Page 166 - Analytical Electrochemistry 2d Ed - Jospeh Wang

5-2 ION-SELECTIVE ELECTRODES 151

be stored in an aqueous solution when not in use, so that the hydrated gel layer of the
glass does not dry out. A highly stable response can thus be obtained over long time
periods. As with other ion-selective electrodes, the operator should consult the
manufacturer's instructions for proper use. Commercial glass electrodes are remark-
ably robust and, with proper care, will last for more than a year. Proper maintenance
of the reference electrode is also essential to minimize errors.
Measurements of pH can also be performed using other types of potentiometric
sensors. Non-glass electrodes offer various advantages for certain pH measurements
(particularly intravascular and intraluminal clinical applications, food assays, and
operation in ¯uoride media), including ease of preparation, low electrical resistance,
and safety in handling. The most common examples are the quinhydrone electrode
(in which the response is due to a proton-transfer redox reaction of the quinone±
hydroquinone couple), and the antimony electrode (based on the redox reaction
between antimony and antimony oxide involving protons). Other metal±metal oxide
couples, such as palladium±palladium oxide, have been applied for pH measure-
ments. Membrane electrodes based on various neutral hydrogen ion carriers (e.g.
tridodecylamine) can also be employed (17). The resulting electrodes exhibit
excellent selectivity, reproducibility, and accuracy, but their dynamic range is
inferior to that of glass electrodes. (The dynamic range appears to depend on
the acidity constant of the incorporated ionophore.) New pH sensors based on
new glass compositions or non-glass formulations are currently being developed
in various laboratories. While such electrodes may be useful for speci®c applica-
tions, glass electrodes are likely to remain the choice for routine analytical
applications.

5-2.1.2 Glass Electrodes for Other Cations From the early days of glass
pH electrodes, it was noticed that alkaline solutions display some interference on the
pH response. Deliberate changes in the chemical composition of the glass membrane
(along with replacements of the internal ®lling solution) have thus led to electrodes
responsive to monovalent cations other than hydrogen, including sodium, ammo-
nium, and potassium (15). This usually involves the addition of B 2 O 3 or Al 2 O 3 to
sodium silicate glasses, to produce anionic sites of appropriate charge and geometry
on the outer layer of the glass surface. For example, the sodium- and ammonium-
selective glasses have the compositions 11% Na 2 O±18% Al 2 O 3 ±71% SiO 2 and 27%
Na 2 O±4% Al 2 O 3 ±69% SiO 2 , respectively. Unlike sodium silicate glasses (used for
pH measurements), these sodium aluminosilicate glasses possess what may be
termed AlOSiO sites with a weaker electrostatic ®eld strength and a marked
preference for cations other than protons. The overall mechanism of the electrode
response is complex but involves a combination of surface ion-exchange and
ion-diffusion steps. To further minimize interference from hydrogen ions, it is
desirable to use solutions with pH values higher than 5. Improved mechanical and
electrical properties can he achieved using more complex glasses containing various
additives.