Kinetic considerations of ion selectivity coefficients of ion-selective electrodes based on neutral carriers

Ion selectivity coefficients of ion-selective electrodes based on neutral carriers are described by means of a mixed potential model of ion transport reactions at the aqueous solution/ion-sensitive membrane interface. The decrease in ion selectivity can be explained by the deviations from the equilibrium conditions, which arise from the ionic partial current across the interface, but the proposed correspondence of the exchange current density of ion transfer reactions with the ion selectivity coefficients is rationalized only for certain conditions of the kinetic parameters. The ion selectivity for liquid membrane transport is discussed starting from three different rate-determining steps. It is shown that the potentiometric selectivities of ion-selective electrodes and the transport selectivities are correlated when the ionic transfer across the aqueous solution/ membrane interface is fast compared with the complex ion transport through the membrane. The significance of a kinetic approach for the design of neutral carriers for ion-selective electrodes is stressed.     

Studies on the matched potential method for determining the selectivity coefficients of ion-selective electrodes based on neutral ionophores: experimental and theoretical verification.

A theory is presented that describes the matched potential method (MPM) for the determination of the potentiometric selectivity coefficients (KA,Bpot) of ion-selective electrodes for two ions with any charge. This MPM theory is based on electrical diffuse layers on both the membrane and the aqueous side of the interface, and is therefore independent of the Nicolsky-Eisenman equation. Instead, the Poisson equation is used and a Boltzmann distribution is assumed with respect to all charged species, including primary, interfering and background electrolyte ions located at the diffuse double layers. In this model, the MPM-selectivity coefficients of ions with equal charge (ZA = ZB) are expressed as the ratio of the concentrations of the primary and interfering ions in aqueous solutions at which the same amounts of the primary and interfering ions permselectively extracted into the membrane surface. For ions with unequal charge (ZA not equal to ZB), the selectivity coefficients are expressed as a function not only of the amounts of the primary and interfering ions permeated into the membrane surface, but also of the primary ion concentration in the initial reference solution and the delta EMF value. Using the measured complexation stability constants and single ion distribution coefficients for the relevant systems, the corresponding MPM selectivity coefficients can be calculated from the developed MPM theory. It was found that this MPM theory is capable of accurately and precisely predicting the MPM selectivity coefficients for a series of ion-selective electrodes (ISEs) with representative ionophore systems, which are generally in complete agreement with independently determined MPM selectivity values from the potentiometric measurements. These results also conclude that the assumption for the Boltzmann distribution was in fact valid in the theory. The recent critical papers on MPM have pointed out that because the MPM selectivity coefficients are highly concentration dependent, the determined selectivity should be used not as "coefficient", but as "factor". Contrary to such a criticism, it was shown theoretically and experimentally that the values of the MPM selectivity coefficient for ions with equal charge (ZA = ZB) never vary with the primary and interfering ion concentrations in the sample solutions even when non-Nernstian responses are observed. This paper is the first comprehensive demonstration of an electrostatics-based theory for the MPM and should be of great value theoretically and experimentally for the audience of the fundamental and applied ISE researchers.     

Determination of a polymer surfactant by potentiometry with an ion-selective electrode using inorganic or organic anions as counterions of the electrode-active compound

Ion-selective electrodes for the determination of a cationic polymer surfactant with membranes containing ion pairs of polysulfonylpiperidinylmethylene hydroxide (PSPMH) with inorganic complex anions or organic counterions of the azo dye series as the electrode-active compound are described. Electrochemical characteristics and analytical potentialities of film ion-selective electrodes based on ion pairs PSPMH-potassium tetraiodomercurate, PSPMH-Magneson IREA, and PSPMH-Stilbazo R are compared. The influence of the concentration of the electrode-active compound in the membrane, the pH and ionic strength of the PSPMH solution, the lifetime of the membrane, and the number of measurements on the characteristics of ion-selective electrodes is studied. A procedure is developed for the potentiometric determination of PSPMH in aqueous solutions using proposed ion-selective electrodes. Presented at the V All-Russian Conference with the Participation of CIS Countries on Electrochemical Methods of Analysis (EMA-99), Moscow, December 6–8, 1999.     

