Liquid Membrane Ion-selective Electrodes Research Articles

Dependence of analytical parameters of ion-selective electrodes on the membrane solvents used in the construction of membranes

The aim of this study was analysis of the suitability of available membrane solvents and their application to obtain liquid membrane ion-selective electrodes that are sensitive to diazepam or fluoxetine. Sensitive and reasonably selective poly(vinyl chloride) membrane electrodes are based on using water insoluble ion-pair of diazepam – [tetrakis(3,5bis-(trifluoromethyl)phenyl]borate or fluoxetine – [tetrakis(4-fluorophenyl)]borate as electroactive material. As membrane solvents for construction of membranes there were used 2-nitrophenyloctyl ether, bis(2-ethylhexyl) sebacate, 1-isopropyl-4-nitrobenzene, bis (1-buthylpentyl) adipate, tris(2-ethylhexyl) phosphate. Evaluation of analytical parameters of the obtained electrodes, their selectivity and evaluation of analytical parameters for membranes subjected to a process of "accelerated aging" in a climate chamber at 60 °C and humidity of 70 % without the VIS and UV light, were developed. There was carried out an analysis of the suitability of the electrodes, which will show the best stability and reproducibility of the analytical parameters for the determination of substances in respect of which exhibit selectivity. The determination was developed using potentiometric titration method.

Interpretation of the potential response of PVC membrane ion-selective electrodes based on the mixed potential theory

Previously, an excellent theory was proposed for understanding the potential response of liquid membrane ion-selective electrodes (ISEs), which was based on a concept of zero-current potential or “mixed potential (MP)” of the interface between the liquid membrane and a sample solution containing interfering ions. Unfortunately, however, the MP theory has not been utilized explicitly in the development of the most popular, polyvinyl chloride (PVC) membrane ISEs, because the theory has not been verified by using the PVC membrane/water interface. In this study, we have successfully employed a micro PVC membrane/water interface to perform voltammetric measurements for the interfacial transfer of ions, including tetramethylammonium (TMA+), tetraethylammonium (TEA+), and tetrapropylammonium (TPrA+) ions. Using the thus obtained voltammetric data (i.e., the standard ion-transfer potentials), the potential response of a PVC membrane TEA+-ISE in the presence of TMA+ or TPrA+ as the interfering ion could be well elucidated by the MP theory. This theory would be available for the sophisticated design of high performance ISEs, which is not determined by conventional “trial-and-error” procedures.

A new ion selective electrode for cesium (I) based on calix[4]arene-crown-6 compounds

A polyvinylchloride (PVC) based liquid membrane ion selective electrode (ISE) for cesium has been developed. 25,27-Dihydroxycalix[4]arene-crown-6 (L1), 5,11,17,23-tetra-tert-butyl-25,27-dimethoxycalix[4]arene-crown-6 (L2) and 25,27-bis(1-octyloxy)calix[4]arene-crown-6 (L3) were investigated for their use as ionophores. The cation exchange resin DOWEX-50W was used to maintain low activity Cs+ in inner filling solution to improve the performance. The best response for cesium was observed with L3 along with optimized membrane constituents and composition. Excellent Nernstian response (56.6mV/decade of Cs(I)) over the concentration range 10−7 to 10−2M of Cs(I) was obtained with a fast response time of less than 10s. Detection limit for Cs(I) using the present ISE is 8.48×10−8M Cs(I). Separate solution method (SSM) was applied to ascertain the selectivity for Cs(I) over alkali, alkaline earth and transition metal ions. The response of ISE for Cs(I) was fairly constant over the pH range of 4–11. The lifetime of the electrode is 10 months which is the highest life for any membrane based Cs-ISE so far developed. The concentration of cesium ion in two simulated high level active waste streams was determined and results agreed well with those obtained independently employing AAS.

Ion selective electrode for cesium based on 5-(4′-nitrophenylazo)25,27-bis(2-propyloxy)26,28-dihydroxycalix[4]arene

