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.     

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.     

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.     

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.     

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.     

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.     

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.     

Ionophore extrem hoher Lipophilie als selektive Komponenten für Flüssigmembranelektroden

Selective ionophores of extreme lipophilicity for liquid membrane electrodes Lipophilic ionophores of the type diether diamides have been prepared. Their lipophilicity is up to 7 orders of magnitude higher than the one of the most lipophilic ion carriers used as selective components in liquid membrane electrodes reported so far. For such ion carriers of extremely high lipophilicity kinetic limitations of the carrier induced ion transfer between aqueous and membrane phase usually dominate and heavily disturb the electromotive behavior of the membrane electrode. These limitations are absent only in those cases where most of the lipophilic segments of the carrier may remain in the membrane phase while the segments with the coordination sites are exposed to the aqueous phase during the transfer process.     

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.     

Effect of membrane composition on the selectivity of electrodes selective for alkylammonium ions with different degrees of substitution

The effect of the nature of an ion exchanger and a plasticizer on the selectivity of ion-selective electrodes for cations of primary-tertiary amines and quaternary ammonium bases was studied. The increments of methylene and ionic groups in the logarithms of selectivity coefficients were calculated. It was demonstrated that the effect of the membrane composition on the selectivity coefficient was governed by changes in the increments of corresponding groups. The maximum change in selectivity, which may attain three orders of magnitude, was observed for pairs of primary-quaternary alkylammonium cations. It was shown that the effect of the nature of the cation exchanger on selectivity coefficients was comparable with that of the plasticizer nature.     

Selective coulometric release of ions from ion selective polymeric membranes for calibration-free titrations

Coulometry belongs to one of the few known calibration-free techniques and is therefore highly attractive for chemical analysis. Titrations performed by the coulometric generation of reactants is a well-known approach in electrochemistry, but suffers from limited selectivity and is therefore not generally suited for samples of varying or unknown composition. Here, the selective coulometric release of ionic reagents from ion-selective polymeric membrane materials ordinarily used for the fabrication of ion-selective electrodes is described. The selectivity of such membranes can be tuned to a significant extent by the type and concentration of ionophore and lipophilic ion-exchanger and is today well understood. An anodic current of fixed magnitude and duration may be imposed across such a membrane to release a defined quantity of ions with high selectivity and precision. Since the applied current relates to a defined ion flux, a variety of non-redox active ions may be accurately released with this technique. In this work, the released titrant's activity was measured with a second ionophore-based ion-selective electrode and corresponded well with expected dosage levels on the basis of Faraday's law of electrolysis. Initial examples of coulometric titrations explored here include the release of calcium ions for complexometric titrations, including back titrations, and the release of barium ions to determine sulfate.     

Changes in zeta potential caused by a dc electric current for thin double layers

An asymptotic solution was obtained to describe one-dimensional, steady-state transport of a symmetric binary electrolyte normal to two large parallel electrodes, in the limit in which the Debye length is infinitesimal compared to the distance separating the two electrodes. Despite the nonzero ion flux, Boltzmann's equation continues to describe the relationship between either ion concentration and the electrostatic potential inside the diffuse part of the double layer, while local electroneutrality applies outside, even for current densities approaching the limiting value. In the absence of ion adsorption or dissociation reactions at the electrodes, the magnitude of any charge or zeta potential arising on the electrodes at zero current is determined by the equilibrium constant for the redox reactions which would exchange ionic charge carriers for electric charge carriers at the electrode surface. Nonzero current causes the ionic strength of the bulk to vary with position. This perturbs the Debye length of the diffuse cloud on either electrode: it is the local ionic strength just outside the cloud which determines the Debye length for that cloud. Nonzero current also changes the zeta potential. The dimensionless rate of change dζ/dJ was as large as 30.     