Determination of selectivity coefficients of ion-selective electrodes by a matched-potential method

A new method is proposed for the determination of selectivity coefficients of ion-selective electrodes. A reference solution of the primary ion is used and potential changes are measured after adding either the primary ion or secondary ion. Increasing concentrations of the secondary ion are added to provide the same potential change as obtained for a fixed added concentration of the primary ion. The ratio of the primary-to-secondary ion concentrations for this potential change represents the selectivity coefficient. This method is illustrated by the use of a sodium glass electrode and a potassium valinomycin electrode. Results obtained are compared with those from conventional determinations of selectivity coefficients. The advantages of this method are discussed.     

Potentiometric selectivity coefficients: Problems of experimental determination

The main problems arising in the determination of selectivity coefficients of ion-selective electrodes using the methods recommended by IUPAC are considered. A method for the determination of the limiting (thermodynamically justified) selectivity coefficients based on the analysis of the time dependence of the selectivity coefficients found experimentally using the separate solutions method is justified theoretically and experimentally. It has been demonstrated that the proposed method ensures the reliable determination of low values of selectivity coefficients (as low as n × 10^?6). The criteria of the applicability of the proposed method are formulated and a particular algorithm of its implementation is proposed.     

Highly selective PVC-membrane electrodes based on Co(II)-Salen for determination of nitrite ion.

A cobalt(II) derivative was used as a suitable ionophore for the preparation of a polymeric membrane nitrite-selective electrode. The electrode reveals a Nemstian behavior over a very wide NO2- ion concentration range (1.0 x 10(-6)-1.0 x 10(-1) M) and a very low detection limit (5.0 x 10(-7) M). The potentiometric response is independent of the pH of solution in the pH range 4.0-9.5. The electrode shows advantages such as low resistance, fast response and, most importantly, good selectivity relative to a wide variety of inorganic and organic anions. In fact, the selectivity behavior of the proposed NO2- ion-selective electrode shows great improvements compared to the previously reported electrodes for nitrite ion. The proposed electrodes could be used for at least 2 months without any significant changes in potentials. The electrode was successfully applied to the determination of nitrate ion concentrations in sausage and milk samples.     

Potentiometric behavior of ion-selective electrodes to large cationic species modulated by decyl alcohol

The effect of 1-decanol on the potentiometric response of three ion-selective electrodes to large cationic species is analyzed. The electrodes were constructed with plasticized PVC membranes. The results suggest that 1-decanol alters the ionic transport through the membrane/water interface to an extent that depends on the strength of the active ion pair. The water solubility of the cation, its molecular weight, and the size of the ion pair seem to be relevant factors in this type of behavior. The potentiometric selectivity coefficients are also dependent on the presence of 1-decanol in the membrane. These results are similar to those already described in ion-selective electrodes with membranes capable of sensing anionic benzene sulfonate-type systems. Thus, the effect of the alcohol appears to be general by affecting mainly the membrane surface polarity.     

Unbiased Selectivity Coefficients of Potentiometric Sensors Using Thin Membrane Layers

Solid-contact ion-selective electrodes with thin membranes can more easily achieve complete ion exchange at the sample-membrane interface, giving unbiased selectivity coefficients. The calculated reconditioning rate corresponds well to experimental data. Alternatively, the redox state of the conducting polymer may be electrochemically switched resulting in primary ion expulsion from the membrane and interfering ion uptake from the sample. Both approaches were tested for tetrabutylammonium-selective electrodes. The approach's limitations are identified, which includes primary ions impurities in interfering ion salts and an interaction between primary ion and redox polymer. These limitations are visualized with ionophore-based sodium- and silver-selective electrodes.     

Voltammetric determination of perchlorate ion at a liquid–liquid microscopic interface

A method is proposed for the electrochemical determination of perchlorate ion by voltammetry at the interface between two immiscible phases (water–o-nitrophenyl octyl ether). A demountable original-design amperometric ion-selective electrode based on a laser-microperforated polymeric membrane was fabricated for voltammetric measurements. The conditions of analytical signal recording in the determination of ClO₄ ^? were determined. The effect of interfering ions was assessed and amperometric selectivity coefficients were calculated. The accuracy of the procedure was verified by the added–found method. The developed electrode was applied to the determination of perchlorate in natural and drinking waters.     

