Cation Exchange Membrane Nafion Research Articles

The preparation of a novel anion-exchange membrane and its application in all-vanadium redox batteries

An anion-exchange membrane was prepared using a homogeneous blend mixture of poly(ether sulfone) (PES) and poly(vinylpyrrolidone) (PVP). Advantageous properties of each component were retained in this PES–PVP membrane, such as good mechanical strength, ion selectivity, and excellent hydrophilicity. This membrane can potentially be an alternative candidate for the separator in all-vanadium redox battery systems, given its good ionic conductivity, low permeability of positively charged vanadium ions, resistive property against oxidants, and low cost. Because cross-mixing of the positive and negative electrolytes can be effectively prevented, the coulombic efficiencies were higher than 99% when using these anion-exchange membranes in all-vanadium redox batteries, which is significantly higher than those obtained using expensive Nafion cation-exchange membranes.

Novel spectroscopic sensor for the hydroxyl radical scavenging activity measurement of biological samples

A novel spectroscopic sensor was developed and validated for hydroxyl radical scavenging (HRS) activity estimation using terephthalate (TP) as probe. This sensor was designed by electrostatic immobilization of the chromogenic oxidizing agent of the CUPric Reducing Antioxidant Capacity (CUPRAC) method, Cu(II)-Neocuproine (Cu(II)-Nc) complex, on a Nafion cation-exchange membrane, and the spectrophotometric assay developed in aqueous–alcoholic solutions was integrated to the CUPRAC sensor. Hydroxyl radicals (•OH) generated from an equivalent mixture of Fe(II)+EDTA with hydrogen peroxide attacked both the probe and the •OH scavengers in 37°C-incubated solutions for 1/2h. The HRS activity was measured using the decrease in CUPRAC absorbance at 450nm – arising from the reduction of Cu(II)-Nc reagent to the Cu(I)-neocuproine chelate – of the hydroxylated probe (TP) undergoing radical attack in the presence of •OH scavengers. The HRS activity was evaluated as the second-order rate constants of biologically active compounds for •OH scavenging and also as the percentage scavenging of a measured compound or sample relative to a reference compound. Using this reaction, a kinetic approach was adopted to assess the HRS activity of amino acids, plasma- and thiol-antioxidants. This assay, applicable to small molecule antioxidants and tissue homogenates, proved to be efficient for serine and albumin for which the widely used TBARS (thiobarbituric acid-reactive substances) test is nonresponsive. Under optimal conditions, about half of the probe (TP) was converted into 2-hydroxyterephthalate (hTP), and this monohydroxylated derivative, being the only product of hydroxylation, was a more specific marker of •OH than the non-specific malondialdehyde end-product of the TBARS test. The sensor gave a linear response to scavenger concentration in the competition kinetic equation.

Water uptake and salt transport through Nafion cation-exchange membranes with different thicknesses

The water uptake of different aqueous salt solutions in different Nafion membranes, and the salt transport through those membranes under the driving force of a concentration gradient, have been studied. These experiments have been performed by using the following salts: lithium chloride, sodium chloride, potassium chloride and cesium chloride. Different homogeneous Nafion membranes, NF111, NF112, NF115 and NF117, have been used in this work, with the aim of studying the membrane thickness influence. It has been observed that the membrane water uptake increases with the membrane thickness and decreases with the size of the cation. The integral permeability coefficient has been determined from the time evolution of the salt concentration in the dilute solution. The results show that the integral permeability coefficient decreases with the membrane thickness. In general, the influence of the type of electrolyte on the integral permeability coefficient is not significant for the membranes with larger thickness (that is, NF115 and NF117). Average apparent cation transport number has been determined for the same salt solutions, and from that membrane apparent permselectivity has been estimated. In general, in membranes with large thickness, the average apparent cation transport number increases with the cation size. Finally, from fluxes and membrane potentials membrane negative ionic permeabilities have been determined finding also that they decrease with membrane thickness. Salt diffusion coefficient has been also determined from membrane negative ionic diffusion coefficient.

