OPTIMUM COMPOSITION OF NEUTRAL CARRIER BASED PH ELECTRODES

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.