Towards Reversibility of Ion Transfer Across the Interface between Valinomycin Membranes and Aqueous Electrolyte Solutions (original) (raw)
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Analytical Chemistry, 2009
Cyclic voltammetry is used to investigate the transfer of alkali-metal cations, protons, and ammonium ions facilitated by the complex formation with valinomycin at the interface between an aqueous electrolyte solution and a room-temperature ionic liquid (RTIL) membrane. The membrane is made of a thin (∼112 µm) microporous filter impregnated with an RTIL that is composed of tridodecylmethylammonium cations and tetrakis[3,5-bis-(trifluoromethyl)phenyl]borate anions. An extension of the existing theory of voltammetry of ion transfer across polarized liquid membranes makes it possible to evaluate the standard ion-transfer potentials for the hydrophilic cations studied, as well as the stability constants (K i ) of their 1:1 complexes with valinomycin, as log K i ) 9.0 (H + ), 11.1 (Li + ), 12.8 (Na + ), 17.2 (K + ), 15.7 (Rb + ), 15.1 (Cs + ), and 14.7 (NH 4 + ). These data point to the remarkably enhanced stability of the valinomycin complexes within RTIL, and to the enhanced selectivity of valinomycin for K + over all other univalent ions studied, compared to the conventional K + ion-selective liquid-membrane electrodes. Selective complex formation allows one to resolve voltammetric responses of K + and Na + in the presence of an excess of Mg 2+ or Ca 2+ , which is demonstrated by determination of K + and Na + in the table and tap water samples.
Russian Journal of Electrochemistry, 2019
Bulk resistance and other electrochemical properties of membranes of K +-selective electrodes (ISEs) containing valinomycin are measured by means of chronopotentiometry and electrochemical impedance. It is shown that the bulk resistance of the membranes, within the Nernstian potentiometric response range, increases along decrease of KCl concentration in solution. Analogous results were reported earlier for Ca 2+ and NO ISEs. This non-constancy of the bulk resistance is in conflict with current views on the mechanism of ISEs response. Tentatively, this paradox is ascribed to heterogeneity of membranes due to water uptake from solution.
Electroanalysis, 2001
The potentiometric response and the electrochemical impedance spectra of thin PVC based membranes selective to Ca 2 and to Na were studied before and after ®rst contact with respective primary ions. The membranes included potassium tetrakis(p-Cl-phenyl) borate, thus being initially in potassium form. Nearly Nernstian potentiometric responses were obtained in interfering electrolytes before the contact with primary ions, while after the ®rst contact the slope of potentiometric curves decreased drastically. However, the impedance spectra were similar regardless of prior conditioning of the membranes, and no evidence for a hindrance towards charge transfer processes was observed. The results suggest that the membraneasolution interface reactions are reversible both for the primary and the interfering ions, and the contamination of the solutions with primary ions is the only reason for the lack of Nernstian response in interfering electrolytes.
Analytical Chemistry, 2004
The goal was to identify formulations for use in valinomycin K + ion-selective electrodes that could routinely achieve a detection limit of <10-6 M, even after repeated use and exposure at higher K + activity (0.1 M) and without the requirement for special pretreatment or conditioning in low K + activity (10-3 M). Electrodes that would be characterized by high potential stability were sought in this work. Valinomycin-containing membranes with diffusion coefficient of ∼10-11 cm 2 s-1 , formulated from methacrylic/acrylic polymers with or without plasticizer, were compared with plasticized PVC membranes (diffusion coefficient 10-8 cm 2 s-1). The methacrylic/ acrylic membranes without plasticizer were shown to give an order of magnitude lower detection limit, when compared with PVC-dioctyl sebacate and o-nitrophenyl octyl ether plasticized methacrylic/acrylic polymers under the same conditions, highlighting the influence of plasticizer on the detection limit. As predicted from current theoretical derivation, the inner contacting layer in the ionselective electrode construction was shown to be highly influential in maintaining the detection limit below 10-6 M with use and with poly(pyrrole) providing the inner contact ion-to-electron transduction function, a further order of magnitude improvement in the lower detection limit could be maintained for both chloride and hexacyanoferrate doped poly(pyrrole), when 2% ionophore was employed in the ion-selective membrane. This formulation showed extraordinary stability and reproducibility in terms of measurement range and drift over extended measurement testing, with close to Nernstian slopes. At higher ionophore concentrations (4%), the apparent selectivity of the electrode was improved at the expense of detection limit and the nature of the poly(pyrrole) dopant ion became important in determining the dominant exchange processes at the poly(pyrrole)/ion-selective membrane interface. Carrier-based ion-selective electrodes (ISEs) have long been in routine analytical use, yet they still attract an important focus for further research, e.g., 1-9 trying to push their capabilities into new dimensions. In the normal construction, the polymeric/ ionophore membrane interfaces with the sample and, on the backside, with an aqueous electrolyte in which an internal reference electrode is placed. 4,8 At low sample activities, zero current ion fluxes across the membrane due to extraction processes at the membrane interfaces can cause analyte ion activity in the phase boundary to be significantly higher than in the sample bulk. 10-20 This influences the measured potential, so that consideration of these processes and their management has become a prime objective to attain lower detection limits and optimum selectivity.
