External and Internal Potentials

The electrical state of any phase a can be characterized by its internal potential, which is a sum of the external (or outer) potential induced by free electrostatic charges of the phase and the surface potential x [6, 23-26]  

When the free electrostatic charge in phase a turns to zero, = 0 and = X . The surface potential of a liquid phase is dictated by a certain interfacial orientation of solvent dipoles and other molecules with inherent and induced dipole moments, and also of ions and surface-active solute molecules. For solid phases, it is associated with the electronic gas, which expands beyond the lattice (and also causes the formation of a dipolar layer) other reasons are also possible. 

A conclusion of fundamental importance, which follows from Eq. (3), states that the electrical potential drop can be measured only between the points, which find themselves in the phases of one and the same chemical composition. This conclusion was first formulated by 

Only these values or the differences of electrochemical potentials referred to phases a and p 

The electrochemical equilibrium requires the equality of electrochemical potentials for all components in the contacting phases. From this condition of electrochemical equilibrium, the dependence of Galvani potential on the activities of potential-determining ions can be derived, which represents the Nernst equation [27] for a separate Galvani potential. That is, for an interface formed by a metal (M) and solution (S) containing the ions of this metal 

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