Page 84 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN005M-206 June 15, 2001 20:25

162 Electrochemistry

THE TECHNOLOGY for the interconversion of chem- tween the anode and the cathode), which is the defining
ical energy and electrical energy has been utilized since difference for electrochemistry and electronics, is due to
the mid-19th century. This conversion is accomplished by the incompatibility of electrons and electrolyte solutions.
ionic-current flow in an electrolyte solution between two During the past four decades, the dynamics and mech-
electrodes connected to each other via an external circuit anisms of electron-transfer processes have been studied
with an electrical load or current source. Batteries, fuel via the application of transition-state theory to the kinet-
cells, and corrosion processes convert the energy of chem- ics for electrochemical processes. As a result, the kinetics
ical reactions into electrical energy. Electrolysis, electro- of both the electron-transfer processes (from solid elec-
plating, and some forms of electroanalysis reverse the trode to the solution species) as well as the pre- and post-
direction of conversion, using electrical energy to produce electron-transfer homogeneous processes can be charac-
a net chemical change. The basic principles and quanti- terized quantitatively.
tative relationships (voltage, current, charge conductance, By the use of various transient methods, electrochem-
capacitance, and concentration) for electrochemical phe- istry has found extensive new applications for the study
nomena were empirically elucidated by Michael Faraday of chemical reactions and adsorption phenomena. Thus,
and other European scientists before the discovery of the a combination of thermodynamic and kinetic measure-
electron (J. J. Thompson, 1893) and the development of ments can be utilized to characterize the chemistry of het-
chemical thermodynamics (G. N. Lewis, 1923). Building erogeneous electron-transfer reactions. Furthermore, het-
on this foundation, the utilization of electrochemical phe- erogeneous adsorption processes (liquid-solid) have been
nomena for thermodynamic characterization and analysis the subject of intense investigations. The mechanisms of
of molecules and ions (electroanalytical chemistry) be- metal-ion complexation reactions also have been ascer-
gan at the beginning of this century [potentiometry (1920) tained through the use of various electrochemical impulse
and polarography (1930)]. Relationships that describe the techniques.
techniques of potentiometry and polarography derive di- The so-called Renaissance of electrochemistry has
rectly from solution thermodynamics. In the case of po- come about through a combination of modern electronic
larography, there is a further dependence on the diffusion instrumentation and the development of a more molecular-
of ionic species in solution. The latter is the basis of con- based theory implemented with the data processing and
ductivity measurements, another area that traces its ori- computational power of computers. Within the area of
gin to the 19th century. These quantitative relationships physical chemistry, numerous thermodynamic studies of
make it possible to apply electrochemistry to the detailed unstable reaction intermediates have made use of mod-
characterization of chemical species and processes in the ern electrochemistry. In addition, extensive studies of the
solution phase. kinetics of electron-transfer processes in aqueous and
Electrochemistry is the science of electron transfer nonaqueous media have been accomplished. The electro-
across a solution/electrode interface. At the cathode, chemical characterization of adsorption phenomena has
electrons (from the electrode) are transformed within the been of immense benefit to the understanding of catalytic
interface via reaction with ions or molecules to produce processes.
−
+
reduced molecules or ions (e.g., H O + e → H·+ H 2 O; Some of the most exciting applications of electrochem-
3
II
−
−
H 2 O + e → H·+ HO ; · O 2 ·+ e → O ·;Cu (bpy) 2+ istry have occurred in the areas of organic and inorganic
−
−
2
2
I
III
II
+
−
−
+ e → Cu (bpy) ;Fe Cl 3 + e → Fe Cl ). [Note: Al- chemistry and of biochemistry. The applications have
−
2 3
though the traditional formulation of the hydro- ranged from mechanistic studies to the synthesis of un-
nium ion (H 3 O ) is pervasive in the chemical literature, stable or exotic species. The control of an oxidation or
+
the positive charge is equally distributed among the reduction process through electrochemistry is much more
three hydrogens, which prompts the formulation used precisethanispossiblewithchemicalreactants.Withinthe
+
here (H O).] At the anode, molecules or ions (from the area of inorganic chemistry, electrochemistry has been es-
3
solution) are transformed within the interface to pro- peciallyusefulforthedeterminationofformulasofcoordi-
duce electrons (at the electrode surface) and oxidized nation complexes and the electron-transfer stoichiometry
+
ions and molecules (e.g., 2 H 2 O → H O + HO·+ e ; of new organometallic compounds. Electrochemical syn-
−
3
III
II
Fe Cl → Fe Cl 3 + e ). The resultant electrons move thesisisincreasinglyimportanttothefieldoforganometal-
−
−
3
from the anode through the wires of the external circuit to lic chemistry.
the cathode as electronic current (amperes; coulombs per During the past 50 years, numerous exciting extensions
second). Within the solution phase the current is carried of electrochemistry to the field of analytical chemistry
by the ions of the supporting electrolyte (positive ions have occurred. A series of selective-ion potentiometric
toward the cathode and negative ions toward the anode). electrodes have been developed, such that most of the
The limitation of ionic current in the solution phase (be- common ionic species can be quantitatively monitored in