Analysis of ion behavior and potentials in a Na+ ion-selective membrane containing methyl monensin.

Ion-selective electrodes (ISEs) are widely used in clinical examinations. The mechanisms of ISEs have been studied to develop a disposable ISE for "dry chemistry" analyses. The behavior of Na+ and Ag+ in an ion-selective polymer liquid membrane (ISM) containing methyl monensin (MMO), which is selective for Na+, has been clarified by x-ray microanalysis and measurement of electrical potential. The results indicate that Na+ is not transported across the membrane because of a stability of the Na(+)-MMO complex, whereas Ag+ (from the Ag/AgCl electrode) is transported with Cl- across the membrane by diffusion. The difference in potential, which follows a Nernst gradient, is generated by the formation of a double layer of electrons at the interface between the ISM and the electrolyte solution. These results are explainable by the stability of the trap in the polymer liquid membrane. MMO acts as a stable trap of Na+ and as a metastable site of Ag+ for movement.