Crystalline Solid-State Ion-Selective Electrodes
Solid-state ion-selective electrodes use membranes fashioned from polycrystalline or single-crystal inorganic salts. Polycrystalline ion-selective electrodes are made by forming a thin pellet of Ag2S, or a mixture of Ag2S and either a second silver salt or another metal sulfide. The pellet, which is 1–2 mm in thickness, is sealed into the end of a nonconducting plastic cylinder, and an internal solution containing the analyte and a reference electrode are placed in the cylinder. Charge is carried across the membrane by Ag+ ions.
The membrane potential for a Ag2S pellet develops as the result of a difference in the equilibrium position of the solubility reaction
Ag2S(s) < = = = = > 2Ag+(aq)+ S2–(aq)
on the two sides of the membrane. When used to monitor the concentration of Ag+ ions, the cell potential is
Ecell = K + 0.05916 log [Ag+]
The membrane also responds to the concentration of S2–, with the cell potential given as
If a mixture of an insoluble silver salt and Ag2S is used to make the membrane, then the membrane potential also responds to the concentration of the anion of the added silver salt. Thus, pellets made from a mixture of Ag2S and AgCl can serve as a Cl– ion-selective electrode, with a cell potential of
Ecell = K – 0.05916 log [Cl–]
Membranes fashioned from a mixture of Ag2S with CdS, CuS, or PbS are used to make ion-selective electrodes that respond to the concentration of Cd2+, Cu2+, or Pb2+. In this case the cell potential is
where [M2+] is the concentration of the appropriate metal ion.
Several examples of polycrystalline, Ag2S-based ion-selective electrodes are listed in Table 11.2. The selectivity of these ion-selective electrodes is determined by solubility. Thus, a Cl– ion-selective electrode constructed using a Ag2S/AgCl mem- brane is more selective for Br– (KCl–/Br– = 102) and I– (KCl–/l– = 106) since AgBr and AgI are less soluble than AgCl. If the concentration of Br– is sufficiently high, the AgCl at the membrane–solution interface is replaced by AgBr, and the electrode’s response to Cl– decreases substantially. Most of the ion-selective electrodes listed in Table 11.2 can be used over an extended range of pH levels. The equilibrium be- tween S2– and HS– limits the analysis for S2– to a pH range of 13–14. Solutions of CN–, on the other hand, must be kept basic to avoid the release of HCN.
The membrane of a F– ion-selective electrode is fashioned from a single crystal of LaF3 that is usually doped with a small amount of EuF2 to enhance the mem- brane’s conductivity. Since EuF2 provides only two F– ions, compared with three for LaF3, each EuF2 produces a vacancy in the crystal lattice. Fluoride ions move through the membrane by moving into adjacent vacancies. The LaF3 membrane is sealed into the end of a nonconducting plastic tube, with a standard solution of F–, typically 0.1 M NaF, and a Ag/AgCl reference electrode.
The membrane potential for a F– ion-selective electrode results from a difference in the solubility of LaF3 on opposite sides of the membrane, with the potential given by
Ecell = K – 0.05916 log [F–]
One advantage of the F– ion-selective electrode is its freedom from interference. The only significant exception is OH– (KF–/OH– = 0.1), which imposes a maximum pH limit for a successful analysis.
Below a pH of 4 the predominate form of fluoride in solution is HF, which, unlike F–, does not contribute to the membrane potential. For this reason, an analysis for total fluoride must be carried out at a pH greater than 4.
Unlike ion-selective electrodes using glass membranes, crystalline solid-state ion-selective electrodes do not need to be conditioned before use and may be stored dry. The surface of the electrode is subject to poisoning, as described earlier for a Cl– ISE in contact with an excessive concentration of Br–. When this happens, the electrode can be returned to its original condition by sanding and polishing the crystalline membrane.