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.     

Sodium Ion-Selective Chalcogenide Glass Electrodes

Abstract

Analytical characteristics and sensing mechanism of sodium ion-selective electrodes based on NaCl-Ga₂S₃-GeS₂ glasses have been investigated. Chalcogenide glass electrodes containing 10 mol.% NaCl in the membrane showed near-Nernstian response in the concentration range from 10-³ to 1 M sodium nitrate solution. These sensors were superior to the conventional pNa oxide glass electrodes in selectivity in the presence of hydrogen ions and in Na⁺ ion sensitivity in fluoride media. Prolonged solution treatment for several days reduces, however, the detection limit of the sensors and the slope of the electrode response. Ionic processes at the membrane surface have been investigated using XPS technique and ²²Na tracer measurements. It was shown that sodium ion-exchange governed Na⁺ ion response of chalcogenide     

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.     

Sulfonamides as Ionophores for Ise. III. Guanidinium Ion-Selective Electrodes Based on Lipophilic Bis-Sulfonamide Podands

Abstract

PVC membrane ion-selective electrodes based on bis-sulfonamide podands and DOS (Bis(2-ethylhexyl)-sebacate) as plasticizer are described. They were found to behave as guanidinium ion sensors, exhibiting high selectivity for guanidinium ions over potassium ions.     

Selectivity coefficient changes for liquid-membrane electrodes

The effects of the activity levels of the measured ion and interfering ions, and of the detection limit of the electrode on the values of selectivity coefficients for liquid-membrane ion-selective electrodes are discussed. The coefficients were determined by the mixed-ion solution method. Depending on the activity of the interfering ion, the activity of the main ion for which the selectivity coefficient is determined may differ. The best conditions of measurement are those which involve the largest contribution from the term containing the selectivity coefficient in the Nikolsky equation; the measurements are then most precise, and the values of the selectivity coefficients describe the electrode behaviour most consistently. When the limit of detection of the electrode is comparable with the other terms, it must be taken into account in calculations. Under the optimal conditions, selectivity coefficients were calculated for the Orion calcium and divalent cation electrodes, with calcium as the main ion and alkali metal ions as interfering ions.     

Highly Selective Sodium Ion Responsive Glass Electrode

Abstract

A highly selective electrode glass for the measurement of sodium ion activity is described. The corresponding glass electrodes shew a selectivity for sodium over hydrogen ions of about 10 and for sodium over potassium ions of about 10,000.     

The interaction of electrolytes with non-ionic surfactant micelles

The effect of NaCl and HCl on two non-ionic surfactant micelles was studied using several techniques, including conductivity and ion-selective electrodes. Both surfactants exhibit opposite behaviour. When Tween 20 is titrated with HCl the conductivity notably increases in comparison with water, whereas that of Triton X-100 solutions do not change with respect to water until a certain HCl concentration is reached, when it increases. The hydrogen ion activity is lower in Triton X-100 solutions and higher in Tween 20 solutions than in pure water. Chloride ion activity is higher in Tween 20 solutions than in water, whereas in Triton X-100 the activity does not significantly differ from that in water. The activity of sodium ion is lower in Tween 20 solutions than in water, whereas that in Triton X-100 solutions does not differ from the titration of water. These phenomena are explained by the changes in conformation of the non-ionic headgroups, which capture water, and in some cases ions, modifying the activity of ions in the intermicellar solution.     

Conformational analysis of 12-crown-3 and sodium ion selectivity of electrodes based on bis(12-crown-3) derivatives with malonate spacers

A conformational analysis was performed on two crown ethers (12-crown-3 and 12-crown-4) using a combination of semi-empirical and ab initio methods. The lowest conformations of 12-crown-3 and 12-crown-4 were found to have exodentate C_3v and C₂ structure, respectively. In the case of the sandwich-type complexation, the nucleophilic cavity of 12-crown-3 rather than that of 12-crown-4 seemed to be optimal for complexation with the Na⁺ ion. Four new bis(12-crown-3) derivatives (1–4) were examined as potential sodium ion-selective ionophores in poly(vinylchloride) (PVC) membrane electrodes. The ion-selective electrode (ISE) based on di(1,5,9-trioxacyclododecanylmethyl) 2-dodecyl-2-methylmalonate (1) had the highest selectivity for the sodium ion among the alkali and alkaline earths studied. The sodium ion selectivity of the ISE based on bis(12-crown-3) derivative 1 was superior to that of the ISE based on the bis(12-crown-4) analogue.     

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.     

Nickel hexacyanoferrate membrane as a coated wire cation-selective electrode

Nickel hexacyanoferrates containing alkali metal cations as counter ions were used to prepare ion-selective electrodes for potentiometric sensing of intercalated species in the coated wire electrode (CWE) configuration. All the electrodes developed display a quasi-Nernstian response towards potassium ion, whereas the highest sensitivity is generally achieved when Cs+ is the counter cation in the sensing material. The selectivity constants of the electrodes were calculated by the matched potential method considering K+ as the primary ion. The selectivity order is Cs+ > K+ > Na+ > Li+ and reflects the effective dimension of the hydrated cations.     

The sodium responsive tungsten bronze electrode: An electrode for sodium ion analysis of DNA solutions

The preparation and construction of tungsten bronze electrodes, responsive to sodium ions, are reported. The electrodes were made from tungsten bronze powder (Na_0.4WO₃), compressed into pellets with a resin binder and sealed in epoxy resin.The electrodes were found to respond almost instantenously to changes in sodium ion concentration and to be relatively stable with time. The pH dependence decreases as the sodium ion concentration increases, indicating suitability for solutions of sea water concentration.Unfortunately the electrodes show a response to potassium ions of apparently equal magnitude to the sodium response between 0.001 mol kg⁻¹ and 0.1 mol kg⁻¹. Preliminary tests indicate that they also respond to lithium, calcium, aluminium, cupric and tetramethylammonium ions at concentrations of 0.1 mol kg⁻¹. At high sodium ion concentrations ( 1 mol kg⁻¹) the effect of cross sensitivity is negligible for these ions. No attempts are made in this short paper to evaluate selectivity coefficients. Some data demonstrating the low but linear response to sodium ion are given. Tests carried out in DNA—saline-bearing solutions suggest that the possible suitability for marine or biological applications.     