A polyvinylchloride (PVC) based liquid membrane ion selective electrode (ISE) for cesium was fabricated with 5-(4'-nitrophenylazo)25,27-bis(2-propyloxy)26,28-dihydroxycalix[4]arene as ionophore. Different membrane constituents were investigated to realise optimum performance of the ISE developed. Of the four plasticizers and two ion additives studied, the best response was observed with membrane having 2-nitro phenyl octyl ether (oNPOE) as plasticizer and potassium tetrakis (perchloro phenyl) borate (KTpClPB) as ion additive. Linear response over concentration range of 10(-5)-10(-1)M CsCl was obtained. The Nernstian slope of the response was 56 mV per decade for Cs with a response time less than 20s. Matched potential method has been applied to find out the selectivity for Cs over several ions like Rb(+), K(+), Na(+), NH(4)(+), Sr(2+), Ba(2+), Ca(2+), Mg(2+), Cu(2+), Pb(2+), Zn(2+), Ni(2+) and Ce(3+). The response of ISE for Cs(+) was fairly constant over the pH range of 3-11. The lifetime of the electrode is 9 months which is the longest life for any membrane-based Cs-ISE so far developed. The concentration of cesium in two simulated high level active waste streams was determined and results agreed well with those obtained independently employing atomic absorption spectrometry.

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.

Use of Highly Hydrophobic Ionic Liquids for Ion-selective Electrodes of the Liquid Membrane Type

The phase-boundary potential at the interface between an aqueous KCl solution (W) and a hydrophobic room-temperature ionic liquid (RTIL), trioctylmethylammonium bis(nonafluorobutylsulfonyl)imide ([TOMA(+)][C(4)C(4)N(-)]), containing dicyclohexano-18-crown-6 (DCH18C6), shows the nernstian response to K(+) in W within a polarized potential window of 500 mV between [TOMA(+)][C(4)C(4)N(-)] and W, demonstrating that hydrophobic RTILs can be used as a nonvolatile ionic medium for liquid-membrane ion-selective electrodes. The complex formation constant of K(+) with DCH18C6 in [TOMA(+)][C(4)C(4)N(-)] is estimated to be on the order of 10(9) from the upper detection limit using a partition equilibrium model in the presence of a neutral ionophore. The response time of the phase-boundary potential is approximately 20 min. Gelled [TOMA(+)][C(4)C(4)N(-)] also shows the nernstian response to K(+), although the upper limit is lower probably due to a change in the solvent properties from the non-gelled [TOMA(+)][C(4)C(4)N(-)]. The response time of the gelled [TOMA(+)][C(4)C(4)N(-)] is approximately 5 min, shorter than that of the non-gelled [TOMA(+)][C(4)C(4)N(-)].

New membrane electrodes based on a functionalized tetraphenylborate covalently bound to the polymeric backbone

Abstract In this study new liquid contact polymeric membrane ion-selective electrodes (ISEs) have been realized with the aim of reducing leaching phenomena of active component and so improving the shelf life of the electrode. A new tetraphenylborate ion exchanger functionalized with allylic moieties has been synthesized and linked to a copolymer acrylonitrile–butadiene, alternative to PVC. The linking reaction takes place by radicalic mechanism and is catalysed by dicumyl peroxide. The leaching of the so obtained polymers has been evaluated by spectrophotometric and potentiometric measurements to check the effectiveness of the reaction. In order to test the performances of the new polymeric material, ISEs with and without ionophore have been prepared using the functionalized matrix as well as traditional PVC as polymeric membrane. Analysis of variance was carried out on the detection limit and on the slope of the calibration curve obtained for every kind of electrode, in order to identify the effects of the covalently bound ion exchanger on the lifetime and performance of the sensors.

.ALPHA.-Hydroxycarboxylate Selectivity by Spontaneous Formation of Dimeric Dialkyltin(IV) Complexes in Membrane of Anion-Selective Electrode

The liquid-membrane ion-selective electrode based on bis(pentafluorobenzyl)tin(IV) dibromide showed higher selectivities to α-hydroxycarboxylates than to other carboxylates without any hydroxyl group and those with a β-hydroxy group. The calibration curve for citrate was linear over a range of 10−4 to 10−2 mol dm−3 with a potential slope of −60 mV/decade. The equilibrium study on the complexation between dimethyltin(IV) and a series of carboxylates indicates the formation of stable, neutral and dimeric complexes only with α-hydroxycarboxylates, [(Me2Sn)2(H−1hc)2] {(H−1hc)−2 : species formed by deprotonation of α-hydroxycarboxylates (hc−) at α-OH groups}. The corresponding species may be spontaneously formed as electroactive species in the liquid membrane containing bis(pentafluorobenzyl)tin(IV) dibromide and acquire the selectivity to α-hydroxycarboxylates.

