Liquid-Based Ion-Selective Electrodes - Potentiometric Methods of Analysis

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

Ca2+(aq) + 2(C₁₀H₂₁O)₂PO4– (m) < = = = = > Ca[(C₁₀H₂₁O)₂PO₂]₂(m)

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.