Polymeric Membrane Electrodes Research Articles

Aluminum(III) Porphyrins as Ionophores for Fluoride Selective Polymeric Membrane Electrodes

Aluminum(III) porphyrins are examined as potential fluoride selective ionophores in polymeric membrane type ionselective electrodes. Membranes formulated with Al(III) tetraphenyl (TPP) or octaethyl (OEP) porphyrins are shown to exhibit enhanced potentiometric selectivity for fluoride over more lipophilic anions, including perchlorate and thiocyanate. However, such membrane electrodes display undesirable super-Nernstian behavior, with concomitant slow response and recovery times. By employing a sterically hindered Al(III) picket fence porphyrin (PFP) complex as the membrane active species, fully reversible and Nernstian response toward fluoride is achieved. This finding suggests that the super-Nernstian behavior observed with the nonpicket fence metalloporphyrins is due to the formation of aggregate porphyrin species (likely dimers) within the membrane phase. The steric hindrance of the PFP ligand structure eliminates such chemistry, thus leading to theoretical response slopes toward fluoride. Addition of lipophilic anionic sites into the organic membranes enhances response and selectivity, indicating that the Al(III) porphyrin ionophores function as charged carrier type ionophores. Optimized membranes formulated with Al(III)PFP in an o-nitrophenyloctyl ether plasticized PVC film exhibit fast response to fluoride down to 40 mM, with very high selectivity over SCN � , ClO4 � ,C l � ,B r � and NO3 � (k pot < 10 � 3 for all anions tested). With further refinements in the membrane chemistry, it is anticipated that Al(III) porphyrin-based membrane electrodes can exhibit potentiometric fluoride response and selectivity that approaches that of the classical solid-state LaF3 crystal-based fluoride sensor.

Strain Accumulation in Polymer Electrolyte Membrane and Membrane Electrode Assembly Materials During a Single Hydration/Dehydration Cycle

A digital image correlation technique was used to measure drying-induced strains in perfluorosulfonic acid (PFSA) membrane and membrane electrode assembly (MEA) materials which are key components of fuel cells. Circular coupons of each material were constrained in a stainless-steel drum fixture and hydration was achieved by immersing the drum/coupon assembly in 80°C water for 5min. During air drying at 25°C and 50% relative humidity, a series of 1280×960 digital images of each coupon surface was recorded with a digital camera and image acquisition system for drying periods up to 26h. Cumulative correlation of an initial image recorded prior to hot water immersion and the final image at the end of drying produced in-plane strain contour maps. Incremental correlation was employed to track strain evolution at the coupon centers and at peripheral points. During the earliest drying stages, where both materials exhibited viscoelastic behavior, peak radial strains of 9.50% and 2.50% were measured in the PFSA membrane and MEA materials, respectively. At longer drying times, peak radial strains reached constant values of 5.70% and 1.25% in the PFSA membrane and MEA materials, respectively. Strain accumulation in the MEA tends to be less uniform than that in the membrane, and corresponding peak strain values in the MEA tend to be less than those in the membrane independent of drying duration. Reinforcement from the electrode layers lowers strain accumulation in the MEA, but it does not preclude crack nucleation in these layers as drying proceeds.

A bromide selective polymeric membrane electrode based on Zn(II) macrocyclic complex

A novel bromide ion-selective PVC membrane sensor based on 2,3,10,11-tetraphenyl-1,4,9,12-tetraazacyclohexadeca-1,3,9,11-tetraene zinc(II)complex ( I) as carrier has been developed. The electrode exhibited wide working concentration range 2.2 × 10 −6 to 1.0 × 10 −1 M and a limit of detection as 1.4 × 10 −6 M with a Nernstian slope of 59.2 ± 0.5 mV per decade. The response time of electrode was 20 s over entire concentration range. The electrode possesses the advantages of low resistance, fast response and good selectivities for bromide over a variety of other anions and could be used in a pH range of 3.5–9.5. It was successfully used as an indicator electrode in the potentiometric titration of bromide ions with silver ion and also in the determination of bromide in real samples.

