Page 24 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology En001f25 May 7, 2001 13:58

Analytical Chemistry 563

chromatographic detectors for sample elution analysis and the oxidized and reduced forms of any species (a ox , a red )
subsequent species collection. by the general form of the Nernst equation,

RT a red
◦
D. Analytical Electrochemistry E = E − ln ,
nF a ox
Electroanalyticalchemistryincludesabroadrangeoftech-
◦
niques that have as their focus the fact that the analyte par- where E is the standard electrode potential, R is the molar
ticipates in a galvanic or electrolytic electrochemical cell. gas constant, T is the absolute temperature, n is the charge
All techniques can be classified into one of three major associated with the redox reaction, and F is the faraday.
areas: those that measure electrical properties of the cell,
those that measure cell electrical properties as a function Indicator and reference electrodes. Complete an-
of a chemical reaction in the electrolyte, and those that alytical electrochemical cells are composed of combina-
physically collect the analyte at an electrode for further tions of an indicator and a reference electrode. Each of
analysis. these electrodes contributes half-reaction chemistry; in
combination they provide a complete redox reaction. The
indicator electrode is the analyte activity sensing element
1. Conductimetry
and develops a potential E Ind relative to the reference elec-
When an electrical potential is applied between two elec- trode, which represents a constant known potential E Ref
trodes immersed in an electrolyte solution, ions are accel- independent of the composition of the sample solution.
erated and migrate to electrodes of opposite charge. The The measured cell potential E cell can be written
rate of ion migration is a function of the applied potential
acting as a driving force countered by frictional forces and E cell = E Ind + E Ref + E Junc ,
electrophoretic and relaxation effects in solution. Conduc-
tance is represented as reciprocal resistance and can be where E Junc represents the liquid junction potential origi-
corrected for the cross-sectional area and length of the nating at the interfaces of the electrodes in the sample so-
conduction solution (specific conductance), or it can be lution due to the unequal distribution of ions as a function
stated as the specific conductance of 1 equivalent of so- of their different rates of migration across the interface.
lute (equivalent conductance). A linear relationship exists Ideally, the reference electrode potential is constant, and
between equivalent conductance and the square root of the junction potential is constant or negligible. A number
the concentration of a strong electrolyte. The measure- of reference electrodes are available, the most common
ment of solution conductance is highly sensitive, though being silver/silver chloride, calomel, or hydrogen gas sys-
not selective for any particular species. The device finds tems.
application as a detector for ion chromatographic instru- Indicator electrodes are generally employed for titration
ments, due to its inherent sensitivity. Conductance can be analysis where the cell potential varies as ions are con-
employed as a parameter to follow certain titrations such sumed or produced in a chemical reaction. At least four
as those involving neutralization, precipitation, and com- important types of indicator electrode can be identified.
plexation reactions that consume or generate ionic species. The simplest systems involve a strip of material suitable
The conductance experiment uses an ac source to avoid for the provision or acceptance of electrons, such as gold,
faradaic currents, with oscillation frequencies from 60 to platinum, or carbon, immersed in a solution containing
1000 Hz. A Wheatstone bridge assembly can be used as both the oxidized and reduced forms of some species so
the detection element if the electrochemical cell acts as that a redox potential can be measured. Another situation
one arm of the bridge, though other electronic assemblies involves the use of a metal that is reversible with respect to
are suitable for direct instrumental measurement of con- analyte ions of the metal so that the metal electrode partic-
ductance. The electrochemical cell consists of two large ipates in mass and charge balance rather than only charge
area platinized platinum electrodes placed in fixed orien- balance. A third class of electrodes uses two interfaces, as
tation with defined separation. in the case of the silver/silver chloride reference system,
where the metal is coated with a sparingly soluble salt and
is suitable for titration analysis as well as determination
2. Potentiometry
of the activity of the metal or the anion in the salt coating.
The measurement of the electromotive force between two Another class of electrodes representing a specialized se-
electrodes can provide information about the activity or ries of membrane-based devices designed to monitor the
concentration of an ion in the analyte solution. The poten- activities of certain ions selectively is described in the fol-
tial of any electrode (E) can be related to the activities of lowing two sections.