Page 171 - Analytical Electrochemistry 2d Ed - Jospeh Wang

156 POTENTIOMETRY

FIGURE 5-12 Valinomycin.

Some common ionophores used for sensing different cations are displayed in Figure
5-13. The development of highly selective lithium electrodes for clinical monitoring
of psychiatric patients (receiving lithium-based drugs) has been particularly challen-
ging considering the large sodium interference. Similarly, highly selective iono-
phores for sodium are needed for addressing the large excess of potassium in the
intracellular ¯uid.
Anion-selective liquid-membrane electrodes have also been developed based on
the coordination of the anionic guest to host materials, such as metallophorphyrin or
hydrophobic vitamin B 12 derivatives, alkyltin compounds or macrocyclic poly-
amines (34±38) (Figure 5-14). Such biomimetically designed ionophores offer
effective sensing of inorganic and organic anions, such as thiocyanate, carbonate,
salicylate, phosphate, or adenosine nucleotides. Unlike anion-exchanger electrodes,
these anion sensors display selectivity patterns greatly different from the Hofmeister
sequence (due to the direct interaction of the host with the speci®c anion). Often, this
interaction involves an exchange of the coordinated anion at the metal center of the
organometallic ionophore with the target anion in the sample solution. Useful
reviews describe in detail individual carrier-based ISEs, (according to the analyte
for which they were developed) and their practical limits. (39,39a). Many exciting
developments based on novel host±guest chemistry (e.g., recognition by steric
shapes) are anticipated.

5-2.3 Solid-State Electrodes
Considerable work has been devoted to the development of solid membranes that are
selective primarily to anions. The solid-state membrane can be made of single