Electrochemical Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 298

Like other sensor device, general-purpose pH electrodes perform well in
certain ranges. Solutions of extreme pH values (pH 10) will

result in non-Nernstian behavior, which are termed alkaline error and acid
error, respectively. Use a lithium glass electrode or a full-range electrode
(0-14) instead, if the pH of very acidic or basic solutions is measured.
Make sure to store a pH electrode in its wetting cap containing electrode fill
solution (3 MKCl, purchased or prepared by dissolving 22.37 g KCl into
100 mL DI water). Do not store electrode in DI water.

< 0.5 and pH >

11.2.2 Measurement of Ions by Ion Selective
Electrodes (ISEs)

Since the development of H þ selective pH electrode, many other ion selective
electrodes (ISEs) have become available for various applications. Three types of
membrane electrodes and the ions each can analyze are summarized in Table 11.1.
Ions with standard methods adopted by the U.S. EPA and APHA are also noted.
The general principles and constructions of solid and liquid phase ion selective
electrodes are similar to those of glass electrodes used for pH analysis. They are all
membrane-based electrodes that respond selectively to ions in the presence of
others. The measurement is based on the quantitative relationship between boundary
potential (E) generated across a membrane and the concentration (in essence,
activity) of the specific ion. All ISEs consist of a cylindrical tube of diameter
between 5 and 15 mm and 5-10 cm long. An ion-selective membrane is fixed at
one end so that the external solution can only come into contact with the outer
surface. The other end is fitted with a low noise cable or gold-plated pin for
connection to the millivolt-measuring device.
The specific compositions of the membrane materials vary among different
ISEs. In the glass electrodes, for example, variations of membrane composition can
be made to increase the affinity of hydrated glass to a specific monovalent cation,
such as K þ and Na þ (Table 11.1). The boundary potential then depends on these
cations, probably through an ion exchange mechanism similar to that described for a
glass pH electrode.
As shown in Table 11.1, there are two other major types of membrane material,
one based on a solid crystal matrix (either a single crystal or a polycrystalline
compressed pellet) and another based on a plastic or rubber film impregnated with a

Table 11.1 Types of ion selective electrodes and their applications a

Membrane Analyte ions

Glass H þð1;2Þ ,Ag þ ,K þ ,Li þ ,Na þ ,NH 4
Solid phase Br ð1Þ ,Cl ð1; 2Þ ,CN ð1; 2Þ ,F ð1; 2Þ ,I , SCN ,S 2ð1; 2Þ ,Ag þ ,Cd 2þ ,Cu 2þ ,Pb 2þ
Liquid phase BF 4 , ClO 4 ,NO ð1;2Þ ,Ca 2þ ,K þð2Þ , hardness (Ca 2þ þMg 2þ )

3

a Standard methods are available from (1) U.S. EPA and (2) APHA (1998).