Koji Matsusaki

Change of anion exchange membranes in an aqueous sodium hydroxide solution at high temperature

Properties of commercial anion exchange membranes were examined after the membranes had been immersed in an aqueous sodium hydroxide solution, 3.0 to 9.0 N, of high temperature, up to 75°C. The anion exchange membranes used had N-methyl-pyridinium groups or benzyl trimethylammonium groups bonded to a styrene-based copolymer as anion exchange groups, with backing polymers. Alkali deterioration of the membranes was classified: decomposition of backing polymers (added inert polymer and backing fabric) and decomposition of anion exchange groups. N-Methyl-pyridinium groups were especially easy to decompose compared with benzyl trimethylammonium groups, and lost ion-exchange ability. Benzyl trimethylammonium groups also lost ion-exchange ability and changed partially in weakly basic anion exchange groups. The anion exchange membranes with various anion exchange groups, benzyl trimethylammonium groups, triethyl-, tri-n-propyl-, tri-n-butyl-, and 1-azonia-4-azabicyclo-[2,2,2]-octane (different hydrophobicity) were prepared from base membranes of chloromethylated polysulfone, and copolymer of chloromethylstyrene and divinylvenzene with polyethylene fabric and powder. Though the membrane based on polysulfone weakened mechanically by immersion the membrane in the alkali solution, polyethylene reinforced-membrane did not change mechanically. However, with increasing chain length of alkyl groups bonded to ammonium groups, loss of the anion exchange capacity was remarkable and 1-benzyl-azonia-4-azabicyclo-[2,2,2]-octane groups were also not stable. The most stable anion exchange groups among these were benzyl trimethylammonium groups.

Composite Membranes Prepared from Cation Exchange Membranes and Polyaniline and Their Transport Properties in Electrodialysis

A cation exchange membrane was modified with polyaniline by polymerizing aniline with ammonium peroxodisulfate on the membrane surfaces, producing a membrane with polyaniline layers on both surfaces or a membrane with a single polyaniline layer on the surface. The modified membranes, composite membranes, showed sodium ion permselectivity in electrodialysis compared with divalent cations at an optimum polymerization time. The electronic conductivity of dry membranes showed a maximum (ca. 5 {times} 10{sup {minus}3} S/cm) at the same polymerization time as the time to attain a maximum value of the sodium ion permselectivity. Because emeraldine-based polyaniline is conductive and has a cationic charge, the sodium ion permselectivity is based on the difference in the electrostatic repulsion forces of the cationic charge on the membrane surface of a desalting side to divalent cations and sodium ions. In fact, the selective permeation of sodium ions appeared only when the layer faced the desalting side of the membrane, and was affected by dissociation of polyaniline. Further oxidized polyaniline, pernigraniline-based polyaniline, did not affect the permselectivity between cations, and the diffusion coefficient of neutral molecules, urea, increased with increasing polymerization time. Sodium ion permselectivity was maintained with repeated electrodialysis.

Interaction between Anionic Polyelectrolytes and Anion Exchange Membranes and Change in Membrane Properties

Abstract Electrodialytic transport properties of anion exchange membranes were measured after formation of anionic polyelectrolyte layers on the membrane surfaces: relative transport number of various anions to chloride ions, current efficiency and apparent diffusion coefficients of neutral molecules. The anionic polyelectrolyte layers were formed by immersing the membrane into an aqueous solution of polycondensation product of sodium naphthalene sulfonate and formaldehyde or polystyrene sulfonic acid. The change in the relative transport number between anions was remarkable in the anion exchange membrane with high ion exchange capacity by forming the layer. Results were: the relative transport number of sulfate ions to chloride ions decreased and those of nitrate ions to chloride ions, fluoride ions to chloride ions and bromide ions to chloride ions increased compared with the corresponding membrane. Although the apparent diffusion coefficient of neutral molecules suggested clogging of the membrane pores by the polyelectrolyte, anions with higher hydrated ionic diameter were able to permeate through the membrane easily. This means that difference of electrostatic repulsion force against two anions is effective on the change in the relative transport number of anions.