Ion-selective liquid electrodes: Problems of description and experimental determination of selectivity

Current state of selectivity theory for liquid-membrane ion-selective electrodes was considered. Analytical expressions for the selectivity coefficients of electrodes based on liquid ion exchangers, neutral carriers, and charged carriers were obtained using phase boundary potential model. The reasons responsible for the dependence of experimentally determined selectivity coefficients on the determination conditions are discussed. The main ways for the electrode design optimization providing dramatic improvement of the selectivity are considered.     

Complexation of aminoglycoside antibiotics with metal cations as a derivatization reaction: Determination of gentamicin by equilibrium electrochemical and spectrophotometric methods

The complexation of aminoglycoside antibiotics with metal cations was proposed as a derivatization method for the further determination of the complex obtained by potentiometry with ion-selective electrodes (ISE), voltammetry at the interface between two immiscible electrolyte solutions (ITIES), and spectrophotometry. It was shown by the spectrophotometric method that gentamicin formed a 1: 1 complex with copper(II). For the potentiometric determination of gentamicin, we obtained ionophores that were ion associates formed by the gentamicin complex of copper(II) and tetraphenylborate derivatives as counterions. The transfer of the gentamicin complex of copper(II) was studied voltammetrically at the ITIES. The results obtained indicate that l antibiotic gentamicin can be directly determined as a complex with copper(II) by potentiometric, voltammetric, and spectrophotometric methods.     

The Synthesis of Styrene/4'-vinyl-benzo-24-crown-8 Copolymer and Its Use in Potassium-Ion Selective Electrodes

A synthetic procedure has been developed for the preparation of styrene/4'-vinyl-benzo-24-crown-8 copolymer (SPV-B24C8). The resulting copolymer was used as a membrane carrier to construct a K+ ion-selective electrode. The electrode exhibits a Nernstian response for K+ ions over a wide concentration range. The nature of the plasticizer, the additive, the concentration of internal solutions in the electrodes and the composition of the membrane were investigated. The performance of the electrode in terms of electrode reproducibility, response stability and regeneration was also studied. The selectivity coefficients of the electrode for potential interferents including alkali, alkaline earth, some transition and heavy metal ions were found to be in the order of 10-3 or less. The electrode was successfully used for the determination of potassium ion in blood serum.     

Side effects in measurements of selectivity coefficients of solid state ion selective electrodes

The influence of various factors such as solubility, the oxidation of the membrane, and the contamination of the solution, on the experimental values of the selectivity coefficients of solid state sulphide ion selective electrodes is discussed. A new method for the evaluation of very small selectivity coefficients, based on the addition of reagents forming complexes or insoluble salts with the main ion, is proposed. By means of this method, selectivity coefficients for silver, copper, cadmium and lead ion selective electrodes have been determined, which are in far better agreement with thermodynamic values than those described in the literature.     

The calibration and response of ion-selective electrodes at low concentrations of primary ions

Electrolytic generation of ions is proposed for the preparation of standard solutions for the calibration of iodide and silver ion-selective electrodes in the concentration range 10^-4–10^-7 M. The responses of these electrodes and also the copper(II) ion-selective electrode were examined in various electrolyte solutions. The current efficiencies of the electrolytic generation of the iodide ions into the various solutions were measured coulometrically. The advantages of this newly proposed calibration technique are discussed.     

