Variability of selectivity coefficients of solid-state ion-selective electrodes

The generalized model for the selectivity mechanism of solid-state ion-selective electrodes has been experimentally verified. The experimental parameters investigated were the concentration of interfering ion, temperature and stirring. Among the systems studied were electrodes sensitive to chloride (bromide, iodide), bromide (chloride, iodide), iodide (chloride, bromide), silver (copper, lead), copper (silver, lead) and lead (silver, copper), the species given in brackets being considered as the interferents. The model has been confirmed except for cases where the concentration of ions formed at the electrode surface by metathesis is too small to be the factor that dictates the electrode potential.     

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.     

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.     

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.     

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.     

Response and selectivity characteristics of alkylammonium ion-selective electrodes

A series of poly(vinyl chloride) matrix coated-wire electrodes based on dinonylnaph-thalene sulfonic acid (DNNS) selective to various alkylammonium ions was prepared and selectivity characteristics were evaluated. A set of tributylammonium electrodes was used to study the interference by a homologous series of alkylammonium ions. Selectivity coefficient (k^pot_i,j) values for alkylammonium ions at a given carbon number decreased in the order primary, secondary, tertiary, quaternary. A linear increase in log k^pot_i,j with carbon number was observed in each series. Two additional sets of electrodes selective to dodecylammonium ion and to di-n-hexylammonium ion were evaluated for response to other C-12 amines. Electrodes for primary amines showed little or no response to secondary and tertiary amines, while electrodes for tertiary amines responded strongly to primary amines and less strongly to secondary amines. The effect of structural modifications was evaluated in a study of N-substituted cydohexylamines. The dependence of the magnitude of k^pot_i,j on the concentration of the interference was determined for both strongly and weakly interfering species. For strong interferences, k^pot_i,j was observed to increase to a maximum value of characteristic concentration. For weak interferences, the k^pot_i,j variation with concentration was markedly less. Guides for estimating the response to an interfering species based on its structure and molecular weight were obtained. The results are compared to analogous solvent-extraction systems. The significance of these results is discussed in terms of quantifying different components in binary mixtures using several electrodes simultaneously.     

Release of copper from commercial solid-state copper ion-selective electrodes

The release of copper from two commercial solid-state cupric ion-selective electrodes [Orion 94-29 Cupric Electrode (I) and Radiometer F1112 Selectrode (II)] was measured by immersion in the following media: 0.1M potassium nitrate (pH = 4.7), 0.5M sodium chloride (pH = 4.7) and 0.1M nitric acid. In the 0.1M potassium nitrate medium, the amount of copper released from both electrodes causes interference when they are used for the determination of cupric ion at the 10⁻⁷M level. In comparison with the 0.1M potassium nitrate medium, the copper release in the 0.5M sodium chloride and 0.1M nitric acid media was increased for electrode II but not for electrode I. The release of copper was not affected by removal of oxygen from the media but can be substantially lowered by coating the electrodes with a thin cation-exchange membrane (Nafion). The mechanism of copper dissolution is investigated.     

Estimation of chloride in oxidizing media by means of ion-selective electrodes

Chloride concentrations down to the ppm level in a large excess of chlorate or perchlorate can be quantitatively estimated by use of chloride ion-selective electrodes (ISEs). Similarly traces of chloride in chromic acid solutions can be estimated with a heterogeneous silicone-rubber based chloride ISE. However, homogeneous chloride ISEs pose a problem for practical applications, because their response in chromic acid solutions changes with time owing to chemical attack on the membrane surface. In permanganate solutions, both homogeneous and heterogeneous type electrodes can be used for monitoring chloride ions. The Orion electrode, however, was found to show a slightly super-Nernstian response in such solutions. The presence of 10(-3)M iron(III) had no adverse effect on the performance of these electrodes in permanganate solutions.     

Determination of pharmaceutical compounds containing covalently-bound halogen by means of ion-selective electrodes

Organic compounds containing covalently bound halogen can be determined by potentiometric techniques with ion-selective electrodes in different ways. Apart from potentiometric titration of the halide after combustion, direct potentiometric determination of the halide produced at appropriate pH in the solution of the compound in the presence of silver(I) catalyst dissolved from the ion-selective electrode is possible in some cases. In the latter case, the chain length is critical; the chain must have at least three carbon atoms for hydrolysis to take place. Titration with a silver nitrate solution is also possible; prior pH adjustment is then needed for myelobromol but not for the other compounds tested.     

Prussian blue solid-state films and membranes as potassium ion-selective electrodes

The use of thin films of Prussian blue and heterogeneous Prussian blue membranes as potassium ion-selective electrodes was investigated. All of the heavier group I cations and NH⁺₄ interfere strongly but there is relatively good selectivity towards Na⁺ with a selectivity coefficient of ca. 5 × 10^?3. The thin-film measurements, based on Prussian blue deposited on platinum, involve conditioning the electrode to a fixed potential according to the method used by Engel and Grabner for copper hexacyanoferrate(III) films. The membrane electrodes were based on mixing Prussian blue with polymeric supporting films such as polystyrene and epoxy. A particularly simple practical configuration involves Prussian blue membranes deposited directly on copper conductors where one membrane serves as a reference electrode. A reversible cell, without liquid junction, is formed with Prussian blue and Ag/AgCl electrodes and this serves as a means for determining an accurate value for the standard reduction potential of Prussian blue, which is found to be 0.238 V vs. Ag/AgCl at 25 °C.     

