Galvani potential difference, interface between

For many years there existed a widely accepted view in the literature that Galvani potentials of interfaces between two immiscible electrolyte solutions could not be measured only the potential differences resulting from the changes in the ionic... 

The density of electronic states in metals has a large concentration in its broad conduction band which is partially filled by electrons. There is a sharp transition between occupied and vacant states at the Fermi energy Ep. At equihbrium between a metal and a redox couple in an electrolyte, the Fermi energy of the metal and the free energy of electrons in the redox system coincide. The Galvani potential difference, Acpo> between the metal and the electrolyte at equilibrium causes, due to the accompai ing charging of the electric double layer, a shift of the electronic states between both sides of the interface relative to each other such that this condition is fulfilled. At equilibrium we have the relation (cf. Equation [2.74]) ... 

The second effect is that of a change in the potentiaf difference effectively influencing the reaction rate. By its physical meaning, the activation energy should not be influenced by the full Galvani potential across the interface but only by the potential difference (cpo ) between the electrode and the reaction zone. Since the Galvani potential is one of the constituent parts of electrode potential E, the difference - j/ should be contained instead of E in Eq. (14.13) ... 

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. 

From the thermodynamic point of view, the occurrence of a heterogeneous ET event at a liquid-liquid interface is determined not only by the relation of redox potentials between reactants in each phase, but also by the Galvani potential difference. Let us consider the general reaction,... 

Potential differences at the interface between two immiscible electrolyte solutions (ITIES) are typical Galvani potential differences and cannot be measured directly. However, their existence follows from the properties of the electrical double layer at the ITIES (Section 4.5.3) and from the kinetics of charge transfer across the ITIES (Section 5.3.2). By means of potential differences at the ITIES or at the aqueous electrolyte-solid electrolyte phase boundary (Eq. 3.1.23), the phenomena occurring at the membranes of ion-selective electrodes (Section 6.3) can be explained. 

The inner potential difference between two contacting phases is cafied in electrochemistry the Galvani potential difference, and the outer potential difference is called the Volta potential difference. The outer potential difference corresponds to what is called the contact potential between the two phases. We call, in this test, the inner potential difference across an interface the interfacial potential. 

It may be convenient to regard the Galvani potential difference between two phases in contact as being due to two effects the orientation of dipoles in the interface between them and the separation of independently mobile charged species across the phase boundary in an analogous way to that discussed for the separation of (f> into outer and surface potentials [5, 6]. 

The significance of Eq. (1.18) is that, at the contact between two different phases, for example two different metals, a certain Galvani potential difference exists, which is generated by the difference between the chemical potential of the electrons in each metal. So, for example, if an interface Cu-Fe is considered and the equilibrium is assumed,... 

Galvani potential difference — A

electrostatic component of the work term corresponding to the transfer of charge across the - interface between the phases a and f whose - inner electric potentials are (/>" and R, respectively, i.e., the Galvani potential difference is the difference of inner electric potentials of the contacting phases. The electrical potential drop can be measured only between the points which find themselves in the phases of one and the same chem-ical composition [i,ii]. Indeed, in this case p ( = p and... 

Charge-transfer processes occurring across the liquid-liquid interface have also been studied by EPR. The Galvani potential difference between the two immiscible solvents, water and 1,2-dichloroethane (DCE), was controlled electrochemically by means of a bipotentiostat. The water phase contained potassium ferrocyanide, which, in the DCE phase, by electrochemical polarization of the interface, can reduce a compound such as tetracyanoquinodimethane to its radical anion or oxidize a compound such as tetrathiafulvalene to its cation radical. Both radicals were detected by EPR spectroscopy [79]. 

Since the discussion here concerns interfaces, it is also important to consider the potential differences which arise between the boundaries of two phases a and p. The inner potential difference, which is known as the Galvani potential difference, can be defined as... 

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... 

An ITIES is formed between water and nitrobenzene (NB) using electrolytes with the picrate (Pic ) anion at the same activity (0.1 M). The electrolyte in water is Li Pic and that in NB, tetraphenylarsonium (TA" ") picrate. Given that the standard Gibbs energy of transfer of Pic is -4.6 kJmoU [18], estimate the Galvani potential difference at the interface. Use the data in table 8.11 to estimate the activity of Li" " in NB. 

In the present chapter, the relationship between the electrode potential and the activity of the solution components in the cell is examined in detail. The connection between the Galvani potential difference at the electrode solution interface and the electrode potential on the standard redox scale is discussed. This leads to an examination of the extrathermodynamic assumption which allows one to define an absolute electrode potential. Ion transfer processes at the membrane solution interface are then examined. Diffusion potentials within the membrane and the Donnan potentials at the interface are illustrated for both liquid and solid state membranes. Specific ion electrodes are described, and their various modes of sensing ion activities in an analyte solution discussed. The structure and type of membrane used are considered with respect to its selectivity to a particular ion over other ions. At the end of the chapter, emphasis is placed on the definition of pH and its measurement using the glass electrode. 

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