Preparation and properties of anion exchange membranes having pyridinium or pyridinium derivatives as anion exchange groups

Anion exchange membranes with pyridinum groups and various pyridinium derivative groups were prepared from a copolymer membrane composed of chloromethylstyrene and divinylbenzene, and pyridine and pyridine derivatives. The anion exchange membranes obtained showed excellent electrochemical properties in electrodialysis. The transport numbers of sulfate ions, bromide ions, nitrate ions, and fluoride ions relative to chloride ions were evaluated in connection with the species of a substituent and the position of the substituent in the pyridinium groups. In general, when a hydrophilic substituent (methanol groups) existed at the 2-position of the pyridinium groups, nitrate ions and bromide ions, which are less hydrated, permeated through the membranes with difficulty, and sulfate ions permeated selectively through the membranes. On the other hand, when hydrophobic groups, for example, ethyl groups, existed at the 2-position of the pyridinium groups, bromide ions and nitrate ionspermeated selectively through the membranes and fluoride ions had difficulty permeating through the membranes. The carbon number of the alkyl chain of 4-alkyl pyridinium groups also affected permeation of nitrate ions and bromide ions due to the change in hydrophilicity of the membranes. Though the hydration of the anions and the species of the substituent at the 2-position of the pyridinium groups were related to selective permeation of the anion through the membranes, permeation of sulfate ions was not as sensitive to the hydrophilicity of the membranes.

Transport numbers of various anions relative to chloride ions in modified anion-exchange membranes during electrodialysis

Modified anion-exchange membranes were prepared by the reaction of a copolymer membrane composed of chloromethylstyrene and divinylbenzene with polyethylenepolyamines such as ethylenediamine, tetraethylenepentamine and polyethyleneimine (molecular weight: 70000) and then trimethylamine. The change in transport numbers of various anions such as bromide, nitrate, sulfate and fluoride, relative to chloride ions, was evaluated by electrodialysis of the mixed solution of the respective sodium salt and sodium chloride. In general, the anion-exchange membranes, modified with polyethylenepolyamines, selectively permeated nitrate and bromide ions rather than chloride ions, compared with the anion-exchange membrane with benzyl trimethylammonium groups though the degree of change differed with the nature of the polyethylenepolyamines. On the other hand, fluoride ions and sulfate ions were difficult to permeate through the modified membranes. In particular, sulfate ions were difficult to permeate through the anion-exchange membrane which had been reacted with tetraethylenepentamine. The change in the relative transport number is mainly related to the increase or decrease in affinity of the respective anions to the modified membranes, which implies a change in the hydrophilicity of the membranes relative to the hydration of the anions.

Transport properties of phosphonic acid and sulfonic acid cation exchange membranes

Phosphonic acid and sulfonic acid cation exchange membranes were prepared from copolymers of chloromethylstyrene and divinylbenzene or styrene and divinylbenzene. The electrodialytic transport properties, especially the relative transport number between two cations and the current efficiency, were compared. Multivalent cations permeate the membranes with phosphonic acid groups more slowly than the membranes with sulfonic acid groups. When a mixed salt solution of multivalent cations and sodium ions was electrodialyzed, the current efficiency was low with the phosphonic acid membrane, due to binding of multivalent cations to phosphonic acid groups as confirmed by measuring the water content of the membranes of the alkaline earth cation form. When a cationic charged layer, which prevents permeation of multivalent cations through the membrane, was formed on the surface of the phosphonic acid membrane, the current efficiency increased and the relative transport number of multivalent cations to sodium ions decreased.