Titrimetric method for determination of O-desulfated heparin in physiological samples using protamine-sensitive membrane electrode as endpoint detector

O-Desulfated heparin (ODSH) is a promising new anti-inflammatory agent for the prevention of reperfusion injury following myocardial infarction or stroke. This partially desulfated heparin derivative has less anticoagulant activity than unfractionated heparin but retains the inherent anti-inflammatory properties of heparin. Thus, ODSH could be administered at the high doses needed to achieve desired anti-inflammatory function without risk of hemorrhage. However, given the very low anticoagulant activity of this species, traditional methods for heparin determination in clinical samples might not be well suited for ODSH measurements. In this article, a novel titrimetric method for detection of ODSH in buffer and plasma is described using a protamine-sensitive polymer membrane electrode as the detector. Titrations of ODSH with the heparin antagonist protamine yield sharp endpoints with sensitivity to ODSH in the micrograms per milliliter range for plasma samples. The stoichiometry for protamine interaction with ODSH is determined to average 1.39 μg protamine/μg ODSH in plasma. This technology is further applied to a toxicokinetic study of ODSH in an animal model, demonstrating the ability to detect the changes in ODSH concentrations in biological samples.

Potentiometric determination of activity coefficients for NH 4Cl in the ternary NH 4Cl/LiCl/H 2O mixed electrolyte system

This work reports the results of the determination of mean activity coefficients for NH 4Cl in ternary (NH 4Cl( m 1) + LiCl( m 2) + H 2O) electrolyte system by potentiometric method at 25 °C. The experimental potentiometric data were obtained by combining a solvent polymeric (PVC) ammonium-selective membrane electrode ( NH 4 + ISE), containing a mixture of nonactin/monactin as ionophore, and an Ag|AgCl electrode. The measurements were performed, at constant ionic strength, in different series of mixed salt solutions characterized by a fixed salt mole ratio r (where r = m 1/ m 2 = 25, 50, 75, 100). The non-ideal behavior of the system was described on the base of the Pitzer ion-interaction model for mixed salts, over the molality ranging from 0.01 to 3 mol/kg.

Determination of activity coefficients for NH 4Cl in 1-propanol/water mixed solvents by potentiometric measurements

Activity coefficients for NH 4Cl in different 1-propanol/water mixed solvents were determined at 25 °C using experimental electromotive force (emf) data of the following galvanic cell: Ag | AgCl | NH 4 Cl ( m ) , H 2 O ( 100 − w % ) , 1 − PrOH ( w % ) | NH 4 + ( ISE ) , in which ISE stands for the ion-selective electrode and w for the mass fraction percent of 1-propanol. The measurements were performed in various mixed 1-propanol/H 2O solvent systems containing 0, 10, 20, 30, 40 and 50 mass fraction percent of 1-propanol, respectively. The experimental emf data were obtained by a solvent polymeric PVC ammonium-selective membrane electrode containing a mixture of nonactin/monactin as ionophore and an Ag/AgCl electrode. The non-ideal behavior of the electrolyte system was described, for each mixed solvent system, on the base of the Pitzer ion-interaction model by determining the related coefficients over the molality range from dilute solution to near salt saturation (i.e. about 5.13 mol/kg in pure water or 2.34 mol/kg in 50% mixed 1-propanol/water solvent).

Zirconium(IV)-salophens as fluoride-selective ionophores in polymeric membrane electrodes

The feasibility of using Zr(IV)-salophens as ionophores to prepare anion-selective polymeric membrane electrodes is examined. It is shown that electrodes formulated with these compounds exhibit greatly enhanced selectivity towards fluoride anion (as compared to a classical quaternary ammonium anion-exchanger) when introduced into plasticized PVC films containing lipophilic anionic site additives. Electrodes constructed with optimal membrane compositions exhibit the following selectivity pattern: F − > ClO 4 − > SCN − > NO 3 − > Br − > Cl −. For these electrodes, near-Nernstian and rapid potentiometric fluoride responses are observed. It is shown that the optimal response to fluoride occurs when the sample solution is buffered in the range of pH 4.5–6.0. At present, the response lifetime of the proposed electrodes is limited to 14 days due to fast deterioration of fluoride response with time. This phenomenon is ascribed to the decomposition of the ionophore within the polymeric membrane phase of the electrode in contact with the aqueous test solution.