Study of the effects of the applied current regime and the concentration of chromic acid on the transport of Ni 2+ ions through Nafion 117 membranes

In the present work galvanostatic experiments are conducted with an electrodialysis cell in order to study the passage of Ni 2+ ions present in chromic acid solutions through a Nafion cation-exchange membrane. The competing transport between H + and Ni 2+ ions through the membranes has been studied under the imposition of under- and overlimiting currents. Results show an increasing competence between both ions when the concentration of protons exceeds that of the Ni 2+ ions. Increased nickel removal rates and current efficiency values were achieved with intensive current regimes due to the onset of electroconvection. However, at low values of the [Ni 2+]/[H +] ratio, this improved transfer through the membrane implies high values of specific energy consumption due to increased membrane voltage drops, thus decreasing the convenience of applying overlimiting currents. The obtained results are related to the values of plateau length calculated from the polarization curves of this membrane/electrolyte system and prove that Ni 2+ ions promote the arising of electroconvective vortices, whereas H + ions hamper electroconvection.

Determination of transport properties of Ni(II) through a Nafion cation-exchange membrane in chromic acid solutions

The transport properties of a cation-exchange membrane in the presence of nickel sulphate solutions were investigated by means of chronopotentiometry. The interest of this study relies on the application to the treatment of chromium plating rinse baths. The presence of chromic acid in the solutions and the initial concentration of nickel sulphate affect significantly the shape of the chronopotentiograms. The different shape of chronopotentiograms as well as the values of different parameters is thereby justified by the calculation of the speciation diagrams of Ni(II). Parameters like the limiting current density, resistance of the ohmic region, plateau length, and nickel transport numbers through the cation-exchange membrane were calculated from the chronopotentiometric and the current–voltage curves. The obtained results show an important competence between Ni 2+ and H + ions, with increases of the nickel transport numbers when the [Ni 2+]/[H +] ratio corresponding to the initial equilibrium exceeds the unity. The increase in the Ni 2+ concentration brings about a facilitated arising of electroconvection, which promotes overlimiting currents and additional mixing at the membrane surface.

Reprint of “pH tuning of Nafion ® for selective detection of tryptophan”

Selective and sensitive detection of the amino acid tryptophan is of importance in food processing, pharmaceutical formulations and in biological fluids. Electrochemical methods of detection of tryptophan are hampered by sluggish electron transfer kinetics and in complex matrices through overlapping peaks from interferents. This study examines the potential of the cation exchange membrane Nafion ® to enhance selectivity and sensitivity of this analyte through a seldom explored feature of this membrane: pH manipulation. A detailed examination of the effect of pH on the selectivity afforded by Nafion ® as a function of the analyte charge is presented. Selective detection of tryptophan and significant increases in sensitivity of its detection was observed in the presence of melatonin, dopamine and other interferents present in a pharmaceutical formulation through manipulation of the pH of the solution. At pH 3.0 at a Nafion ®-modified electrode, changes in the protonation of melatonin and tryptophan lowered the anodic potential of the analytes in a non-uniform manner increasing the peak resolution and permitting analyses with detection limits of 1.6 ± 0.1 nM and 1.6 ± 0.2 nM, respectively.

Solvent effects during multicomponent ion uptake into a Nafion cation-exchange membrane

A multicomponent ion partition coefficient model has been used to predict competitive alkali metal ion uptake into a Nafion ® cation-exchange membrane from solvents composed of methanol, methanol/water, and acetonitrile/water. The analysis was similar to that used previously for monovalent and divalent uptake and transport in Nafion, where the membrane was modeled as an array of parallel cylindrical pores of constant radius with a continuous distribution of fixed charges along the pore wall. Ion–ion electrostatic interactions, electric-field induced solvent dipole alignment, and ion solvation free energy effects were taken into account. In the present study, the model was modified to account for a different solvent composition in the membrane, as compared to that in the external solution (solvent uptake data were collected experimentally). The model accurately predicted competitive K +/Na + uptake from methanol and methanol/water mixed solvents and Cs +/K + uptake selectivities for acetonitrile/water solvents. In the latter case, model calculations suggested that some acetonitrile absorbed into the amorphous PTFE phase of Nafion, when the external acetonitrile concentration was greater than 33 vol%. The addition of water to methanol caused a drop in the potassium/sodium selectivity (to its pure water value), whereas the addition of acetonitrile to water caused the Cs +/K + selectivity to decrease.

