best ion-selective electrodes available. Conduction through the membrane is facilitated by the
movement of F– ions between anionic lattice sites which in turn is influenced by the F– ion activities on
each side of the membrane. If the electrode is filled with a standard solution of sodium fluoride, the
membrane potential is a function of the fluoride activity in the sample solution only. Thus,
where k incorporates the logarithm of the activity of the internal fluoride solution. The electrode has a
Nernstian response down to 10–^5 M F– (0.19 ppm) and selectivity ratios of less than 0.001 for all other
anions except OH–.
Electrodes responding to other halides, sulphide, cyanide, silver, lead, copper and cadmium are made
using membranes fabricated from pure or doped silver sulphide (Ag 2 S). The membrane potential is
affected by the movement of Ag+ ions between cationic lattice sites which in turn is determined by the
activities of the Ag+ ion in the internal and sample solutions. As the activity of the former is fixed, that
of the latter alone influences the membrane potential. The electrode will also respond to the presence of
S^2 – ions because of their effect on the Ag+ ion activity via the solubility product expression:
Silver halide and thiocyanate membranes would respond in a similar way to a silver sulphide
membrane, Ag+ ions being the mobile species, but by themselves make unsuitable membrane materials.
A Nernstian response is, however, retained when they are incorporated into a Ag 2 S matrix, the
membrane behaving as if it were a pure halide or thiocyanate conductor, i.e.
where X– = Cl–, Br–, I– or SCN–
As Ksp = [Ag+] [X–], the membrane potential is given by
where k" includes the logarithm of the activity of the internal silver solution and the solubility product
of the silver salt incorporated into the membrane.
Similarly, doping Ag 2 S with a divalent metal sulphide enables the electrode to respond to the
corresponding metal (e.g. Pb, Cu or Cd). In all cases the electrode responds by virtue of the solubility
product equilibria