Application of the Mixed-Potential Theory to the Interpretation of the Potential Response of a PVC Membrane Ion-Selective Electrode for Desipramine

The potential response of an ion-exchanger type PVC ion-selective electrode (ISE) for a drug ion, desipramine(+) (DES+), was analyzed by a mixed-potential (MP) theory proposed previously. The transfer of DES+ and its analogous ions, imipramine(+) (IMP+) and neostigmine(+) (NEO+), at a micro o-nitrophenyl octyl ether/water interface was studied by ion-transfer voltainmetry; also, the standard ion-transfer potentials (Delta(W)(O)phi(o)(j)) of the ions were then determined. The application of MP theory with the Delta(W)(O)phi(o)(j) values successfully explained the under-Nernstian response of DES+-ISE due to interference from IMP or NEO+. In this study, a universal method based on numerical calculations was developed for evaluating MP associated with plural interfering ions, which was impossible in the previous method based on analytical equations. Using the universal method, we could well predict the detection limit of DES+-ISE theoretically. The MP theory is promising for the sophisticated design of ISEs, which is not due to conventional "trial-and-error" procedures.