A Chronoamperometric Method To Estimate Changes in the Membrane Composition of Ion-Selective Membranes

Abstract

A new chronoamperometric method is used to estimate changes in the membrane composition of mobile-site, ionophore-based membranes. The characteristic features of the chronoamperometric curves (initial current, slope, break time) of valinomycin-based, potassium-selective membranes loaded with potassium tetrakis(4-chlorophenyl)borate are correlated with the mobile-site and free ionophore concentration in the membrane. limiting cases for strong and negligible ion pair formation are distinguished. Replicate measurements indicate a relative standard deviation in the calculated values less than 10%. The practical applicability of the method was tested with membranes incorporated into conventional ion-selective electrode bodies or cast onto microfabricated planar sensor structures.

Influence of Incorporated Lipophilic Particles on Ion Fluxes Through Polymeric Ion‐Selective Membranes

“…More recently, chronoamperometry was also found to be useful for quantifying the concentration of ionophore and ionic sites [29,38]. With this method, a voltage step of, typically, 1 ± 10 V induces an initial plateau (fixed sites, [29]) or a slow linear decrease (mobile sites [38]) of the current followed by a breakdown related to the depletion of the excess free ionophore in the surface layer of the membrane. Beyond this breakpoint, the current is controlled by the back-diffusion of free ionophore within a steadily increasing diffusion layer.…”

Current response of ion-selective solvent polymeric membranes at controlled potential

“…1). In the case of plasticized polymer membranes used for conventional ISEs, the diffusion coefficient of primary ions in the membrane phase is lower than in aqueous solutions by at least two orders of magnitude [37][38][39][40]. Thus, the membrane resistance is high and the value of I o comparatively low.…”

Multispectral imaging of ion transport in neutral carrier‐based cation‐selective membranes

“…But, when the applied voltage falls into section B of the current-voltage curve, the current transients show a characteristic breakpoint (t 5 s) followed by a cottrelian decay of the current (Fig. 6, upper right) (36,37,40). The time instant of the breakpoint depends on the applied voltage (V 5 I init 3 R bulk ) as well as on the concentration (C free ionophore 5 C ionophore 2 C site ) and diffusion coefficient (D free ionophore ) of the free, uncomplexed ionophore.…”

“…The time instant of the breakpoint depends on the applied voltage (V 5 I init 3 R bulk ) as well as on the concentration (C free ionophore 5 C ionophore 2 C site ) and diffusion coefficient (D free ionophore ) of the free, uncomplexed ionophore. Equation (4) has been used for determining the diffusion coefficient of the free ionophores in ISE membranes and to assess the residual life time of ionophore-based sensors through the determination of the loss of ionophore and sites during extended use (36,37).…”

“…While the detection limit improvement requires extremely small compensation currents in the range of nAs, (33), systematic studies were also made for the fundamental understanding of membrane processes in the large current domain. The importance of this domain is underlined by recent progress in the chronoamperometric assessment (36,37) of the concentration of the active components (sites and ionophore) in ISMs. The longterm stability and the lifetime of ion sensors are both related to changes in the optimal membrane composition because of sample contact.…”