Synthesis, transport and ionophore properties of α,ω-diphosphorylated aza podands: III. Liquid ion-selective electrodes based on phosphorylated aza podands

Aza podands having side α-aminomethylphosphine oxide groups were prepared and used as electrode-active agents in liquid membrane ion-selective electrodes. A series of liqiud membrane electrodes sensitive to Cu2+ and Hg2+ cations were prepared on the basis of N,N-bis[di(n-hexyl)phosphorylmethyl]piperazine. The electrodes containing copper complexes with aza podands as ionophores exhibit anionic function toward lipophilic anions. Potentiometric and argentometric determination of iodide ions was performed with an iodide-selective electrode based on the mercury complex.

Studies on a Pb2+ - selective electrode with a macrocyclic liquid membrane: Potentiometric determination of pb2+ ions

This paper presents experimental and theoretical data regarding the design, characterization and analytical applications of a non-expensive, liquid-membrane ion-selective electrode for Pb2+ ions. The membrane is a solution of the active complex formed by Pb2+ ions with dibenzo-18-crown-6-ionophore (DB-[18]-C-6) extracted in propylene carbonate (PC). The successful application of the developed electrode for the determination of Pb2+ ions in aqueos solution samples by direct potentiometry and potentiometric titration is presented. For the presented analytical results, there are insignificant systematic errors between the direct potentiometric method with the developed ion-selective electrode and atomic absorption spectrometry. .

Effect of the phase volume ratio on the potential of a liquid-membrane ion-selective electrode.

Two-phase liquid system IA(w)|IX(o) comprising the interface between the aqueous solution (w) of uni-univalent electrolyte IA and an organic solvent solution (o) of a uni-univalent electrolyte IX with the common cation I(+) is considered as a simple model of a liquid-membrane ion-selective electrode (ISE). Taking into account the electroneutrality and mass balance conditions, the equilibrium Galvani potential difference (pd) between the aqueous and organic phases, phi = phi(w) - phi(o), is calculated numerically as a function of the ratio of the initial electrolyte concentrations, x = / = 10(-)(4)-10(4), for the selected values of the phase volume ratio r = V(o)/V(w) = 10(-)(3), 1, and 10(3), and the standard ion transfer potentials of the present ions ranging from -0.5 to 0.5 V. Numeric results corroborate the symbolic expressions derived for the cases when X(-) and A(-) are extremely lipophilic and hydrophilic ions, respectively, or when the concentration ratio x is extremely large or small. In contrast to the extraction system, where both electrolytes are initially present in the aqueous phase, the effect of the phase volume ratio on the equilibrium pd in the ISE model is rather weak, unless the counterions X(-) and A(-) differ little in their lipophilicity from the target ion I(+). It is shown that both the ISE and extraction model exhibit the Nernstian behavior only in a limited range of the concentration ratio x depending on the value of the standard ion transfer potentials of the counterions. When this ratio is extremely large or small, equilibrium pd approaches the limiting value given by the distribution potential of the electrolyte IA or IX, respectively. Similar conclusions can be drawn for the two-phase liquid system AI(w)|XI(o) with the common anion I(-).

Extraction of electrolytes from aqueous solutions and their spectrophotometric determination by use of acid-base chromoionophores in lipophylic solvents.

The formation of non-absorbing complexes in an organic phase has been exploited for the spectrophotometric determination of ionic analytes in aqueous solutions. The method is based on liquid-liquid extraction of aqueous solution with lipophylic organic phases containing an acid-base chromoionophore, a neutral lypophilic ligand (neutral carrier) selective to the analyte and a cationic (or anionic) exchanger. The method avoids all difficulties of the preparation of the very thin membranes used in optodes, so that it can advantageously be used for the study of the role physical-chemical parameters of the system in order to optimize them and to prepare, if necessary, an optimized optode. Two lipophylic derivatives of Nile Blue and 4',5-dibromofluorescein have been synthesized, in order to ensure their permanence within organic phase. Two different neutral carriers previously characterized by us as ionophores for liquid-membrane Ion Selective Electrodes have been employed. Three different ionic exchangers have been tested. Furthermore, a model allowing the interpolation of experimental data and the determination of the thermodynamic constant of the ionic-exchange equilibrium has been developed and applied.

