Page 174 - Analytical Electrochemistry 2d Ed - Jospeh Wang

5-2 ION-SELECTIVE ELECTRODES 159

FIGURE 5-15 Migration of the ¯uoride ion through the LaF 3 lattice (doped with EuF 2 ).
The vacancies created within the crystal cause jumping of neighboring F into the vacancy.

Other useful solid-state electrodes are based on silver compounds (particularly
silver sul®de). Silver sul®de is an ionic conductor, in which silver ions are the
mobile ions. Mixed pellets containing Ag 2 S±AgX (where X ˆ Cl, Br, I, SCN) have
been successfully used for the determination of one of these particular anions. The
behavior of these electrodes is determined primarily by the solubility products
‡
involved. The relative solubility products of various ions with Ag thus dictate the
selectivity (i.e., k ˆ K SP Agi† =K SP Agj† ). Consequently, the iodide electrode
ij

(membrane of Ag 2 S=AgI) displays high selectivity over Br and Cl . In contrast,
the chloride electrode suffers from severe interference from Br and I . Similarly,

mixtures of silver sul®de with CdS, CuS, or PbS provide membranes that are
responsive to Cd 2‡ ,Cu 2‡ ,orPb 2‡ , respectively. A limitation of these mixed-salt
electrodes is that the solubility of the second salt must be much larger than that of
silver sul®de. A silver sul®de membrane by itself responds to either S 2 or Ag ‡
ions, down to the 10 8 M level.
Sensors for various halide ions can also be prepared by suspending the
corresponding silver halide in an inert support material, such as silicone rubber
(40). Such support material provides a ¯exible, heterogeneous membrane with
resistance to cracking and swelling. The resulting membrane is called a hetero-
geneous or precipitate-impregnated membrane. For example, a chloride-selective
electrode is based on a heterogeneous membrane prepared by polymerizing mono-
meric silicone rubber in the presence of an equal weight of silver chloride particles.
A 0.5 mm thick disk of this heterogeneous membrane is sealed to the bottom of a
glass tube; potassium chloride and a silver wire are placed in the tube. The
sensitivity of the electrode is limited by the solubility of silver chloride. Chloride
concentrations from 5 10 5 to 1.0 M can be measured. Such an electrode operates

over the pH range 2±12, and at temperatures between 5 and 5000 C. Ion-selective

electrodes for thiocyanate (SCN ) or cyanide (CN ) can be prepared in a similar
fashion. Such electrodes rely on a ``corrosion'' reaction between the silver halide
(AgX) and the target ion, for example:

AgX ‡ 2CN ! Ag CN† ‡ X 5-17†
2