16
2 Electrochemistry and Electrodeposition
understanding of the polarizability of the electrodes, see Sect. 2.8 on the electrode
kinetics.
In the case of redoxi electrodes, the electron-conducting phase serves as an electron exchange surface, and both the reduced and the oxidized form of the redox pair
are solvated in the ion conducting phase.
When several elementary electrode reactions take place on an electrode, a mixed
potential can be measured. The special case when each single electrode reaction can
lead to an equilibrium can only be achieved with extreme concentration ratios of the
components taking part in various electrode reactions since a 60 mV difference in the
electrode potential for a reaction with z = 1 requires an order of magnitude change
in the activity of a component.
When several electrode reactions take place on one electrode but the net current
is zero at the same time, the common case is that each reaction has a dominant
direction. This happens during corrosion, electroless metal deposition and cementation processes. Such electrodes can never be in equilibrium, and their electrode
potential is set by the kinetic properties of the reactions going on, providing that the
total current is zero. A more detailed picture of such processes will be elucidated in
Sect. 2.8 where the basic kinetic relationships are explained.
2.5 The Nature of the Electron Conductor/Solution
Interface
In an equilibrium bulk solution of an electrolyte, the temporal average of the concentration of any charge carrier is the same as its analytical concentration. However,
when we fix a coordinate system to a particular ion, we find that ions of the opposite
charge are more abundant in the close vicinity of this ion. The distribution of ions
around each other is influenced by electrostatic forces and the thermal fluctuations,
and the elucidation of this problem requires the treatment of the Poisson–Boltzmann
equation. This field, together with the estimation of the activity coefficients of the
electrolytes, is dealt with by the Debye–Hückel theory (not discussed here in detail;
see, e.g., Sects. 2.4–2.5 of [12]).
The presence of an interface between two phases of different conductivity mechanism can modify the even concentration distribution of the charged species for
various reasons. In the forthcoming part of this chapter, the train of thoughts will
refer to an electrode in which the electron-conducting part is an inert metal that
itself does not undergo any charge transfer reaction and serves as either a source or
a well of electrons. Similarly, the electrolyte dissolved in the solution is taken as
non-reactive. The practical consequences of the discussion will be valid for reactive
electrodes, too; nevertheless, the understanding of the basic phenomena will be easier
by considering a case without any side process.
Shortly after the elaboration of the theory of electrolytic dissolution of ionic
compounds in solution, Helmholtz assumed (1879) that the surface of the solid
2 Electrochemistry and Electrodeposition
understanding of the polarizability of the electrodes, see Sect. 2.8 on the electrode
kinetics.
In the case of redoxi electrodes, the electron-conducting phase serves as an electron exchange surface, and both the reduced and the oxidized form of the redox pair
are solvated in the ion conducting phase.
When several elementary electrode reactions take place on an electrode, a mixed
potential can be measured. The special case when each single electrode reaction can
lead to an equilibrium can only be achieved with extreme concentration ratios of the
components taking part in various electrode reactions since a 60 mV difference in the
electrode potential for a reaction with z = 1 requires an order of magnitude change
in the activity of a component.
When several electrode reactions take place on one electrode but the net current
is zero at the same time, the common case is that each reaction has a dominant
direction. This happens during corrosion, electroless metal deposition and cementation processes. Such electrodes can never be in equilibrium, and their electrode
potential is set by the kinetic properties of the reactions going on, providing that the
total current is zero. A more detailed picture of such processes will be elucidated in
Sect. 2.8 where the basic kinetic relationships are explained.
2.5 The Nature of the Electron Conductor/Solution
Interface
In an equilibrium bulk solution of an electrolyte, the temporal average of the concentration of any charge carrier is the same as its analytical concentration. However,
when we fix a coordinate system to a particular ion, we find that ions of the opposite
charge are more abundant in the close vicinity of this ion. The distribution of ions
around each other is influenced by electrostatic forces and the thermal fluctuations,
and the elucidation of this problem requires the treatment of the Poisson–Boltzmann
equation. This field, together with the estimation of the activity coefficients of the
electrolytes, is dealt with by the Debye–Hückel theory (not discussed here in detail;
see, e.g., Sects. 2.4–2.5 of [12]).
The presence of an interface between two phases of different conductivity mechanism can modify the even concentration distribution of the charged species for
various reasons. In the forthcoming part of this chapter, the train of thoughts will
refer to an electrode in which the electron-conducting part is an inert metal that
itself does not undergo any charge transfer reaction and serves as either a source or
a well of electrons. Similarly, the electrolyte dissolved in the solution is taken as
non-reactive. The practical consequences of the discussion will be valid for reactive
electrodes, too; nevertheless, the understanding of the basic phenomena will be easier
by considering a case without any side process.
Shortly after the elaboration of the theory of electrolytic dissolution of ionic
compounds in solution, Helmholtz assumed (1879) that the surface of the solid