Gallium(III)‐Schiff Base Complexes as Novel Ionophores for Fluoride Selective Polymeric Membrane Electrodes

AbstractGallium(III)‐Schiff base complexes are examined as potential ionophores for the development of fluoride ion‐selective polymeric membrane electrodes. To probe the effect of ionophore structure, several different salen and salophen compounds have been synthesized and tested in plasticized poly(vinyl chloride) membranes (H21–5). A salophen ligand structure highly substituted with t‐butyl groups (H21) was found to be optimal with respect to membrane compatibility, and its corresponding Ga(III) complex yields membrane electrodes with greatly enhanced potentiometric selectivity for fluoride ion over a host of other anions. Addition of lipophilic anionic sites into the membrane phase provides optimized response and selectivity, indicating that the Ga1 ionophore functions as a charge carrier. The use of dioctylsebacate as the membrane plasticizer helps reduce any dimer‐monomer equilibria in the membrane phase, yielding sensors with rapid and fully reversible EMF response. Optimal potentiometric detection of fluoride is achieved in solutions buffered to relatively low pH values (pH 3.0), to reduce anionic response to hydroxide. The use of gallium(III)‐Schiff base complexes is shown to offer a promising research direction to ultimately achieve polymeric membrane fluoride sensors that exhibit potentiometric response equivalent to the classical lanthanum fluoride‐based solid state type membrane electrode.

Analysis of Simulated Martian Regolith Using an Array of Ion Selective Electrodes

AbstractA prototype miniature array of polymer membrane and solid state ion selective electrodes was developed for the purpose of performing an in‐situ analysis of the soluble ionic species in Martian regolith (soil). The array contains a total of 27 electrodes for K+, Na+, Ca2+, Mg2+, NH$\rm{ {_{4}^{+}}}$, Ba2+, NO$\rm{ {_{3}^{-}}}$, Cl−, and Li+, each in triplicate. Barium electrodes were used to indirectly monitor sulfate through precipitation by the addition of barium chloride while the lithium electrodes served as a reference for the array by having a constant lithium concentration as a background for all solutions. The array was tested with several types of simulants, soils, and sawdust from a Mars meteorite, all with varying salt content, meant to approximate the various hypotheses regarding the ionic composition of the Martian soil. The activities of anions and cations determined with the array were compared to ion chromatography data.

Application of some recently synthesized 9, 10-anthraquinone derivatives as new class of ionophores responsive to lead (II) ion

Lead-selective solvent polymeric membrane electrodes, based on some recently synthesized 9, 10-anthraquinone derivatives, are described. The electrode exhibits a good Nernstian response for Pb (II) ions over a wide concentration range of 1.0/spl times/10/sup -6/-1.0/spl times/10/sup -2/ M with a slope of 28.9 mV decade/sup -1/. The potential-pH profile of membrane based on 1-hydroxy-2-({2-[2-(2-hydroxyethoxy)ethoxy]ethoxy}methyl)anthra-9, 10-quinone (A/sub 3/) demonstrated a lack of H/sup +/ interference within a wide pH range (1.5-6.8). The detection limit is 6.7/spl times/10/sup -7/ M. The developed sensor has a very short response time (2.0 s), and it can be used as a working electrode in a flow injection system. The lifetime of the proposed sensor is 120 days (without any considerable divergence in potentials) with good reproducibility (SD=/spl plusmn/0.1 mV). The proposed sensor revealed good selectivity for Pb (II) over a wide variety of other metal ions. It can be used as an indicator electrode in the potentiometric titration of lead ions, with EDTA, oxalate, chromate, and hydroxide ions, and in direct determination of lead in a wastewater sample.

