Page 90 - 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

168 Electrochemistry

such that changes in pH would be proportional to changes ence potentials that do not vary more than a few millivolts.
in the partial pressure of NH 3 . These electrodes as well as The silver/silver chloride electrode also finds application
SO x -, NO x -, HF-, H 2 S-, and HCN-sensing electrodes are in nonaqueous titrations, although some solvents cause
commercially available. the silver-chloride film to become soluble. Aqueous refer-
The gas-sensing electrodes also are used for the poten- ence electrodes are as convenient for nonaqueous systems
tiometric measurement of biologically important species. as any of the prototypes that have been developed to date.
An enzyme is immobilized at or near the gas probe. The When there is a need to rigorously exclude water, double-
gas sensor measures the amount of characteristic gas pro- salt bridges (aqueous/nonaqueous) are a convenient
duced by the reaction of the analyzed substance with solution.
the enzyme. For example, an enzyme electrode for urea For measurement of redox couples, a frequently over-
[NH 2 C(O)NH 2 ] determination is constructed by the im- looked but convenient reference electrode is a conven-
mobilization of urease onto the surface of an ammonia- tional glass pH electrode (assuming the sample solution
selective electrode. When the electrode is inserted into a system contains a constant level of acidity). Such an elec-
solution that contains urea, the enzyme catalyzes its con- trode provides an extremely inert and stable reference po-
version to ammonia, tential that is completely indifferent to most redox species.
urease However, the glass electrode requires the use of an elec-
NH 2 C(O)NH 2 + H 2 O 2NH 3 + CO 2 . (43)
trometer amplifier such as that contained in pH meters.
The generated ammonia diffuses through the gas-
permeable membrane and is detected by the probe of the
C. Applications of Potentiometry
electrode. A steady-state signal is reached when the rate
of NH 3 diffusion from the electrode equals the rate of its Although all potentiometric measurements (except those
generation. involving membrane electrodes) ultimately are based on a
In general, a necessary part of a potentiometric mea- redox couple, the method can be applied to oxidation-
surement is the coupling of a reference electrode to the reduction processes, acid-base processes, precipitation
indicating electrode. The ideal reference electrode has a processes, and metal-ion complexation processes. Mea-
number of important characteristics: (1) a reproducible surements that involve a component of a redox couple
potential, (2) a low-temperature coefficient, (3) the capac- require that either the oxidized or reduced conjugate of
ity to remain unpolarized when small currents are drawn, the species to be measured be maintained at a constant
and (4) inertness to the sample solution. If the reference and known activity at the electrode. If the goal is to mea-
electrode must be prepared in the laboratory, a convenient sure the activity of silver ion in a solution, then a silver
and reproducible system is desirable. wirecoupledtotheappropriatereferenceelectrodesmakes
Although the standard half-cell reactions are all refer- an ideal potentiometric system. Likewise, if the goal is
enced to the standard hydrogen electrode (NHE or SHE), to monitor iron(III) concentrations with a platinum elec-
this is an exceedingly awkward reference electrode. It has trode, a known concentration of iron(II) must be present
been selected because its potential falls in the middle of in the sample solution such that potential changes are only
those for the most common half reactions in water and dependent on the iron(II) concentration.
because with rigorous care highly reproducible potentials Table I summarizes a number of redox couples that are
can be duplicated by equally careful workers in other lab- well behaved in aqueous solutions and provide a means for
oratories. Furthermore, because it consists of a platinized monitoring the indicated species by potentiometric mea-
platinum electrode over which hydrogen gas is passed in surements. This can be either in the form of monitoring
combination with a known activity of hydronium ion, it a titration or as a direct absolute measurement of activity.
can be combined in a number of cases with other half-cells Although the tabulations of standard potentials infer that
without a liquid junction. These three factors undoubtedly the listing should be much more comprehensive, most of
justify its selection as the ultimate reference electrode for the couples tabulated are not well behaved in an electro-
fundamental measurements. The electrode reaction is a chemical sense and do not provide a Nerstian response
typical redox half reaction which includes the oxidized under normal laboratory conditions. The vast majority of
and reduced forms of hydrogen. By controlling the hy- the data tabulated is based on calorimetric data.
drogen partial pressure at a fixed level, this becomes an The major application of the potentiometric method
indicating electrode for hydronium-ion activity. is for acid-base measurements in both aqueous and non-
For most potentiometric measurements, either the sat- aqueous solvent systems. Although the glass electrode is
urated calomel reference electrode or the silver/silver- universally the most common indicating electrode sys-
chloride reference electrode is used. These electrodes can tem for such measurements, many other electrodes have
be made compact, are easily produced, and provide refer- been developed. However, except in extremely specialized