Analytical Chemistry, 2012
It is well known that the selectivity of an ion-selective electrode (ISE) depends on the stoichiometry of the complexes between its ionophore and the target and interfering ions. It is all the more surprising that the possibility for the simultaneous occurrence of multiple target ion complexes with different complex stoichiometries was mostly ignored in the past. Here we report on the simultaneous formation of 1:1 and 1:2 complexes of a fluorophilic crown ether in fluorous ISE membranes, and how this results in what look like super-Nernstian responses. These increased response slopes are not caused by mass transfer limitations and can be readily explained with a phase boundary model, a finding that is supported by experimentally determined complex formation constants and excellent fits of response curves. Not only Cs + but also the smaller ions Li + , Na + , K + , and NH 4 + form 1:1 and 1:2 complexes with the fluorophilic crown ether, with cumulative formation constants of up to 10 15.0 and 10 21.0 for of the 1:1 and 1:2 complexes, respectively. Super-Nernstian responses of the type observed with these electrodes are probably not particularly rare, but lacking in the past an adequate discussion in the literature remained ignored or misinterpreted. Preliminary calculations also predict sub-Nernstian responses and potential dips of a similar origin. The proper understanding of such phenomena will facilitate the development of new ISEs based on ionophores that form complexes of higher stoichiometries. buhlmann@umn.edu. Supporting Information Available Figures showing potentiometric K + responses in presence of a background of Li + , Na + , or NH 4 +. Illustration of effect of K IL and K JL on response curves of membranes with 71 mol % ionic sites. Derivation of the expression for the width of super-Nernstian response ranges. Illustration of the site-to-ionophore ratio on the EMF slope in the super-Nernstian response range. Examples of complex stabilities that result in a sub-Nernstian response slope and potential dip. This material is available free of charge via the Internet at http://pubs.acs.org.
Electroanalysis, 1993
Ion pairing and acid dissociation constants, as well as limiting conductances, have been estimated in ion-selective electrode membranes based on a 33 wt% poly(viny1 chloride) (PVC), 66 wt% dioctyladipate matrix using the Fuoss equation of conductance for weakly dissociable species. Dissociation constants of 1.66, 3.51, and 0.69 X lo-' were determined for NaC104, C6H,COOK (KBz), and benzoic acid (HBz), respectively, in a membrane matrix with 1% valinomycin and 0.01% KB(C6H,), present as additives. The first two values are consistent with tight ion pairing in a low dielectric solvent and indicate no special role for PVC in ion solvation. The weak acid dissociation is 4 to 5 orders of magnitude higher than expected; HBz also shows unusual interactions with the membrane matrix at low added concentrations. The effect of HBz and methylbenzoate (MeBz) on surface charge transfer resistance and membrane bulk resistance shows that HBz is dissociating to form ions, and a possible mechanism for the unusual interactions has been deduced. MeBz caused a significant increase in surface charge transfer resistance, indicating a mechanism by which membrane permeation by lipophilic, neutral components of biological samples could degrade sensor performance.
Desalination and Water Treatment, 2013
In this study, we compare the current-voltage (I-V) curves of the same counter-ion with and without buffer coions. We compare the polarization curves of the counter-ion K + without and with borate coions which results from boric acid dissociation. The boric acid was added progressively from 0.005 to 0.1 N. The role of the mixture (borate coions/boric acid) is to buffer the solution at pH = 9.23, the protons, whatever their origin, do not contribute to the over-limiting current transport. We have a later onset of the over-limiting current and an increase in the plateau length of the membrane's polarization curves. On the other hand, the addition of boric acid to a potassium sulfate solution does not affect the limiting current density, also the plateau length decreases and its slop increase with increasing boric acid concentration, this unexpected results helps us to better understand the classical electrochemical concentration polarization phenomena in electrodialysis process.
Ionic exchange and selectivity of NASICON sensitive membranes
Sensors and Actuators B: Chemical, 1992
The complex impedance spectra of NASICON Na+ ion-sensitive membrane contacted with solutions of different concentrations of the primary ion and of the main interfering ions (Li+, K+, Ca2+ and H30+) were measured over the range 65 kHz-0.01 Hz. The impedance data can be correlated to the potentiometric selectivity coefficients determined by the separate solution method for 0.1 M solution.
Studies of ion transfer across liquid membranes by electrochemical techniques
Annual Reports Section "C" (Physical Chemistry), 2012
The fundamentals and recent advances in ion transfer across the interface between two immiscible electrolyte solutions (ITIES) are reviewed. The different strategies developed to overcome the limitations of the traditional experimental studies with ITIES and to broaden its scope of applications are discussed. Special attention is given to studies of ion transfer through liquid membranes which contain two ITIES, one or both of which can be polarized. Theoretical and experimental studies on the application of different galvanostatic and potentiostatic electrochemical techniques to the study of such systems are described, emphasizing their unique characteristics. The article also includes sections devoted to facilitated ion transfer, liquid/liquid micro-interfaces and the use of weakly supported media. Highlights Recent theoretical and experimental studies on the implementation of different electrochemical techniques for ion transfer processes across liquid membranes with different configurations have allowed in-depth characterization of these processes. These studies have paved the way for new practical applications.
The Journal of Physical Chemistry B, 1999
A theoretical description of the steady-state potential response of ionophore-based ion-selective electrodes is presented that, so far, is the most general formalism available. The treatment considers membrane systems with any number of ionophores, differently charged cations, anions, and fixed or stationary ionic sites. The theory accounts for thermodynamically controlled selectivity characteristics, as well as for various diffusioninduced effects resulting from transmembrane ion fluxes at zero current. The phenomena discussed in detail include apparent super-and sub-Nernstian responses and detection limits. An extension of the treatment for time-dependent phenomena is also given. The present approach can be applied for optimizing the selectivity coefficients and improving the detection limits of ionophore-based ion-selective electrodes.