Page 170 - Analytical Electrochemistry 2d Ed - Jospeh Wang

5-2 ION-SELECTIVE ELECTRODES 155

compounds (e.g., tailor-made cyclic polyethers) capable to envelop various target
ions in their pocket. Electron-donor atoms, present in the polar host cavity, further
facilitate and in¯uence the interaction with the target ion. For example, while
oxygen-containing crown ethers form stable complexes with alkali or alkaline earth
metals, sulfur-containing ones are best suited for binding heavy metals. The extent of
this interaction is determined by the ``best-®t'' mechanism, with larger ions not able
to ®t the molecular cavity while smaller ones are weakly coordinated. Often, a
subunit group is added to the crown compound to impart higher selectivity (through
steric or blockage effects) and improved lipophilicity. The ion recognition process is
thus in¯uenced by the cavity (ring) size, the number and positioning of the electron-
donor atoms, and the nature of the subunit. For example, 14-crown-4-ether
compounds (i.e., 4 oxygens in a 14-atom ring) offer selective recognition of lithium.
Overall, these ionophores serve as reversible and reusable binding reagents that
selectively extract the target analyte into the membrane. Such a binding event creates
the phase boundary potential at the membrane±sample interface. To assure reversible
binding, it is essential to keep the free energy of activation of the analyte±ionophore
reaction suf®ciently small (28). Molecular modeling techniques are being used to
guide the design of ionophores toward target analytes. The speci®c design takes into
consideration the selectivity demands imposed by clinical or environmental samples.
A host of carriers, with a wide variety of ion selectivities, have been proposed for
this task. Most of them have been used for the recognition of alkali and alkaline
earth metal cations (e.g., clinically relevant electrolytes). A classical example is the
cyclic depsipeptide valinomycin (Figure 5-12), used as the basis for the widely used
ISE for potassium ion (29). This doughnut-shaped molecule has an electron-rich
pocket in the center into which potassium ions are selectively extracted. For
example, the electrode exhibits a selectivity for K ‡ over Na ‡ of approximately
5000. The basis for the selectivity seems to be the ®t between the size of the
potassium ion (radius 1.33 A Ê ) and the volume of the internal cavity of the
macrocyclic molecule. The hydrophobic side chains of valinomycin stretch into
the lipophilic part of the membrane. In addition to its excellent selectivity, the
electrode is well behaved and has a wide working pH range. Strongly acidic media
‡
can be employed because the electrode is 18,000 times more responsive to K than
‡
to H . A Nernstian response to potassium ion activities, with a slope of 59 mV per
pK , is commonly observed from 10 6 to 10 1 M. Such attractive performance
‡
characteristics have made the valinomycin ISE extremely popular for clinical
analysis (with 200 million assays of blood potassium being carried out annually
in the United States using this device).
Many other cyclic and noncyclic organic carriers with remarkable ion selectivities
have been used successfully as active hosts of various liquid membrane electrodes.
These include the 14-crown-4-ether for lithium (30); 16-crown-5 derivatives for
sodium; bis-benzo-18-crown-6 ether for cesium; the ionophore ETH 1001 [(R,R)-
1
1
N,N -bis(11-ethoxycarbonyl)undecyl-N,N -4,5-tetramethyl-3,6-dioxaoctanediamide]
for calcium; the natural macrocyclics nonactin and monensin for ammonia and
sodium (31), respectively; the ionophore ETH 1117 for magnesium; calixarene
derivatives for sodium (32); and macrocyclic thioethers for mercury and silver (33).