Potentiometric Determination of Non‐Ionic Surfactants by Liquid Membrane Electrodes

AbstractIt is well known that non‐ionic surfactants (NIS) influence remarkably the potentiometric measurements with liquid membrane ion selective electrodes (ISEs), interfering particularly on performance of ISEs for earth‐alkali metals, for which the loss of selectivity with regard to alkali metals has been documented. These studies indicate that such interferences are due to the extraction of surfactants within the membrane, where a competition takes place between the originally present ionophore and the surfactant which also acts as a ligand for alkali metals. The interpretation of such phenomena enabled one to exploit this interference for analytical purposes by membrane/solution extraction experiment monitored by UV measurements and by impedance FRA analysis on coated wire electrodes. Using Ca/Mg ISEs based on the neutral ionophore ETH 4030, it has been established that the logarithm of the Ca/Mg over Na potentiometric selectivity constant is linearly correlated with the concentration of NIS like Tegopren 5863 and Triton X‐100. The proposed method has been applied for the development of a new potentiometric analytical procedure for the determination of Tegopren 5863 in synthetic seawater (SSW), ranging from 0.25 to 5 ppm. Our procedure consists in the exposure of the electrode to stirred SSW containing the surfactant; the progressive extraction of Tegopren 5863 causes a growth in electrode's sensitivity to Na+ and K+, losing selectivity for Ca2+ and Mg2+. In turn this induces an increase of EMF, as all these ions are present in the studied matrix. The potential drift was monitored for 15 hours, showing that the process reaches thermodynamic equilibrium after about 12 hours of exposure. This method presents a value of 210 ppb of Tegopren 5863 as detection limit.

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.

Lower Rim Substituted tert-Butyl calix[4]arenes (I). The Structure and Complexing Properties in Ion-Selective PVC Membrane Electrodes

Some di- and tetraamide tert-butylcalix[4]areneswere synthesized and described. Their ionophoric properties were studied in liquid membrane ion-selective electrodes. The correlation between thechemical structure (conformation in solution determined by 1H NMR) and potentiometric ion-selectivity and complex formation constant havebeen studied. The PVC membrane electrodes based on tetraamides 8–11 show high sodium selectivity, are stable and long lasting. Disubstituted amides 1–7 are selective for larger and more lipophilic ions, as for exampleguanidinium ion. The crystal structure of the diamide 4 was determined by single crystal X-ray analysis. Crystals of 4 are triclinic, spacegroup P-1, with: a = 16,669(8), b = 17.795(10), c = 20.984(8) A, α =91.08(4)°, β = 91.60(3)°, γ = 90.73(4)° and Z = 4. Ionophore 4 posseses a distorted cone conformation and is substituted at the proximal phenol rings.

Structural Selectivity of Calix(m)pyrroles(n)furans (m + n = 4) as Ionophores in Ag(I) Ion-Selective Electrodes

A series of octamethylcalix[m]pyrroles[n]furans (m + n = 4), such astrans-octamethylcalix[2]pyrroles[2]furans L1),cis-octamethylcalix[2]pyrroles[2]furans (L2) and octamethylcalix[1]pyrrole[3]furans (L3) have been studied as sensors in liquid membrane ion-selective electrodes for Ag(I) ion. The electrode based on L1, trans-N2O2 porphyrinogen, gave the best results with a wide working concentration range of 1.0 × 10-1 - 1.0 × 105.6 M and a Nernstian slope of 57.0 mV/decade. This electrode exhibited a fast response time of 30 s and high selectivity over a number of mono-, di- and tri-valent cations, with only Tl(I) and Hg(II) ion interferences. The effect of anion excluders on the performance of the membrane electrodes has been also studied. The electrode based on L1 showed no significant potential changes in the range 2.5 < pH < 7.5. The crystalstructure of L3, NO3 porphyrinogen, was determined by single crystal X-ray analysis. The crystallographic analysis of L3 reveals that its structure is a saddle-shaped 1,3-alternate conformation with enough space to accommodate Ag(I) in the three dimensional cavity.

Construction of a Triphenyltetrazolium Liquid Membrane Ion Selective Electrode and its Analytical Application to the Assay of Vitamin C

The construction of a liquid triphenyltetrazolium cation (TT+) ion-selective electrode (ISE) based on [TT+]3[P(W3O10)4] salt dissolved in 2-nitrotoluene is described. The liquid membrane electrode exhibits a rapid and almost Nernstian response to triphenyltetrazolium cations in the concentration range from 2×10−4 to 1×10−2 mol l−1. The Nernstian slope is 56–58 mV decade−1 which remains constant for at least two months. The response is virtually unaffected by pH changes in the range 3–12. Major interferents are periodate and malate. Deviation from linearity is also observed in the presence of bromide, iodide and thiocyanate, due to their precipitation with triphenyltetrazolium cations. Analytical applications such as direct potentiometry for the determination of TT+ in aqueous solutions and indirect potentiometry for the assay of ascorbic acid (vitamin C) in pharmaceutical preparations are described. Ascorbic acid in the range of 150–500 mg l−1, under specified experimental conditions, can be determined with mean relative error of 1.9%.

