A simple formalism for the quantitative description of the upper and l
ower detection limit of pH selective solvent polymeric membrane electr
odes containing a neutral carrier and a lipophilic anionic additive is
presented, which is based on the consideration of phase transfer equi
libria at the sample/membrane interface. It is shown that the lower an
d upper detection limits are controlled by the activity and lipophilic
ity of the interfering ions and by the basicity of the ionophore. When
interfering cations or anions have completely penetrated the organic
phase boundary layer through ion exchange or coextraction equilibria,
respectively, the electrode response is expected to be a Nernstian fun
ction of the interfering ion activity alone. The measuring range may b
e shifted by incorporating ionophores of different basicity in the mem
brane, but cannot be extended with this approach. Instead, a high conc
entration of ionophore, together with 50 mol-% anionic additive relati
ve to the ionophore, and, most importantly, a more hydrophobic membran
e matrix with less cation binding characteristics has to be chosen for
achieving a maximum measuring range of the potentiometric sensor.