PH tuning of Nafion ® for selective detection of tryptophan

Selective and sensitive detection of the amino acid tryptophan is of importance in food processing, pharmaceutical formulations and in biological fluids. Electrochemical methods of detection of tryptophan are hampered by sluggish electron transfer kinetics and in complex matrices through overlapping peaks from interferents. This study examines the potential of the cation exchange membrane Nafion ® to enhance selectivity and sensitivity of this analyte through a seldom explored feature of this membrane: pH manipulation. A detailed examination of the effect of pH on the selectivity afforded by Nafion ® as a function of the analyte charge is presented. Selective detection of tryptophan and significant increases in sensitivity of its detection was observed in the presence of melatonin, dopamine and other interferents present in a pharmaceutical formulation through manipulation of the pH of the solution. At pH 3.0 at a Nafion ®-modified electrode, changes in the protonation of melatonin and tryptophan lowered the anodic potential of the analytes in a non-uniform manner increasing the peak resolution and permitting analyses with detection limits of 1.6 ± 0.1 nM and 1.6 ± 0.2 nM, respectively.

Electrosynthesis of Potassium Permanganate in a Cation Exchange Membrane Cell

Potassium permanganate is produced by the electro-oxidation of alkaline potassium manganate in a divided electrolytic cell. A filter press type flow electrolyzer has been designed, fabricated, and operated with perfluoro sulfonic acid based cation exchange membrane Nafion 417, precious metal/precious metal oxide coated titanium anode, and stainless steel cathode. Optimum process parameters are evaluated and reported. Performance characteristics of coated titanium anodes are compared with conventional nickel anodes, to fix the most suitable one for the job on hand. Anode materials have been electrochemically characterized by potentiodynamic studies in alkaline potassium manganate solution. Transport properties of the membrane Nafion 417 for K+ ions and water molecules are studied using a small membrane cell operating on batch mode. It has been estimated that each K+ ion transported across the membrane carries with it 1.5 to 1.7 water molecules. The anolyte contains K2MnO4 and KOH. Studies reveal that the relative current efficiency value for electro-oxidation of manganate and the competing anodic oxygen evolution may have a direct bearing on the source of K+ ion transported across the cation exchange membrane.

Analysis of Preparation Conditions of H2SO4 and NaOH from Sodium Sulfate Solutions by Electrodialysis

Preparation of sulfuric acid and sodium hydroxide from solutions containing sodium sulfate was studied by the electrodialysis method in a three-chamber (?three-compartment) electrodialyzer equipped with two membranes, an anion-exchange membrane AESD-2a and a cation-exchange membrane Nafion 427.

Competitive absorption of quaternary ammonium and alkali metal cations into a Nafion cation-exchange membrane

The competitive absorption of salt mixtures containing tetramethylammonium (TMA) and/or tetraethylammonium (TEA) cations, respectively, into a Nafion™ 117 cation-exchange membrane, under equilibrium conditions, was measured experimentally and modelled theoretically. Uptake experiments were performed, with Nafion equilibrated in an aqueous 0.1 M nitrate salt solution containing either: (a) one of the two quaternary ammonium cations and Li +, Na +, or Cs + or (2) a mixture of TEA and TMA. Equilibrium absorption of the various two-component salt solutions in Nafion was modelled using the molecular-level partition coefficient theory developed by Bontha and Pintauro [Chem. Eng. Sci. 49 (1994) 3835]. Without the use of adjustable parameters, the model predicted accurately cation concentrations for all salt systems except Cs +/TMA. The observed and computed absorption selectivity ordering in Nafion was (CH 3CH 2) 4N +>(CH 3) 4N +>Cs +>Na +>Li +. The failure of the model to predict cation uptake for Cs +/TMA mixtures was attributed to small errors in the computation of an ion hydration parameter for TMA, coupled with the fact that the Cs + and TMA cations were nearly the same size.

Transport of U(VI), Th(IV) and lanthanides through cation exchange membrane impregnated with HDEHP-kerosene using electric field

Summary Electrodialysis has been investigated as a method to enhance the transport of U(VI), Th(IV) and lanthanides through Nafion cation exchange membrane I and impregnated with HDEHP-kerosene II. The recovery factor of U(VI) through Nafion cation exchange membrane with and without the applied electric field was 0.8 and 0.14, respectively. The transport process of U(VI) through ion exchange membrane was studied as a function of variation of nitric acid concentration in the feed solution, HDEHP concentration in the membrane stripping solution concentration, pH of the feed solution and effect of electric field. From this study the cationic flux of U(VI) through Nafion I and impregnated with HDEHP-kerosene II was 5.2 × 10−8 and 8.7 × 10−8 g eq cm−1, respectively. The technique of electrodialysis is efficient in preconcentration of U(VI) to 2.8 fold of the initial concentration. The recovery factors of lanthanide elements La(III), Eu(III) and Er(III) through the systems I and II were 0.8 in system I and 0.86, 0.8 and 0.73 in system II, respectively. The recovery factors of La(III), Eu(III), Er(III) and U(VI) through the system II were 0.82, 0.75, 0.71 and 0.27, respectively. The recovery factor of Th(IV) and U(VI) at pH = 1.5 was found to be 0.99 and 0.3, respectively.