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 Preparation and Analytical Applications of Various Metal Ion-Selective Membrane Electrodes Based on Polymeric,Inorganic and Composite Materials—A Review

Ion-selective membrane electrodes commonly known as electrochemical sensors are important in view of the ability to make direct or indirect measurement of various metal ions. The fact is that the use of ion-selective electrodes for such type of measurements requires relatively inexpensive equipment, which makes ion-selective electrodes attractive to scientists in many disciplines. Thus, potentometric sensors can offer an inexpensive and convenient method for the analysis of heavy metal ions in solutions providing acceptable sensitivity and selectivity. For this purpose, many organic, inorganic, chelating, intercalating and composite materials were studied as electroactive materials for the preparation of ion-selective membrane electrodes. The present study provides a detailed review of literature for the fabrication, characterization and analytical applications of ion-selective membrane electrode based on different electro active components.     

Unbiased selectivity coefficients obtained for the pulsed chronopotentiometric polymeric membrane ion sensors

We report here on the successful observation of the unbiased thermodynamic selectivity of ion-selective sensors working in normal pulse chronopotentiometric mode (pulstrodes). In contrast to ion-selective electrodes, the pulstrodes do not require careful counterbalancing of the transmembrane ionic fluxes to achieve unbiased thermodynamic selectivity. The pulstrodes can work under asymmetric conditions, which are often encountered in practice. The composition of the inner filling solution did not affect the sensor response, indicating that the transmembrane flux of primary ions was indeed effectively suppressed in the absence of ion exchanger. For the K-selective sensor considered here, an improvement of Mg discrimination by a factor of 1000 was demonstrated.     

Hydrogen ion-selective membrane electrodes based on alkyldibenzylamines as neutral carriers

New synthetic hydrogen ion-selective carriers, derivatives of dibenzylamine, have been used as neutral carriers in liquid membrane electrodes to measure pH range from 2 to 10. A H(+)-selective membrane electrode based on octyldibenzylamine gave a better linear response to pH than other alkyldibenzylamines as neutral carriers. It gave a linear response over the pH range 2-10 and a slope of 56.5 mV/pH at 20 degrees C. The electrode had fairly low electrical resistance, good potential stability and reproducibility. The selectivity coefficients towards sodium, potassium and calcium ions as well as other characteristics of the membrane electrode have been studied.     

Application of H-point standard addition method and partial least squares to the simultaneous kinetic-potentiometric determination of hydrazine and phenylhydrazine

Simultaneous determination of hydrazine (HZ) and phenylhydrazine (PHZ) by H-point standard addition method (HPSAM) and partial least squares (PLS) regression was carried out based on kinetic data from novel potentiometry methods. The rate of chloride ion production in the reaction of HZ and PHZ with N-chlorosuccinimide (NCS) was monitored by a chloride ion-selective electrode. The experimental data show the good ability of ion-selective electrodes (ISEs) as detectors not only for the direct determination of chloride ion but also for simultaneous kinetic-potentiometric analysis using HPSAM and PLS methods. The methods are based on the differences observed in the production rate of chloride ions. The results show that simultaneous determination of HZ and PHZ can be performed in concentration ranges of 0.5 - 20.0 and 0.8 - 25.0 microg mL(-1), respectively. The total relative standard error for applying the PLS method to 8 synthetic samples in the concentration ranges of 1.0 - 16.0 microg mL(-1) for HZ and 2.0 - 16.0 microg mL(-1) for PHZ was 3.96. In order for the selectivity of the method to be assessed, we evaluated the effects of certain foreign ions upon the reaction rate and assessed the selectivity of the method. Both methods (PLS and HPSAM) were evaluated using a set of synthetic sample mixtures and then applied for simultaneous determination of HZ and PHZ in water samples.     

Photoresponsive ion-selective optical sensor

A novel concept of photoresponsive ion-selective optical sensor is described. Photochemical reactions can be utilized to generate and control ion fluxes in an ion-selective optode in the same manner as nonequilibrium electrochemical methods have been used in ion-selective electrodes. In contrast to their equilibrium counterparts, the photoresponsive pH-selective ion optodes are sensitive to both the buffer capacity of the sample and activity of hydrogen ions. Active optical probes are especially attractive for intracellular applications because they can be fabricated as submicron-sized beads. Common optical techniques, such as fluorescence microscopy and flow cytometry, can be combined with active ion probes with only minor modification of the existing experimental setup.     

Hydrogen ion-selective poly(vinyl chloride) membrane electrode based on a calix[4]arene.

A hydrogen ion-selective poly(vinyl chloride) membrane electrode was constructed using 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetracyanomethoxycalix[4]arene as a neutral carrier. The electrode showed an apparent Nernstian response in the 2-11.5 pH range with a slope of 54.0 +/- 0.2 mV/pH at 20 +/- degrees C. This electrode showed a rapid response of the emf to changes in the pH, high ion selectivity with respect to lithium, sodium and potassium, and characteristics similar to those reported for the conventional pH glass membrane electrode. It can be used as a potentiometric indicator electrode in hydrofluoric acid solutions. The effects of iodide, thiocyanate, perchlorate and bromide on the characteristics of the electrode were also considered.     

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.