interior of the phase relative to the uncharged infinity, but it does not include any term describing the interactions of the electron when it is inside the conducting electrode. Thus, going back to the question posed before, the inner potential can be considered as a kind of absolute potential, but it is not useful in practical experiments. Separation of its components, A% and A f, helped in understanding the nature of the potential drop across the metal/solution interface, but it failed when we tried to measure it and use it to predict, for example, the direction of reactions. Does this mean then that the electrochemist is defeated and unable to obtain absolute potentials of electrodes ... 

The interfacial tension depends on the forces arising from the particles present in the interphase region. If the arrangement of these particles (Le., the composition of the interface) is altered by varying, for example, the potential difference across the interface, then the forces at the interface should change and thus cause a change in the interfacial tension. One would expect therefore that the surface tension y of the metal/solution interface will vary with the potential difference V supplied by the external source. 

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