Remote Fiber-Optic Flow Cell for the Detection of Uranium(VI) in Groundwater

Optimization and initial characterization of a remote probe for extremely low levels (<20 μg/L) of uranium(VI) in groundwater are described. The monitor takes advantage of a small length of Nafion cation exchange membrane tubing, which permits diffusion of the uranyl ion into an internally renewable reagent solution of Arsenazo III. The sensitive, colorimetric metal complexation of uranium(VI) by Arsenazo III is measured by recording the absorbance change in a fiber-optic SMA Z-cell that is coupled to a red-light-emitting diode. The entire system is packaged on a 2 × 11 in. breadboard and attached to a 100-ft power and data transmission cord. The final optimized pH of 1.5 was utilized for maximizing sensitivity to uranium while minimizing interferences from other metal ion impurities. The addition of 1 mM diethylenetriaminepentaacetic acid to the reagent further assisted in the elimination of metal ion interferences. Two Nafion membrane tubings with different internal diameters and wall thicknesses are evaluated. Using a 20 min diffusion time and the Nafion 20 membrane, the detection limit obtained for uranium(VI) in groundwater was 1.4 ppb. Of the twenty metal ion interferents examined, all gave essentially no response, with the exception of calcium and thorium, for which a 30 mg/L and a 100 μg/L solution, respectively, gave signals representative of 4.7 and 6.0 ppb uranium(VI).

Electrochemical Treatment of Industrial Wastewater Containing Phenols

The use of phenolic resins for manufacturing different textile products in a company, our case example, generates wastewater with high phenol concentration (30-400 ppm) and chemical oxygen demand (COD) between 3,000 and 15,000 ppm. Moreover, the effluent also contains high chloride concentrations ranging between 4000 and 7000 ppm, ions with a concentration of ca. 2000 ppm, and suspended solids. A study at the laboratory scale of the anodic electrochemical treatment of this industrial effluent has been carried out. The influence of variables such as current density, pH, and charge passed on phenol degradation and COD reduction was studied. As anodes, or or Pt/Ti were used. As the separator, a 117 Nafion cation exchange membrane was employed. The results showed that the final COD value was lower than 1000 ppm. After electrochemical treatment the final concentration of phenol was lower than 1 ppm, and the COD decrease was 70-80%. Unidentified aliphatic carboxylic acids and alcohols were the main products of oxidation. For electrolyses carried out at alkaline pH, the only halocompound detected in the anolyte, gas, and aqueous phases, was chloroform regardless the electric charge passed, the current density employed, and the anode used. © 2002 The Electrochemical Society. All rights reserved.

Proton Transport in Membranes Prepared from Sulfonated Polystyrene-Poly(vinylidene fluoride) Blends

The preparation of new cation-exchange membranes from polymer composites based on poly(vinylidene fluoride), sulfonated polystyrene-co-divinylbenzene, and antimonic acid is reported. The thermal properties of the composites have been characterized by differential scanning calorimetry. Values of the transport number of protons in the membranes were obtained from the observable electric potential. It is defined from the potential difference measured between the electrodes reversible to one of the constituent ions in equilibrium with the system. When compared with Nafion cation-exchange membranes, the membranes described in this work exhibit interesting proton transport properties that could make them suitable as polymer electrolytes in fuel cells. © 2001 The Electrochemical Society. All rights reserved.

An optical membrane redox chemical sensor for the determination of ascorbic acid

The redox indicator, ferroin, tris(1,10-phenanthroline)iron(II) was incorporated into the perfluorosulfonated cation-exchange membrane Nafion and, together with optical fibers, a photodiode, and a light-emitting diode, was used for the construction of a redox optical sensor (optode). The optode was incorporated in the flow-through cell of a computer-controlled sequential injection (SI) system and used for the determination of ascorbic acid (vitamin C). A linear relationship between the logarithm of the ascorbic acid concentration and the membrane absorbance was obtained for the concentration range from 2.50 × 10−3 to 1.00 × 10−1 mol L−1 and a sample frequency of 17 h−1. The concentration of ascorbic acid determined in a number of commercial pharmaceutical products agreed well with iodometric titration results. This result shows that the SI system incorporating the ferroin/Nafion optode is suitable for the automated analysis of ascorbic acid in pharmaceutical products. © 2000 John Wiley & Sons, Inc. Lab Robotics and Automation 12:200–204, 2000

