Interpretation of chronopotentiometric transients of ion-selective membranes with two transition times

Passing currents through ion-selective membranes has contributed to the development of a variety of novel methods. In this work, chronopotentiometric (CP) transients with two transition times (breakpoints) are presented for the first time, with the theoretical interpretation of such voltage transients. The validity of our theory has been confirmed in experiments utilizing ETH 5294 chromoionophore-based pH sensitive membranes with and without lipophilic background electrolyte and ETH 5234 ionophore-based calcium-selective membranes in which the ionophore forms 3:1 complexes with Ca2+ ions. The conditions under which two breakpoints can be identified in the chronopotentiometric voltage transients are discussed. Spectroelectrochemical microscopy (SpECM) is used to show that the two breakpoints in the CP curves emerge approximately when the free ionophore and ion–ionophore complex concentrations approach zero at the opposite membrane–solution interfaces. The two breakpoint times can be utilized to follow simultaneously the concentration changes of the free ionophore, the ion–ionophore complex, and the mobile anionic sites in cation-selective membranes. In membranes with known composition, the time instances where breakpoints occur can be used to estimate the free ionophore and the ion–ionophore complex diffusion coefficients.