Page 162 - Analytical Electrochemistry 2d Ed - Jospeh Wang

5-2 ION-SELECTIVE ELECTRODES 147

actually not the ISE but rather changes in the reference electrode junction potential,
that is, the potential difference generated between the reference electrolyte and
sample solution. The junction potential is caused by an unequal distribution of
anions and cations across the boundary between two dissimilar electrolyte solutions
(which results in ion movement at different rates). When the two solutions differ
only in the electrolyte concentration, the liquid junction potential is proportional to
the difference in transference numbers of the positive and negative ions and to the
logarithm of the ratio of the ions on both sides of the junction:

RT a 1†
i
E ˆ t t † ln 5-9†
1
2
F a 2†
i
Changes in the reference electrode junction potential result from differences in the
composition of the sample and standard solutions (e.g., upon switching from whole
blood samples to aqueous calibrants). One approach to alleviate this problem is to
use an intermediate salt bridge, with a solution (in the bridge) of ions of nearly equal
mobility (e.g., concentrated KCl). Standard solutions with an electrolyte composi-
tion similar to that of the sample are also desirable. These precautions, however, will
not eliminate the problem completely. Other approaches to address this and other
changes in the cell constant have been reviewed (13).

5-2 ION-SELECTIVE ELECTRODES
The discussion of Section 5-1 clearly illustrates that the most important response
characteristic of an ISE is selectivity. Depending on the nature of the membrane
material used to impart the desired selectivity, ISEs can be divided into three groups:
glass, liquid, or solid electrodes. More than two dozen ISEs are commercially
available and are widely used (although many more have been reported in the
literature). Such electrodes are produced by ®rms such as Orion Research, Radio-
meter, Corning Glass, Beckman, Hitachi, or Sensorex.

5-2.1 Glass Electrodes

Glass electrodes are responsive to univalent cations. The selectivity for these cations
is achieved by varying the composition of a thin ion-sensitive glass membrane.

5-2.1.1 pH Electrodes The most common potentiometric device is the pH
electrode. This electrode has been widely used for pH measurements for several
decades. Besides use for direct pH measurements, the pH glass electrode is
commonly employed as the transducer in various gas and biocatalytic sensors,
involving proton-generating or proton-consuming reactions (see Chapter 6). Its
remarkable success is attributed to its outstanding analytical performance, and in
particular its extremely high selectivity for hydrogen ions, its remarkably broad
response range, and to its fast and stable response. The phenomenon of glass