Divalent/monovalent cation uptake selectivity in a Nafion cation-exchange membrane: Experimental and modeling studies

The equilibrium uptake of monovalent/divalent cation salt mixtures by a Nafion perfluorosulfonic acid cation-exchange membrane has been investigated using experimental measurements and a theoretical model. Membrane concentrations were determined for Nafion equilibrated in 0.15 M solutions containing Ni 2+, Cu 2+, Ca 2+, or Mg 2+ with a co-absorbed monovalent cation (Li +, K +, or Cs +). An equilibrium ion absorption model that accounts for ion hydration effects, the dielectric saturation of water molecules in a membrane pore, and the neutralization of fixed-charges by ion pairing with divalent cations, was matched to the experimental data. When the extent of ion-pair formation was used as an adjustable parameter, the model predicted accurately both the monovalent and divalent cation concentrations (with an average error of 7.7%). Both theory and experiments showed that the monovalent cation selectivity was in the same order as observed previously during the uptake of monovalent/monovalent cation salt mixtures (i.e. the monovalent cation with the larger hard sphere radius was preferentially absorbed). The computed mobile divalent cation concentration in a Nafion pore was found to be dependent on the extent of monovalent cation absorption. The number of divalent cation/sulfonate fixed-charge-site ion pairs was found to be independent of the divalent cation type, but was controlled by the type and concentration of the co-absorbed monovalent cation. The fraction of ion-paired fixed charges was correlated with the membrane pore concentration of mobile divalent cations via a Frumkin adsorption isotherm.

A Renewable-Reagent Fiber-Optic Sensor for Measurement of High Acidities

A renewable-reagent fiber-optic HNO(3) sensor was developed for HNO(3) measurement in the 0.1-10.0 M range. The HNO(3) sensor employs a tubular Nafion cation-exchange membrane to extract acid species from an external HNO(3) sample into an internal flowing reagent solution. In high-concentration HNO(3) samples, incomplete HNO(3) dissociation results in a significant concentration of neutral HNO(3) species in addition to protons. As both neutrals and protons are potentially membrane-permeable species, various reagent compositions were tested to examine the contributions of both acid transfer mechanisms. Continuous reagent flow limited internal acid accumulation and transferred reagent to the sensor optical detection cell. All reagent compositions included cresol red as a colorimetric indicator, which was measured within the sensor detection cell. Careful fiber-optic alignment provided sufficient light throughput in a backscatter illumination mode to allow use of a photodiode array detector for visible spectral acquisition. The use of Ca(2+) as a reagent countercation produced notable reductions in HNO(3) sensor response to interferent cations and temperature changes. Sensor measurement of HNO(3) samples in the tested concentration range produced average relative standard deviations of less than 0.4%. Control over reagent flow rate should allow for extension of the HNO(3) sensor measurement range to 16.0 M HNO(3).

Equilibrium Partitioning of Monovalent/Divalent Cation-Salt Mixtures in Nafion Cation-Exchange Membranes

Equilibrium Partitioning of Monovalent/Divalent Cation-Salt Mixtures in Nafion Cation-Exchange Membranes

A micro carbon electrode for nitric oxide monitoring.

A nitric oxide (NO) probe, consisting of a micro carbon fiber working electrode, 10 microns diameter, a platinum counter electrode, and a silver/silver chloride (Ag/AgCl) reference electrode, has been developed. The carbon fiber working electrode is covered with a Nafion cation exchange membrane. Using differential pulse voltammetry (DPV), we found the NO to N2O reduction current peak at approximately -1.35 V versus Ag/AgCl. This has been reported by others. The DPV current outputs are linearly related to dissolved NO concentrations [NO] in the 2-10 microM range. Catecholamines were found not to interfere with the reduction signal. The Nafion membrane also prevents interference by NO2-, NO3-, and amino acids at normal physiologic pH (pH 7.4). The effects of O2 are accounted for through sampling and subtracting background currents from the peak current. To increase sensitivity and shorten response time, a method of integrated pulse amperometry (IPA) was used for the study. The IPA charge outputs (delta C) are linear to the dissolved [NO] in the 50-350 nM range. The carbon fiber electrode has the potential of being miniaturized to a smaller electrode, allowing detection of NO released from the subendothelial space.