Cyclic voltammetry of highly hydrophilic ions at a supported liquid membrane

Cyclic voltammetry has been used to study the coupling of ion transfer reactions at a liquid membrane. The liquids are either supported by a porous hydrophobic membrane (polyvinylidene difluoride, PVDF) when the organic solvent is non-volatile (o-nitrophenyloctylether) or are merely a free standing organic solvent layer such as 1,2-dichloroethane comprised between two hydrophilic dialysis membranes supporting the adjacent aqueous phases. The passage of current across the liquid membrane is associated with two ion transfer reactions across the two polarised liquid liquid interfaces in series. It is shown that it is possible to study the transfer of highly hydrophilic ions at one interface by limiting the mass transfer of the other ion transfer reaction at the other interface. Indeed, for systems comprising an ion M in one aqueous phase and a reference ion R partitioned between the membrane and the other aqueous phase, the observed and simulated cyclic voltammograms have a half-wave potential determined by the Gibbs energy of transfer of M transferring at one interface and by the limiting mass transfer of R at the other interface. This new methodology opens a way to measure the Gibbs energy of transfer of highly hydrophilic or hydrophobic ions, which usually limits the potential window at single liquid liquid interfaces (ITIES).     

Coated-wire electrodes containing polymer immobilized ionophores blended with poly(vinyl chloride)

Polymers containing covalently attached 18-crown-6 or 2.2.2 cryptand units were incorporated into plasticized PVC membranes and the composite membranes were examined as potassium ion sensor elements. Ionophores were linked to carboxy-PVC and to poly(acrylic acid) via amide linkages to an alkyl spacer unit. Coated-wire electrodes (CWEs) from the immobilized ionophores gave acceptable responses, but conventional ion-selective membrane electrodes (ISEs) prepared by solvent casting were inactive. Dip-cast membranes did give active ISEs. Potassium electrode performance was independent of the loading of the ionophore within the acrylate support polymer, but depended upon the spacer length. Ion selectivity varied with the ionophore loading within the support polymer. Selectivity is a composite of the ionophore selectivity and ion-exchange interactions with the acrylate backbone, giving selectivities akin to carboxylate substituted crown ethers, notably enhanced monovalent/divalent ion discrimination relative to the ionophore in solution. Polymer immobilization extended the lifetime of active electrodes.     

Adding Plasticizer Dispersible Conducting Polymer to the Ion-selective Membrane Composition – Revitalizing Single Piece Ion-selective Electrodes Concept

Typically, ion-selective electrodes used in current triggered electrochemical sensing apply a conducting polymer layer covered with an ion-selective membrane. In this work we propose an ion-selective membrane containing a dispersed conducting polymer. Thus obtained system allows elimination of the Achilles hell of heterogeneous ion-selective membranes containing solid particulates dispersed within the ion-selective polymeric membrane. The herein proposed system, even for high conducting polymer loading equal to 5 % w/w, is characterized with insensitivity towards redox interferences, as well as potentiometric detection limits, selectivity well comparable with that for other ion-selective electrodes constructions. Under voltammetric conditions, with increasing loading of the conducting polymer in the membrane cathodic peak potentials are shifted towards more negative values, yet the linear dependence of the peak potential on logarithm of concentration of the analyte in the solution is preserved.     

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.     

Separation and Preconcentration of Strontium from Calcium in Aqueous Samples by Supported Liquid Membrane Containing Crown Ether

Supported liquid membrane containing crown ether (DC18C6) and dinonylnaphthalenesulfonic acid (DNNS) is proposed for separation and preconcentration of strontium in an aqueous environmental sample. The effects of carriers in membrane on permeability and selectivity of Sr²⁺ were characterized. Crown ether has a key role in the selective transport of strontium ion while DNNS facilitates the transport of both Sr²⁺ and Ca²⁺. A strontium ion can be selectively transported to an acidic strip solution against its concentration gradient across the membrane. This SLM can be used as an efficient method for separation and preconcentration of strontium in a neutral aqueous sample.