Perchlorate-selective polymeric membrane electrode based on a cobaloxime as a suitable carrier

A cobaloxime ([chlorobis(dimethylglyoximeato)(triphenylphosphine)] cobalt (III), [Co(dmgH) 2pph 3Cl]) incorporated in a plasticized poly(vinyl chloride) membrane was used to develop a perchlorate-selective electrode. The influence of membrane composition on the electrode response was studied. The electrode exhibits a Nernstian response over the perchlorate concentration range 1.0 × 10 −6 to 1 × 10 −1 mol l −1 with a slope of −56.8 ± 0.7 mV per decade of concentration, a detection limit of 8.3 × 10 −7, a wide working pH range (3–10) and a fast response time (<15 s). The electrode shows excellent selectivity towards perchlorate with respect to many common anions. The electrode was used to determine perchlorate in water and human urine.

Tricoordinate Schiff Base Copper(II) Complex as Neutral Carrier for Highly Selective Thiocyanate Electrode

A new solvent polymeric membrane electrode based on a tricoordinate Schiff base copper(II) complex as neutral carrier is described, that demonstrates excellent selectivity to thiocyanate relative to many common anions and over a wide linear range of 2.6 × 10−6–1.0 × 10−1 M with a detection limit of 8.0 × 10−7 M and a near‐Nernstian slope of −57.6 mV/decade in pH 4.0 of phosphate buffer solution at 25°C. The electrode displays an anti‐Hofmeister selectivity sequence in the following order: SCN− > Sal− > ClO4 − > I− > NO2 − > Br− > NO3 − > Cl− > SO3 2− > SO4 2− > H2PO4 −. The response mechanism is discussed in view of the AC impedance technique and the ultraviolet (UV) spectroscopy technique. The electrode was successfully applied to the determination of thiocyanate ion in wastewater and human saliva. The support of this work by the National Natural Science Foundation of China (29705001), the Chinese Education Ministry Foundation for excellent young teachers, and the Natural Science Foundation of Chongqing City, China, is gratefully acknowledged.

Enhanced sensing performance of the amperometric gas sensor by laser-patterning of the polymer membrane electrode

Ultraviolet laser patterning has been employed for enhancing the sensing performance of the electrochemical gas sensors. Two kinds of catalyst layers (platinum for CO and gold for SO 2) coated on the polytetrafluoloethylene film have been selectively removed by KrF laser irradiation. The laser fluence, repetition rate and number of shots were carefully controlled to avoid the damage of the membrane substrate. The threshold fluences for complete removal of gold film and PTFE-bonded platinum layer are about 60 and 500 mJ/cm 2, respectively. The sensors (especially the SO 2 sensors) with patterned catalyst electrodes exhibit excellent sensing capabilities in sensitivity as well as the response and recovery times. The sensitivity of the SO 2 sensor with patterned sensing electrode is about 0.63 μA/ppm, 14 times improved as compared with the unpatterned sensor. The response and recovery times are shortened from 11 and 17 s to 6 and 9 s, respectively. The signal-to-noise ratio is improved by two times. Due to the original good porous microstructure of the PTFE-bonded platinum layer, the improvement of the patterned CO sensor is not as much as that of the patterned SO 2 sensor. However, the sensitivity is doubled. The great enhancement in the sensing performance is attributed to the increase of the electrochemically active surface area and the higher diffusion efficiency.

A New Neutral Carrier for Silver Ions Based on a Bis(Thiothiazole) Derivative and its Evaluation in Membrane Electrodes