Electrodialytic transport properties of anion-exchange membranes prepared from poly(vinyl alcohol), poly(N-ethyl 4-vinylpyridinium salt) and β-cyclodextrin

Abstract Anion-exchange membranes were prepared from hydrophilic components, poly(vinyl alcohol), β-cyclodextrin and poly( N -ethyl 4-vinylpyridinium bromide) by the casting method, following the cross-linking reaction with formaldehyde. Water content of the membranes increased with increasing content of β-cyclodextrin in the membranes, which means that the membranes became more hydrophilic. The prepared anion-exchange membranes were used in the electrodialysis of various mixed salt solutions to evaluate transport numbers of various anions such as sulfate, bromide, fluoride and nitrate ions, relative to chloride ions. The transport numbers of less hydrated anions, bromide and nitrate ions, relative to chloride ions, decreased and those of strongly hydrated anions, sulfate and fluoride ions, increased with increasing content of β-cyclodextrin in the membranes. Measurements of ion-exchange equilibrium constants of sulfate or nitrate ions to chloride ions and the ratio of mobility of sulfate or nitrate ions to that of chloride ions revealed that the change in the transport numbers between anions was based on the synergistic effect of both change in the ion-exchange equilibrium constant of anions to chloride ions and that in the mobility ratio between them, which was different from behavior of other anion-exchange membranes composed of mainly aromatic or heterocyclic groups. The β-cyclodextrin played a role in giving the membranes a strongly hydrophilic atmosphere and in changing the mobility ratio between anions in the anion-exchange membranes.

Anion exchange membranes prepared by amination of cross-linked membranes having chloromethyl groups with 4-vinylpyridine and trimethylamine

Copolymer membranes composed of chloromethylstyrene and divinylbenzene were reacted with 4-vinylpyridine and then trimethylamine to prepare highly tight anion exchange membranes. When 4-vinylpyridine reacted with chloromethyl groups of the copolymer membranes, 4-vinylpyridine polymerized in the membrane matrix to form a ladder-like polymer, of which one polymer was cross-linked. Consequently, relationships of ion exchange capacity and water content to electrical resistance of the obtained membranes were completely different from those of the anion exchange membranes with benzyl trimethylammonium groups cross-linked with divinylbenzene. With increasing reaction time of the copolymer membranes with 4-vinylpyridine, the electrical resistance of the membranes markedly increased with a small decrease in ion exchange capacity and with the decrease in water content. And fixed ion concentration of the membranes reacted with 4-vinylpyridine and then trimethylamine was higher than that of the membranes with benzyl trimethylammonium groups. These are thought to be because of the formation of a ladder-like polymer in the membrane matrix and the introduction of pyridine unit, which is more hydrophobic and bulkier than trimethylamine. The remarkable decrease in the pore size of the membranes was confirmed by the measurement of the diffusion coefficient of neutral molecules, urea and glucose, through the membranes.

Removal of chloride interference in the determination of chromium by atomic absorption spectrometry with electrothermal atomization

Abstract Two mechanisms of chloride interference are described. The first arises from coordination of chloride to chromium(III), which can be prevented by addition of a masking agent such as tetraammonium—EDTA, The other is due to chloride salts remaining at the atomization step; this can be prevented by volatilizing the chlorides or converting them to oxides before atomization.

Electrodialytic transport properties of cation exchange membranes in the presence of cyclodextrins

Electrodialysis of mixed salt solutions, sodium chloride and sodium sulfate, and sodium chloride and sodium nitrate, was carried out in the presence of α-cyclodextrin using commercial anion-exchange membranes. It was confirmed by several methods that the compound existed in the membrane matrix when the membrane had been immersed in its aqueous solution, though the molecular weight of α-cyclodextrin is relatively high. In electrodialysis, sulfate ions, large and strongly hydrated anions, easily permeated through the membranes and nitrate ions, less hydrated anions, permeated with difficulty through the membranes in the presence of α-cyclodextrin. Because α-cyclodextrin is a hydrophilic compound, which has many ether and alcoholic groups, the hydrophilicity of the anion-exchange membranes is thought to increase. Thus, sulfate ions easily permeate and nitrate ions permeate with difficulty. This proves that the hydrophilicity of the anion-exchange membranes controls permselectivity between anions through the membranes.