Sophisticated Design of PVC Membrane Ion-Selective Electrodes Based on the Mixed Potential Theory

The mixed potential (MP) theory was successfully utilized to design an ionophore-based polyvinyl chloride (PVC) membrane K+ ion-selective electrode (ISE). Prior to the application of the MP theory, the transfer of K+ and interfering ions (Na+, Li+, and H+) facilitated by bis(benzo-15-crown-5) (BB15C5) or dibenzo-18-crown-6 (DB18C6) at a micro PVC membrane/water interface was studied by ion-transfer voltammetry (ITV). The reversible half-wave potentials were then obtained for the facilitated transfer of the ions. Using such voltammetric data and the literature data about diffusion coefficients of ions, we could well-predict the potential responses of the BB15C5- or DB18C6-based K+ ISE, as the function of the concentrations of primary and interfering ions, and also of the counterion for K+ [e.g., tetrakis(4-chlorophenyl)borate] added to the membrane. Thus, the MP theory has been proven to be useful to optimize the membrane composition for a higher ion selectivity and a lower detection limit. It has also been found that the leaching of ions from an inner solution is too small to affect the detection limit, at least for the designed PVC membrane ISE.