Membrane electrodes based on 2,2′-dithiobis(benzothiazole), DTBBT, as a neutral carrier for silver ions are described. Silver-selective membrane electrodes formulated with 2 wt% DTBBT ionophore and 50 mol% TFPB in an FPNPE plasticized poly (vinyl chloride) exhibited near-Nernstian responses towards silver ions (60.3±0.5mV decade−1) over a wide silver ion activity range of 0.83 µM to 94 mM. Increasing the amount of anionic sites, TFPB, to 100 or 150 mol% (relative to the DTBBT weight) resulted in super-Nernstian responses toward silver ions. Membrane electrodes prepared using a low dielectric constant plasticizer, however, exhibited sub-Nernstian responses. Polymer membrane electrodes with optimal composition (i.e., 2 wt% DTBBT, 50 mol% TFPB in FPNPE plasticized poly(vinyl chloride)) exhibited high potentiometric selectivity towards silver ions over alkali, alkaline earth, transition metal ions, as well as heavy metal ions such as Hg2+ and Pb2+. A good correlation was found between the potentiometric selectivity coefficients and the change in the UV-visible spectra of the ionophore upon exposure to different metal ions. The overall performance of the silver-selective membrane electrodes based on DTBBT ionophore, which is available at low cost, was found to be comparable to the performance of silver electrodes prepared with Fluka silver ionophore-IV. A DTBBT-based silver electrode was used as an indicator electrode for titrations of silver ions using standard sodium chloride solutions. Sharp inflections occur at the end point, and the data obtained showed 99.4% recovery with a standard deviation of 0.7% (n=3). In addition, the applicability of the DTBBT-based silver-selective electrode is illustrated by measuring the silver concentrations in natural water spiked with silver nitrate and by analyzing the silver in electroplating wastewater samples. The results obtained utilizing a DTBBT-based silver electrode showed very good agreement with the standard methods of analysis.

A novel polymeric membrane electrode for the potentiometric analysis of Cu 2+ in drinking water

Based on a novel ionophore, an ion-selective electrode has been developed for the determination of Cu 2+ in drinking water. Its selectivity behavior is characterized and its lower detection limit optimized for measurements with different electrolyte backgrounds. The lower detection limit was 2 × 10 −9 M Cu 2+ for samples with low ionic background and 1 × 10 −7 M Cu 2+ with ionic background typically found in drinking water.

Bis-dimethylaminobenzaldehyde Schiff-Base Cobalt(II) Complex as a Neutral Carrier for a Highly Selective Iodide Electrode

A new solvent polymeric membrane electrode based on N,N'-bis-(dimethylaminobenzaldehyde)-glycine cobalt(II) [Co(II)-BDMABG] as a neutral carrier is described, which displays a preferential potentiometric response to iodide ion and an anti-Hofmeister selectivity sequence in the following order: I- > ClO4- > Sal- > SCN- > NO2- > Br- > NO3- > Cl- > SO3(2-) > SO4(2-). The electrode exhibits a near-Nernstian potential linear range of 9.0 x 10(-7)-1.0 x 10(-1) M with a detection limit of 6.8 x 10(-7) M and a slope of -53.0 mV/decade in pH 2.0 of a phosphate buffer solution at 20 degrees C. The response mechanism is discussed in view of the A.C. impedance technique and the UV spectroscopy technique. The electrode was successfully applied to the determination of iodide in Jialing River and Spring in Jinyun Mountains with satisfactory results.

Application of a taste evaluation system to the monitoring of Kimchi fermentation☆

As an objective method, taste evaluation with an instrument is able to supplement the subjective sensory evaluation and to be applied to the optimization of food processing. Kimchi, a Korean traditional pickle fermented with lactic acid bacteria, is expanding its consumption worldwide. The fermentation control of it has been routinely done by measuring titratable acidity and pH. In this study, an eight-channel taste evaluation system was prepared, followed by an application to the monitoring of Kimchi fermentation. Eight polymer membranes which individually responded to cationic or anionic substances were prepared by mixing electroactive materials such as tri-n-octylmethylammonium chloride, bis(2-ethylhexyl)sebacate as the plasticizer and polyvinyl chloride in the ratio of 1:66:33. Each membrane prepared was separately installed onto the sensitive area of an ion-selective electrode to produce the respective taste sensor. The eight-channel sensor array and a double junction reference electrode were connected to a 16-channel high input impedance amplifier. The amplified sensor signals were stored to a personal computer via a multi-channel A/D converter. Two sensor groups composed of the cation-selective and anion-selective polymer membrane electrodes showed characteristic concentration-dependency to various artificial taste substances. As a whole, the response potentials of the sensor array increased during the fermentation period at 4, 10 and 25 degrees C. Even the response potentials of the anion-selective taste sensors slightly increased possibly due to the protonation of anions by liberated H+ ions, thereby leading to a decrease in the anion concentration. When the signal data were interpreted by principal component analysis (PCA), the first PC at 4 degrees C explained most of the total data variance. A close correlation was found between the values of titratable acidity and the first PC, which indicated a possible applicability of the multi-channel taste sensor of this study to the process monitoring of various pickle.

