Selectivity Coefficients Of Ion-selective Electrodes Research Articles

Evaluation of Egorov's improved separate solution method for determination of low selectivity coefficients by numerical simulation.

The group of Egorov has recently proposed an elegant method to determine unbiased selectivity coefficients of ion-selective electrodes (Egorov et al. Anal. Chem. 2014, 86, 3693). Once the electrode is exposed to a solution containing only interfering ions, the time-dependent experimental selectivity coefficients are plotted as a function of the inverse fourth root of time and extrapolated to zero. The principal assumption of the approach is the progression of the diffusion layer in the membrane phase with square root of time. This letter critically evaluates the usefulness of this methodology by finite element analysis. The results suggest that the improvement of observed selectivity values are highly significant for an initially uniform distribution of primary ions across the membrane, strongly supporting the methodology. When strong inward ion fluxes of primary ions are present instead, a modification of the method by extrapolation of logarithmic selectivity coefficients appears to give the best results.

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.

Influence of the nature of liquid anion exchanger on the selectivity of anion selective electrodes

Selectivity coefficients of ion-selective electrodes on the basis of liquid anion exchangers, a quaternary phosphonium salt, and 20 higher quaternary ammonium and other nitrogen-containing salts differing in the length of hydrocarbon radicals, the nature of the quaternary nitrogen atom, and the presence of polar groups near to the exchange center with respect to 17 mineral, organic, and metal complex anions are presented. The effect of the nature of the anion-exchanger on the anion selectivity is discussed.

On the potentiometric investigation of the thermodynamic properties of mixed electrolyte systems: a new mixed solution method for the determination of potentiometric selectivity coefficients of ion-selective electrodes

With the aim of extending the use of ion-selective electrodes (ISEs) for the thermodynamic investigation of concentrated mixed electrolyte systems based on potentiometry, a mixed solution method (MSM) is reported for the determination of potentiometric selectivity coefficients () of ISEs. In this new method, the measurements are performed simultaneously in the presence of all relevant interfering ions in mixed electrolyte systems. As a result, the determination of is carried out in a realistic matrix environment with composition and ionic strength close to that of real mixed electrolyte samples and under experimental conditions where the ISE reveals its real practical mixed ion response behaviour.

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.

The ion adsorption effect on selectivity of liquid state, O, O′-didecylodithiophosphate chelate based ion-selective electrodes

Ion-selective electrodes with liquid membranes including O, O′-didecylo-dithiophosphate complexes of Tl(I), Pb(II), Cd(II) and Ni(II) are characterised and results of the study on their selectivity are reported. A short review of problems related to determination and interpretation of selectivity coefficients of ion-selective electrodes is presented with particular emphasis on the drawbacks of the hitherto used methods. A new method is proposed, which in the experimental part is close to that of mixed solutions recommended by IUPAC but can be applied also when the latter is of no use. The method proposed for determination of selectivity coefficients simultaneously allows concluding about the mechanism of potential generation. A few examples of relations between selectivity coefficients of the electrodes and concentrations of disturbing ions in solutions, are given. An interpretation of the above relations as results of the processes of ion adsorption at the interface of the electrode membrane and water solution is proposed. The results obtained have confirmed the hypothesis given by Pungor, according to which the main role in the mechanism of generation of ion-selective electrodes potential is played by the processes of ion chemisorption at the interface of the membrane and water solution.

Selectivity problems of membrane ion-selective electrodes

A brief review of problems related to the determination of selectivity coefficients of ion-selective electrodes and their interpretation is given, along with a short critical description of the so far applied methods of their determination. A new method is proposed, the experimental part is similar to the mixed solution method recommended by IUPAC but avoids its drawbacks and can be used in cases when the latter cannot be applied. The method proposed permits the determination of the electrode selectivity as a function dependent on the activity of the ions studied, permitting conclusions on the mechanisms of the potential formation and selectivity. The performance of the method is illustrated for a nitrate electrode. Significant dependencies of the selectivity coefficients on concentration of the disturbing ions were interpreted as a result of the process of ion adsorption at the interface between the electrode membrane and the solution studied. The experimental evidence supports the conclusions from the studies of Pungor et al., who claimed that processes at this interface play the main role in the mechanism of generation of the potential of ion-selective electrodes.

Determination of ion-selective electrode characteristics by non-linear curve fitting

A simple practical method of determining potentiometric selectivity coefficients of ion-selective electrodes (ISEs) is described in which electrode characteristics (slope, potentiometric selectivity coefficients and cell constant) can be determined by fitting the experimental data obtained using the fixed interference (FI) method to an appropriate model by non-linear least-squares regression. The proposed method is simple to implement practically, and data processing can be easily achieved through use of the optimisation add-on, Solver, bundled with Microsoft Excel. The flexibility of the method is demonstrated by modelling the response of a valinomycin potassium-selective electrode with the Nikolskii–Eisenman equation and a recently proposed alternative to the Nikolskii–Eisenman equation for cases where the ionic charges on the primary and interfering ions are unequal.

A computer program for evaluation and graphical representation of the potentiometric selectivity coefficients of ion-selective electrodes

A QuickBasic (Microsoft) program for the Apple Macintosh system has been written to facilitate the evaluation and graphical representation of the potentiometric selectivity coefficients ( K i, j pot) of ion-selective electrodes (ISEs). The program also offers the option for communication with Worksheet and Graphics Software, which is useful for data input and further graph editing.

Selectivity coefficients for ion-selective electrodes: Recommended methods for reporting KA,Bpot values (Technical Report)

Abstract

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.

Dissociation and complexation constants of some β-diketone ionophores

The dissociation constants of dibenzoylmethane (DBM), 1-phenyl-3-(p-chlorophenyl)-1,3-propanedion (ETH 245) and 1,13-bis-[4′-(3″-phenyl-1″,3″-dioxopropyl)-phenyl]-tridecan (ETH 1224) were potentiometrically evaluated in 80% dioxane medium at 30 °C. The stability constants of DBM complexes with calcium, magnesium and barium as well as those of ETH 245 with magnesium were determined and correlated with the selectivity coefficients of ion-selective electrodes containing DBM as ionophore.

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.

Automated determination of detection limits and selectivity coefficients of ion-selective electrodes by using a microcomputer-controlled potentiometric system

The detection limit and the potentiometric selectivity coefficients of ion-selective electrodes are determined automatically with a microcomputer-controlled potentiometric system. Measurements of these parameters for three commercially available electrodes of the liquid membrane type (chloride, nitrate and calcium electrodes) gave results in good agreement with those reported in the literature. The non-linear least-squares fit evaluation of data (potential activities) and the selection of the appropriate transfer functions are described. The reproducibility of the results is discussed.

Determination of selectivity coefficients of ion-selective electrodes by means of linearized multiple standard addition techniques

The application of multiple known-addition or dilution techniques to the evaluation of potentiometric selectivity coefficients of ion-selective electrodes is proposed. The validity of the common form of the Nikolskii—Eisenman equation is assumed and experimental data are processed by using Gran-type linear functions. Six possible methods are derived which make it possible to work under bi-ionic conditions for the electrode response. Optimization of the experimental procedures is discussed.