Page 167 - Analytical Electrochemistry 2d Ed - Jospeh Wang

152 POTENTIOMETRY

5-2.2 Liquid-Membrane Electrodes
Liquid-membrane type ISEs, based on water-immiscible liquid substances impreg-
nated in a polymeric membrane, are widely used for direct potentiometric measure-
ments (17,18). Such ISEs are particularly important because they permit direct
measurements of several polyvalent cations as well as certain anions. The polymeric
membrane [commonly made of plasticized poly(vinyl chloride) (PVC)] separates the
test solution from the inner compartment, containing a standard solution of the target
ion (into which a silver=silver chloride wire is dipped). The ®lling solution usually
contains a chloride salt of the primary ion, as desired for establishing the potential of
the internal silver=silver chloride wire electrode. The membrane-active (recognition)
component can be a liquid ion exchanger or a neutral macrocyclic compound. The
selective extraction of the target ion at the sample=membrane interface creates the
electrochemical phase boundary potential. The membranes are commonly prepared
by dissolving the recognition element, a plasticizer (e.g., o-nitrophenyl ether) that
provides the properties of liquid phase, and the PVC in a solvent such as
tetrahydrofuran. (The recognition element is usually present in 1±3% amount.)
Slow evaporation of the solvent over 1±2 days leaves a ¯exible membrane of 10±
100 mm thickness, which can be cut (with a cork borer) and mounted on the end of
plastic tube. The ion-discriminating ability (and hence the selectivity coef®cient)
depend not only upon the nature of the recognition element but also upon the exact
membrane composition, including the membrane solvent and the nature and content
of the plasticizer. The extraction properties of the membrane can be further improved
by adding ion-pairing agents to the plasticizer. The PVC matrix provides mechanical
strength and permits diffusion of analytes to the recognition sites. The hydrophobic
nature of the membrane prevents leaching of the sensing element and the plasticizer
into the aqueous sample solution, and thus extends the operational lifetime. In
contrast, release of the primary ion (from the inner solution) leads to its higher
activity at the layer adjacent to the membrane (relative to the bulk sample), and
hence to increased detection limits of carrier-based liquid-membrane electrodes (19).
Such a localized accumulation of ions makes it impossible to measure dilute
samples. By choosing an internal electrolyte with low activity of the primary ion
and preventing its leakage, it is possible to greatly lower the detection limits by up to
six orders of magnitude down to the picomolar range (20). Active research in various
laboratories is elucidating these interfacial ion ¯uxes in connection with the
development of potentiometric sensors for trace analysis. Lowering of the detection
limits would require careful attention to the selectivity of the resulting ISE. The
selectivity of most ISEs is currently not suf®cient to reach the picomolar level.

5-2.2.1 Ion-Exchanger Electrodes One of the most successful liquid-
membrane electrodes is selective toward calcium. Such an electrode relies on the
ability of phosphate ions to form stable complexes with the calcium ion. It uses a
liquid cation exchanger, consisting of an aliphatic diester of phosphoric acid
[ RO† PO with R groups in the C 8 ±C 16 range] that possesses high af®nity for
2
2
calcium ions. The ion exchanger is held in a porous, plastic ®lter membrane that