Studies of solid-state ion-selective electrodes prepared from semiconducting organic radical-ion salts

The primary redox reactions for solid-state ion-selective electrodes prepared from electronically semiconducting salts of 7,7,8,8-tetracyanoquinodimethane (tcnq) can be identified by considering the redox properties of their constituent ions or molecules. Three different processes involving the couples, Mⁿ⁺/M⁰, 2tcnq^o/(tcnq^-)₂ and (tcnq^-)₂/2tcnq^2- are possible depending on salt composition. Ionic product values determined by potentiometric and atomic absorption methods are in excellent agreement for several such salts; K_s(K₂tcnq₂)=5.8±1.2·10^-11(pot.), 1.7±1·10^-11 (a.a.s.); K_s(Cdtcnq₂) = 3.0±0.5·10^-9 (pot.), 2.9±0.3·10^-9(a.a.s.); K_s(Pbtcnq₂) = 1.3±0.3·10^-10 (pot.), 0.96±0.2·10^-10(a.a.s.); and indicate that the lower activity limit for electrode response is controlled by the solubility of the sensor material itself. Comparisons of predicted and observed standard electrode potentials provide quantitative support for an ion-exchange mechanism of interference. The behaviour of electrodes prepared from Cu₂tcnq₂ (copper(I)) and Cutcnq₂ (copper(II)) is explained on the basis of an interference mechanism and considerations of solid-state equilibria.     

New solid-state ion-selective electrodes — Sensors for chemical analysis of solutions

Summary From the historic point of view one can consider chalcogenide glass ion-selective electrodes (CGISEs) and ion-selective field effect transistors (ISFETs) as new solid-state sensors for chemical analysis. The paper describes the development of the sensors, the methods of investigation, analytical properties and the sensing mechanism of CGISEs and ISFETs.

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Neue ionen-selektive Festkörperelektroden — Sensoren für die chemische Analyse von Lösungen

     

Limits of detection and selectivity coefficients of liquid membrane electrodes

An equation has been derived theoretically describing the emf of a liquid membrane electrode: E = E(0) - (RT/F) ln[(C + radicalC (2) + A (x))/2]. Experimental data are in good agreement with this equation. The parameter A(x) is related to the limit of detection. When the ion-exchanger dissociates completely in the membrane phase, A(x) is given by 4sigma(2)/b(x). Here sigma represents the concentration of the ion-exchanger in the liquid membrane and b(x) is a quantity related to the ion-selectivity.     

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.     

Interpretation of the selectivity and detection limit of liquid ion-exchanger electrodes

An equation has been derived which describes the e.m.f. of a liquid ion-exchanger membrane electrode in conditions of low concentration levels of the primary and interfering ions. The equation is based on the assumption that if the external solution contains no excess of ions which may exchange with the organic phase, then the concentration of the exchanger at the interface decreases, and this is responsible for formation of a diffusion layer inside the membrane. Therefore the potential response depends on the initial concentration of the ion-exchanger in the membrane phase, on the thicknesses of the diffusion layer on both sides of the interface, and on the diffusion coefficients of the species in both phases. This equation explains the non-Nernstian behaviour of the electrode in the presence of interferents, as well as the variation of the conditional selectivity coefficients. The parameters mentioned also influence the detection limit of an electrode. The electrode behaviour has been tested in unstirred solutions and in solutions stirred at different rates. Through its influence on the diffusion layer thickness, the stirring also influences the electrode potential and the characteristics of the electrode.     

Studies of the chlorpheniramine solid-state ion-selective electrode

An Ag/AgCl solid-state electrode was prepared by using urea-formaldehyde resin as the frame material and KCl powder as the active material. Using the prepared Ag/AgCl solid-state electrode as substrate and chlorpheniramine tetraphenylborate ion-pair complex as the active component, a new type of solid-state chlorpheniramine ion-selective electrode was constructed. The properties of the electrode were studied in detail. The electrode shows a rather good stability and can be used in the potentiometric determination of chlorpheniramine.     

Properties of naproxen ion-selective electrodes

Naproxen membrane electrodes based on different plasticizers and the quaternary ammonium salts (QASs) dimethyldidecylammonium bromide, methyltrioctylammonium chloride, or tetraoctylammonium chloride, were prepared. The following basic parameters were investigated for the optimal electrode: measurement range (10⁻⁴ − 10⁻¹ mol L⁻¹), slope of the linear range of the calibration curve (−58.3 mV decade⁻¹), limit of detection (6.0 × 10⁻⁵ mol L⁻¹), lifetime (2.5 months), dependence of the electrode potential on pH (5.5 − 9.0), reproducibility of potential (1.2 mV) and selectivity coefficients in relation to selected organic and inorganic anions. The electrode was utilized for determination of naproxen in tablets by the calibration curve method and the standard addition method.