Validation criteria for developing ion-selective membrane electrodes for analysis of pharmaceuticals

 The problem of validation criteria for developing ion-selective membrane electrodes for the analysis of pharmaceuticals arises from the connection between the reliability of ion-selective membrane electrodes construction and the reliability of the analytical information. Liquid membrane selective electrodes are more suitable for validation than the solid variety. The influence of the stability of ion pair complexes from the membrane on various parameters (e.g. response, limit of detection, and selectivity) is discussed. Validation criteria are proposed. Received: 18 September 1997 · Accepted: 17 November 1997     

Copper ion-selective chalcogenide glass electrodes: Analytical characteristics and sensing mechanism

New copper ion-selective electrodes based on chalcogenide glasses, Cu_xAg_25?xAs_37.5Se_37.5, display high copper(II) ion sensitivity with Nernstian response in the range pCu 1–6, short response time, high selectivity, potential stability and reproducibility. These electrodes are 10–30 times more sensitive in strongly acidic media than crystalline copper ion-selective sensors and are superior to the copper(I) selenide electrode in selectivity and resistance to acids and oxidation. A model is proposed to explain the ion sensitivity of these chalcogenide glass sensors. The sensitivity depends on direct exchange of copper(II) ions between solution and the modified surface layer of the glass. The modified surface layer is formed as a result of partial destruction of the glass network on soaking in solution; its atomic density is 2.0–2.5 times less than that of the original glass. The structural defects and hollows make fast copper(II) ion migration within the modified surface layer possible. Exchange sites in this layer can be formed by both disproportionation and oxidation of copper(I) in the glass network, as well as by diffusion of copper(II) ion from solution in the case of glasses with low copper content. Experimental confirmation of this model is provided by x-ray, photo-electron and scanning Auger electron spectroscopy.     

An Ion-Selective Electrode for Ethylenediaminetetraacetatobismuthate(III) Anions

The main characteristics of liquid-membrane ion-selective electrodes (ISEs) based on high-molecular quaternary ammonium salts (QASs) were studied. The electrodes are reversible to the ethylenediaminetetraacetatobismuthate(III) anion. It was found that the selectivity of the ISEs to the potential-determining ion depends on the symmetry of the QAS. A mechanism of the ISE membrane response was proposed.     

Neutral-carrier-type potassium ion-selective electrodes based on polymer-supported liquid-crystal membranes for practical use.

Polymer-supported liquid-crystal membranes have been designed for neutral-carrier-type potassium ion-selective electrodes, aiming for practical applications of high-performance liquid-crystalline membrane ion sensors. Two types of polymer-supported liquid-crystal membranes were tested for their usefulness; one is microporous poly(tetra fluoroethylene) (PTFE) membranes impregnated by thermotropic liquid-crystalline compounds, and another is poly(methyl methacrylate) (PMMA) membrane dispersing the same liquid-crystalline compounds. Both of the polymer-supported liquid-crystal membranes containing a liquid-crystalline benzo-15-crown-5 neutral carrier as well as a lipophilic anion excluder work well as ion-sensing membranes for potassium ion-selective electrodes, the ion selectivities of which can be switched by the measurement temperatures. Specifically, PTFE-impregnated liquid-crystal membranes are better than the PMMA-dispersed ones in the sensitivity and selectivity of the resulting ion electrodes. A potassium ion assay in blood sera has proved that neutral-carrier-type ion-selective electrodes based on the polymer-supported liquid-crystal membranes are reliable for practical uses.     

Novel PVC-based copper(II) membrane sensor based on 2-(1'-(4'-(1'-hydroxy-2'-naphthyl)methyleneamino)butyl iminomethyl)-1-naphthol.

A copper(II) ion-selective PVC membrane sensor based on 2-(1'-(4'-(1'-hydroxy-2'-naphthyl)methyleneamino)butyl iminomethyl)-1-naphthol (BHNB) as a novel Schiff base containing a sensing material has been successfully developed. The sensor exhibits a good linear response of 29 mV per decade within the concentration range of 10(-1)-10(-6) M of Cu2+. The sensor shows good selectivity for copper(II) ion in comparison with alkali, alkaline earth, transition and heavy metal ions. The BHNB-based sensor is suitable for use with aqueous solutions of pH 3.5-7.0 and displays minimal interference by Sr(II), Cd(II), Hg(II), Zn(II) and Pb(II), which are known to interfere with other previously suggested electrodes. The proposed membrane electrode was used as a sensor for determining the Cu(II) content in black tea samples. It was also applied as an indicator electrode in the potentiometric titration of Cu2+ ions with EDTA.