OPTIMUM COMPOSITION OF NEUTRAL CARRIER BASED PH ELECTRODES

A simple formalism for the quantitative description of the upper and lower detection limit of pH selective solvent polymeric membrane electrodes containing a neutral carrier and a lipophilic anionic additive is presented, which is based on the consideration of phase transfer equilibria at the sample/membrane interface. It is shown that the lower and upper detection limits are controlled by the activity and lipophilicity 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 function of the interfering ion activity alone. The measuring range may be shifted by incorporating ionophores of different basicity in the membrane, but cannot be extended with this approach. Instead, a high concentration of ionophore, together with 50 mol-% anionic additive relative to the ionophore, and, most importantly, a more hydrophobic membrane matrix with less cation binding characteristics has to be chosen for achieving a maximum measuring range of the potentiometric sensor.