Page 89 - 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

Electrochemistry 167

exchange membranes (prepared from organic, polymer-
based, ion-exchange resins) that give a selective response
to specific cations and anions. The main advantages of the
membranes are resistance to high acid and alkali concen-
trations and high conductivity. The membranes, however,
show little selectivity between ions. A related form of the
membrane electrode is the inorganic-crystal electrode. An
example is the lanthanum-fluoride (LaF 3 ) electrode that
senses fluoride-ion concentrations, which has become the
standard sensor for fluoride determinations in water anal-
ysis. It represents a combination of a membrane electrode
with ion-exchange characteristics and an incomplete form
of a second-class electrode in which lanthanum fluoride
is the insoluble material that responds to the free fluoride-
ion concentration in the test sample. Other examples of
this form of electrode include AgX/Ag 2 S and MS/Ag 2 S
electrodes, which give response to monovalent anions and
divalent cations, respectively.
In contrast to solid-membrane electrodes, liquid-
membrane electrodes can extract counterions from the so-
lution phase into the membrane phase. Selectivity is pro-
vided by the charged nature of the membrane carriers and
arises from the competitive degree of extractability of var-
ious counterions. Totally liquid systems can be employed
but are impractical. Instead, a porous support or an inert
FIGURE 1 Glass electrode (a) and its cell schematic (b) in asso-
ciation with a reference electrode. polymer support is used in most commercial electrodes.
Gas-selective electrodes are a particularly important ap-
plication of the glass electrode. For example, the carbon-
the outside surface of the glass membrane is exposed to dioxide electrode is a self-contained system with a glass
an ionic solution, a response for the hydronium-ion ac- electrode and a concentric silver/silver-chloride electrode
tivity [H O] is obtained that follows the Nernst expres- enclosed by a CO 2 permeable membrane. The latter holds
+
3
sion. Although there has been considerable debate about a thin film of bicarbonate solution in contact with the glass
the mechanism of response for the glass electrode, the membrane, which provides a junction to the silver/silver-
current thinking invokes an ion-exchange process that in- chloride reference electrode. The electrode has found ex-
volves the hydroxyl groups on the surface of the glass. tensive application in monitoring CO 2 levels in blood and
Thus, the population of protons on the outside surface of probably will find increasing application in other systems
the membrane affects the population on the inside of the that require continuous measurement of CO 2 partial pres-
membrane, which generates a membrane potential that is sures. The electrode response is based on the reaction
indicated by the silver/silver chloride. At one time there + −
CO 2 + 2H 2 O H O + HOC(O)O (42)
was a belief that hydronium ions actually penetrated the 3
glass membrane from the outside to the inside. However, such that changes in the partial pressure of carbon dioxide
experiments with labeled systems establish that this is not cause an attendant change in the concentration of hydro-
true. Further support for the ion-exchange mechanism is nium ion. [Note: Although the coventional formulation of
−
provided by the realization that glass electrodes are not bicarbonate ion (HCO ) is common, the formulation used
3
specific for hydronium ions, but only give a selective re- here (HOC(O)O ) provides a better sense of its structure
−
sponse.Inotherwords,othermetalions,inparticularthose and chemistry.] Thus, with a fixed concentration of bi-
of sodium and lithium, cause a response from glass elec- carbonate the electrode provides a direct potentiometric
trodes through an equivalent ion-exchange process. response to the partial pressure of carbon dioxide. Other
The unwanted response of glass electrodes to metal gas-monitoring electrode systems should be possible that
ions, particularly alkali metal ions, has prompted the de- are based on similar processes. For example, an ammo-
velopment of specialized glass membranes that have an nia (NH 3 ) electrode might well be developed with the
enhanced selective response for monovalent cations. A converse of the reactions indicated for the CO 2 electrode.
+
parallel approach has been used in the development of ion- Thus, an ammonium-ion (NH ) electrolyte would be used
4