The first use of a Rh(III) complex as a novel ionophore for thiocyanate-selective polymeric membrane electrodes

The feasibility of a newly synthesized Rh(III) complex, Rh[(trpy)(bpy)Cl](PF 6) 2, as a novel ionophore for the preparation of anion-selective polymeric membrane electrodes was tested. The ionophore exhibited anti-Hofmeister behavior with enhanced potentiometric selectivity toward thiocyanate ion compared to other anions. The influence of some experimental parameters such as membrane composition, nature and amount of plasticizer and additive and concentration of internal solution on the potential response of the SCN − sensor were investigated. The electrode exhibits a Nernstian response for SCN − over a wide concentration range (1.0 × 10 −5 to 1.0 × 10 −1 M) with a slope −58.7 ± 0.5 mV per decade and a detection limit of 4.0 × 10 −6 M (0.23 ppm). It could be used in a pH range of 3.0–8.0 and has a fast response time of about 15 s. The proposed sensor was used for the determination of thiocyanate ions in real samples such as urine and saliva of smokers and nonsmokers and, as an indicator electrode, in potentiometric titrations of SCN − ion.

Activity coefficients for NH 4Cl in ethanol–water mixed solvents by electromotive force measurements

Pitzer ion-interaction model and electromotive force (emf) data of a galvanic cell without liquid junction were used for the determination of activity coefficients of NH 4Cl in different ethanol–water mixed solvents at 25 °C. The used galvanic cell is schematically represented by: Ag∣AgCl∣NH 4Cl (m), H 2O (100-w%), EtOH (w%)∣NH 4 + (ISE), in which ISE stands for ion-selective electrode and w for the mass fraction percent of ethanol. The mixed ethanol–water systems contained, respectively, 0, 10, 20, 30, 40, and 50 mass fraction percent EtOH. The experimental emf data were obtained using a solvent polymeric PVC ammonium selective membrane electrode containing a mixture of nonactin/monactin as ionophore and an Ag/AgCl electrode. Activity coefficients were determined over the molality range from dilute solution to near salt saturation (i.e., about 5.38 mol/kg in pure water or 2 mol/kg in 50% mixed EtOH–water solvent).

Solid-contact polymeric membrane electrodes with detection limits in the subnanomolar range

Ion-selective electrode (ISE) membranes with a solid contact rather than a traditional liquid inner contact are anticipated to give lower detection limits because of diminished ion fluxes. Often, however, ill-defined solid contacts give rise to instabilities and interferences by oxygen gas. Here, a poly(methyl methacrylate)/poly(decyl methacrylate) (MMA/DMA) copolymer as membrane matrix was solvent-cast on a layer of poly( n-octyl)thiophene (POT) deposited on Au as internal contact. The resulting Ca 2+- and Pb 2+-selective electrodes were not sensitive to O 2 and no evidence was found for the presence of a water film between membrane and internal contact. On the other hand, a water film was present when electropolymerized poly( n-octyl)thiophene was used. In the absence of a conducting polymer, O 2 induced a significant change in the EMF but no clear evidence was found for a water film between POT and the MMA/DMA membrane. The lower detection limit of the new Au-POT-MMA/DMA solid-contact electrodes was somewhat better than that observed with a corresponding liquid-contact ISE and reached subnanomolar levels. Importantly, the response times for the solid-contact ISE are, at low concentrations, much faster than for the liquid-contact ISE.