Page 88 - 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

166 Electrochemistry

in the concentration of the electroactive species. Another metal as a surface. However, during the past 50 years, it
important point is that the potential response is directly has become evident that platinum electrodes are not as in-
dependent on the temperature of the measuring system. ert as long believed and that their potentiometric response
Thus, if the correct temperature is not used in the Nernst is frequently dependent on the history of the surface and
expression, large absolute errors can be introduced in the the extent of its activation. The evidence is convincing that
measurement of the activity for an electroactive species. platinum electrodes, and, in fact, all metal electrodes, are
covered with an oxide film that changes its characteristics
with time. Nonetheless, the platinum electrode continues
B. Electrode Systems to enjoy wide popularity as an “inert” indicator of redox
reactions and of the activities of the ions involved in such
The indicator electrodes for potentiometric measurements
reactions.
traditionally have been categorized into four separate
For those redox couples that involve a metal ion plus
classes. “First-class” electrodes consist of a metal im-
the metal, the logical electrode system is the metal itself.
mersed in a solution that contains the metal ion. These
In other words, if the measured quantity is to be cupric
electrode systems provide a direct response to the ion or
ion [copper (II)], a practical indicator electrode is a piece
species to be measured,
of copper metal. All second-class electrodes involve an
RT
◦ n+ n+ − active metal in combination with an insoluble compound
E = E + ln [M ]; M + ne M (s).
nF or salt. Thus, the silver/silver-chloride electrode actually
(40)
is a silver/silver-ion electrode system that incorporates the
Therefore, the primary electrode reaction includes the
means to control the silver-ion concentration through the
sensed species. Such electrodes give a direct response ac-
chloride-ion concentration [Eq. (41)].
cording to the Nernst equation for the logarithm of the
Anumberofthemostcommonpotentiometricelectrode
activity of the species.
systems and their applications are summarized in Table I.
Electrodes classified as “second-class” electrode sys-
One of the most important and extensively used indicator
tems are those in which the electrode is in direct contact
electrode systems is the glass-membrane electrode that is
with a slightly soluble salt of the electroactive species
used to monitor hydronium-ion activity. Although devel-
such that the potentiometric response is indicative of the
oped in 1909, it did not become popular until reliable elec-
concentration of the inactive anion species. Thus, the
trometer amplifiers were developed in the 1930s (modern
silver/silver-chloride electrode system, which is represen-
pH meters use high-input-impedance digital voltmeters).
tative of this class of electrodes, gives a potential response
Figure 1 gives a schematic representation of this elec-
that is directly related to the logarithm of the chloride ion
trode and indicates that the primary electrode system is
activity,
a silver/silver-chloride (or mercury/mercurous-chloride)
RT electrode in contact with a known and fixed concentra-
◦ − − −
E = E − ln [Cl ]; AgCl (s) + e Ag (s) + Cl , tion of hydrochloric acid (usually about 0.1 M). When
nF
(41)
even though it is not the electroactive species. This is true
+
I
because the chloride-ion concentration, through the solu- TABLE I Redox Potentials (E ) for the M (OH 2 ) /M and
◦
2
I
−
bility product, controls the activity of the silver ion, which M OH/M, HO Couples of Cu, Ag, and Au in H 2 O and in MeCN
(0.1 M Tetraethylammonium Perchlorate)
is measured directly by the potentiometric silver-electrode
◦
system. E ,VvsNHE a
Becauseanypotentiometricelectrodesystemultimately M M (solv) /M M OH/M, HO −
I
+
I
n
must have a redox couple (or an ion-exchange process
in the case of membrane electrodes) for a meaning- A. H 2 O Cu +0.52 −0.36
ful response, the most common form of potentiometric Ag +0.80 +0.34
electrode systems involves oxidation-reduction processes. Au +1.7
+
Hence, to monitor the activity of ferric ion [iron(III)], an H O·/H 2 O;
2
−
excess of ferrous ion [iron(II)] is added such that the con- HO·/HO (GC) +2.72 +1.89
centration of this species remains constant to give a di- B. MeCN Cu +0.19 −0.79
rect Nernstian response for the activity of iron(III). For Ag +0.54 −0.30
such redox couples the most common electrode system Au +1.58 −0.19
+
has been the platinum electrode. This tradition has come H O·(MeCN)/H 2 O;
2
−
HO·/HO (GC) +3.2 +0.92
about primarily because of the historic belief that the plat-
inum electrode is totally inert and involves only the pure a SCE =+0.24 V vs NHE.