Liquid-Based Ion-Selective Electrodes
Another approach to constructing an ion-selective electrode is to use a hydrophobic membrane containing a selective, liquid organic complexing agent. Three types of organic liquids have been used: cation exchangers, anion exchangers, and neutral ionophores. When the ana- lyte’s concentration on the two sides of the membrane is different, a membrane potential is the result. Current is carried through the membrane by the analyte.
One example of a liquid-based ion-selective electrode is that for Ca2+, which uses a porous plastic membrane saturated with di-(n-decyl) phosphate (Figure 11.13). As shown in Figure 11.14, the membrane is placed at the end of a noncon- ducting cylindrical tube and is in contact with two reservoirs. The outer reservoir contains di-(n-decyl) phosphate in di-n-octylphenylphosphonate, which soaks into the porous membrane. The inner reservoir contains a standard aqueous solution of Ca2+ and a Ag/AgCl reference electrode. Calcium ion-selective electrodes are also available in which the di-(n-decyl) phosphate is immobilized in a polyvinyl chloride (PVC) membrane, eliminating the need for a reservoir containing di-(n-decyl) phosphate.
A membrane potential develops as the result of a difference in the equilibrium position of the complexation reaction
on the two sides of the membrane, where (m) indicates that the species is present in the membrane. The cell potential for the Ca2+ ion-selective electrode is
The selectivity of the electrode for Ca2+ is very good, with only Zn2+ showing greater selectivity.
The properties of several representative liquid-based ion-selective electrodes are presented in Table 11.3. An electrode using a liquid reservoir can be stored in a dilute solution of analyte and needs no additional conditioning before use. The life- time of an electrode with a PVC membrane, however, is proportional to its expo- sure to aqueous solutions. For this reason these electrodes are best stored by cover- ing the membrane with a cap containing a small amount of wetted gauze to maintain a humid environment. The electrode must then be conditioned be- fore use by soaking in a solution of analyte for 30–60 min.
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