Apparently “Non-Nernstian” Equilibrium Responses Based on Complexation Between the Primary Ion and a Secondary Ion in the Liquid ISE Membrane

We have previously reported on a phase boundary model explaining the apparently “twice-Nernstian” responses to divalent cations as they have been observed for some liquid-membrane ion-selective electrodes (ISEs) based on acidic ionophores. The rationalization of those responses, which are characterized by slopes that are two times larger than Nernstian response slopes for divalent cations, took into account the equilibrium distribution of two types of ions, i.e., the divalent cations as primary ions and H+ ions as “secondary ions”, across the sample–membrane interface. In the present work, we have intentionally chosen an extremely simple type of electrodes to show that apparently “non-Nernstian” responses can also occur if the primary and the secondary ion have opposite charge signs. It is shown that apparently “non-Nernstian” equilibrium responses can occur in the case of ionophore-free ion-exchanger electrodes if the sample solutions contain besides the primary ion also a secondary ion, and if 1:n or n:1 complexes between these two types of ions are formed in the electrode membrane phase. In contrast to the well-known responses of ion-exchanger electrodes in solutions that contain metal ion complexes, an interaction between the primary and secondary ion in the sample solution is not required for the observation of apparently “non-Nernstian” equilibrium responses. A set of very simple equations allows the determination of the response slopes that are expected for particular sets of complex stoichiometries and charges of the primary and secondary ion. Due to the very general formulation of the theory, these results are applicable both for electrodes with primary and secondary ions of opposite charge signs and for electrodes with primary and secondary ions of the same charge sign.

Modeling of the Electrified Interface of Liquid Membrane Ion-Selective Electrodes

Computer modeling of the interface between two immiscible electrolyte solutions confirms that non-Nernstian behavior of cation-sensitive ion-selective electrodes (ISEs) can occur by the following molecular mechanisms: (i) extraction of weakly hydrophilic anions into the membrane, known as the anion effect or Donnan exclusion failure; (ii) leaching of the membrane's hydrophobic anion at low aqueous electrolyte concentrations; (iii) interference effects of competing cations. All three effects are known to occur under experimental conditions. Our theory employs a lattice-based self-consistent field model extended to include ion−carrier complexation by means of a multistate mechanism. The interface is created in the computer model by a self-assembling mechanism controlled by an unfavorable interaction parameter between the (PVC-lookalike) polymer of the membrane phase and the aqueous solution. Electrostatic potential profiles and charge-density profiles are available for the whole range of parameters considered in the computation of the calibration lines. The width of the polar/apolar interface is only a few molecular diameters thick, but at low ionic strength conditions the electric double layer extends far into both water and the apolar membrane phase.

Flow injection potentiometry by poly(vinyl chloride)-membrane electrodes with substituted azacrown ionophores for the determination of lead(II) and mercury(II) ions

A flow cell with a poly(vinyl chloride) (PVC) neutral-ionophore liquid-membrane ion-selective electrode has been developed for flow injection potentiometry (FIP). The flow system was optimised and five substituted azacrown ethers: 7,16-dithenoyl-1,4,10,13-tetroxa-7,16-diazacyclooctadecane (DTODC), 7,16-di-(2-thiopheneacetyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTAODC), 7,16-dithenyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTDC), 1,10-dioxa-4,7,13,16-tetraazacyclooctadecane (TC) and 4,7,13,16-tetrathenoyl-1,10-dioxa-4,7,13,16-tetraazacyclooctadecane (TTOTC) synthesised and investigated as the ionophores in Pb 2+ (DTAODC, TTOTC) and Hg 2+ (DTDC, DTODC, TC, TTOTC) selective electrodes. The short contact times between analyte and ionophore in FIP allow the measurement of a strongly complexing ion such as Hg 2+ that saturates the electrodes in batch analysis, or in continuous flow analysis that comes to a steady state. For the mercury-selective electrodes with ionophores with amide functional groups (TTOTC and DTODC) a carrier of 10 mM potassium nitrate was found to increase the speed of response and recovery to baseline. The linear calibration range for a DTAODC lead-selective electrode was pPb=2.0 to 5.0 with a slope of 32.5 mV decade −1 and for a TTOTC mercury-selective electrode, pHg=3.0 to 5.5 with a slope of 28.4 mV decade −1. Highly reproducible measurements were obtained (RSD <2%) at a flow rate of 3.0 ml min −1 giving a typical throughput of 40 samples h −1 for Pb 2+ and 30 samples h −1 for Hg 2+.