Assessment of Ion‐Ionophore Complex Diffusion Coefficients in Solvent Polymeric Membranes

Abstract

Chronoamperometric (CA) and chronopotentiometric (CP) transients have been used previously for the determination of the diffusion coefficients of free ionophores in ion-selective liquid membranes. In this work, the CA and CP transients are utilized to determine the diffusion coefficients of a variety of ion-ionophore complexes in these membranes. The conditions under which the ion-ionophore complex diffusion coefficients can be determined from the CA and CP transients are discussed. For most ionophores, the ion-ionophore complex diffusion coefficients were found to be slightly smaller or similar to the previously calculated free ionophore diffusion coefficients. However, for a couple of ionophores, the ion-ionophore complex diffusion coefficients were found to be significantly smaller than expected. Significant ion-pair formation in the membranes can explain this disparity.

Chronopotentiometry of pure electrolytes with anion-exchange donnan exclusion membranes

“…If this applied current is higher than the steady-state diffusion current [13], a detectable potential change at a transition time s signals the localized depletion of the transported ion at the membrane surface. With ion-selective membranes, this technique had been chiefly employed in fundamental studies to estimate ion or ionophore diffusion coefficients, either in the solution or membrane phase [14,15]. More recently, attractive analytical applications have emerged since the square root of transition time follows a linear relationship with the bulk concentration of the ion of interest.…”

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

“…Although ion-selective membranes are mainly used for zero-current equilibrium measurements, a variety of non-equilibrium techniques have been devised to probe membrane characteristics or adjust membrane responses. For example, from the transition time (the instance of the breakpoint or breakpoint time) that appears in chronoamperometric (CA) [1–4] and chronopotentiometric (CP) [5–7] transients, the concentration or the diffusion coefficient of the free ionophore inside the membrane has been determined.…”

“…A breakpoint emerges in the CA and CP transients at a transition time when the free ionophore concentration approaches zero at the membrane boundary. It has been shown [1–3, 5–7] that the transition time is proportional to the diffusion coefficient and the square of the free ionophore concentration so that it can be used to determine either of these parameters from Eq. 1.…”

“…In membranes with constant composition, the CA method has been used to determine the diffusion coefficients of a variety of ionophores [1, 3]. Recently, it was shown that chronopotentiometry can be used in the same way, with substantial theoretical and experimental advantages [5–7]. The theoretical advantage is related to the validity of the constant current assumption in the chronoamperometric theory.…”

“…The theoretical description of the CP transient has revealed that for some membranes the ion-ionophore complex boundary concentration can approach zero on the negative side of the membrane before the free ionophore concentration approaches zero concentration on the positive side [5–7]. For these membranes, the breakpoint in the transient is related to the ion-ionophore concentration in the membrane, so that the ion-ionophore complex concentrations or the diffusion coefficient of the ion-ionophore complex or anion can be determined from the transition time.…”