Water Transport Number Research Articles

Polyethylene/poly(styrene-co-divinylbenzene) sodium sulphonate membranes : Part 1. Synthesis and properties

Transport properties of cation-exchange membranes have been studied. The self-diffusion coefficients of sodium and chloride ions, the transport numbers of sodium ions and water, and the conductivities were measured in 0.1 M sodium chloride at 25°C. The concentration potentials were determined in the system 0.05/0.15 M sodium chloride. The membranes were prepared by sulphonation of oriented polyethylene (PE) film modified with 30 wt % of styrene—divinylbenzene copolymer (poly(St-co-DVB)). The copolymer was introduced by interpolymerization of the monomers within the film without its dissolution. A sequence of membranes having similar ion-exchange capacity but differing in water content was then obtained from sheets of normal PE/poly(St-co-DVB)SO 3Na membranes by expanding them by heating in water followed by a thermal treatment in air. The deviations of the measured transport properties from the behaviour of homogeneous membranes and the analysis of the Kedem—Katchalsky relationships for composite membranes have led to the conclusion that in PE/poly(St-co-DVB)SO 3Na membranes a structure prevails with a series arrangement of layers with different properties.

Electro osmosis in the system ion‐exchange membrane — sulphuric acid solution

AbstractElectro osmotic phenomena in NAFION‐120 or MRF‐26 sulfonic acid membrane bisecting the same H2SO4 solution were investigated. It was found that the transference numbers of hydrogen ions, t̄H in the NAFION membrane remain nearly constant and close to one even at a concentration of 1.8 mol/kg H2O of the external H2SO4 solution, and in MRF‐26 membrane—when the concentration reaches 1.5 mol/kg H2O. The transport numbers of water and consequently the volume electro osmotic flow in NAFION membrane change with increasing H2SO4 concentration in a similar way as t̄H, however, in MRF‐26 membrane the changes are remarkable in the whole concentration range.

The dependence of electro-osmotic flow on current density and time

The dependence of the water transport number on current density is examined for three membranes whose characteristics cover a wide spectrum: poly(vinylbenzenesulfonate), porous Vycor glass and cellulose. Experiments and theory show that non-linear volume—time plots in electro-osmotic experiments arise from displacements of the membrane in the electric field, and that reliable water transport numbers can be obtained at a given current density. When the current density is varied, experiments show that the observed water transport number can: (a) increase at low current densities because of osmotic flow superimposed on water transport by the electric field; (b) decrease at higher current densities because of accumulation of salt in the membrane; (c) decrease more at current densities near and above the limiting value because of an increased contribution of hydrogen and hydroxide ions to transport. These phenomena arise from a combination of diffusion films at both membrane—solution interfaces and from the dependence of counteflon and water transport numbers on external salt concentration.

The effect of electro-osmotic water transport on current efficiency and cell performance in chlor-alkali membrane electrolysis

A theoretical study is made of the transport processes taking place in the ion-exchange membrane serving as the “separator” in a chlor-alkali electrolysis cell. Special attention is paid to the effects of electro-osmotic water transport (EOWT). Starting from the extended Nernst—Planck flux equations, we derive expressions for characteristic quantities such as the current efficiency of alkali production, n OH, and the voltage drop over the membrane as functions of current density, external concentrations of sodium chloride and sodium hydroxide, and electro-osmotic water transport number, t o. It appears that EOWT has a favourable effect on n OH and that the effect on ohmic resistance and voltage drop is slight, whereas the transport of chloride ions to the catholyte increases. The effect of EOWT on n OH can be represented by a single parameter k o, which is proportional to t o (0≦ k o ≦1). Three regions of k o values can be distinguished, the boundaries of which depend on the external concentrations; in each region the maximum current efficiency depends in a different way on the external concentrations: in the lowest region of k o values η OH, increases linearly with k o and decreases with increasing values of C OH (concentration of NaOH in the membrane on the cathodic side); in the highest region up to η OH = 1, η OH increases linearly with k o and increases with increasing values of C Cl (concentration of NaCl in the membrane on the anodic side), whereas in the intermediate region η OH depends on k only. The concentration of NaOH in the membrane, which at maximum efficiency is constant through-out the membrane, also differs in the three regions of k o values (see Fig. 6). Making use of non-published experimental results on the uptake of water, alkali and sodium chloride by Nafion 170 and 427 membranes, we calculate values of η OH as a function of the external alkali concentration for different values of t o and compare with experimental results obtained in an electrolysis cell. It appears that t o ≌ 1 gives the best agreement between theory and experiment. The same result is deduced from experimental values of the total water transport number, by applying corrections for water of hydration carried along by the migrating ions. Calculation shows that with Nafion 170 membranes under practical conditions, i.e., concentrated alkali (≧ 10 M) and 5 M NaCl, EOWT caused the maximum current efficiency to increase from 0.45 at t o = 0 to about 0.75 or 0.80 at t o = 1; the value of k o lies in the “Intermediate region” and the (constant) concentration of NaOH in the membrane has a value about halfway between C Cl and C OH. At a current density of 3 kA/m 2 the maximum value of η OH is attained within about 2%.

Transport Numbers of Sodium and Water in Nuclepore Membranes from Membrane Potential

Transport Numbers of Sodium and Water in Nuclepore Membranes from Membrane Potential

Preferential solvation of ions and solvent transport

Abstract

Washburn numbers. Part 4.—The Erdey-Grúz experiment. Relative solvent transport numbers for ion constituents in mixtures of water with raffinose, glycine, allyl alcohol, dimethylsulphoxide and dioxan

Erdey-Gruz's diffusion experiment has been used to obtain the sum of the transport numbers of water by ions, relative to a second solvent component, Σn°w=(n+)°w+(n–)°W. The theory of the experiment is described in detail; the corrections for the effect of salt on solvent activity, lacking in Erdey-Gruz's work, have been made. The experimental method involved a diaphragm cell and analysis of changes in mixed-solvent concentration by interferometry.The Σn°w are combined with Washburn numbers, w°w, some of which are new values obtained from cells with ion-selective electrodes. With w°w=(n+)°wt+–(n–)°wt–, the ionic n°w are obtained unambiguously for systems containing low concentrations of glycine, allyl alcohol, dimethylsulphoxide (DMSO) and dioxan. These data were also calculated for a 0.75%(w/w) raffinose + water mixture using transport and diffusion data from the literature.The n°w values for the raffinose + water system, which probably approach closely the dynamic solvation number, N, of the ion in pure water, are Li+(16), Na+(10), K+(6), Cl–(4) and H+(1). In the remaining systems these figures are reduced by the competition of the organic molecule for solvation of the ion. In the glycine + water system the ions transport glycine preferentially.

Transport Properties of Nafion® Membranes in Concentrated Solution Environments

The transport properties of perfluorinated cation exchange membranes in chlor‐alkali cell solution environments have been studied using a newly designed membrane test cell. A radiotracer‐weight method was used to measure sodium ion and water transport numbers for two Nafion® perfluorosulfonate membranes. A homogeneous film shows rapidly decreasing sodium ion transport numbers with increasing catholyte concentration, reaching a value of 0.56 mol F−1 with 12.5M solution at 80°C. A fabric‐backed material with a sulfonamide exchange site layer yields considerably higher values with a more complicated concentration dependence. Water transport numbers for the latter membrane are also higher; a value of about 3 mol F−1 is found for typical experiments in which the anolyte solution is . Results are discussed in terms of the mechanism of ionic membrane transport in relation to polymer water and electrolyte sorption.

Electrochemical behavior of a snake‐cage cation exchange membrane. Membrane potential

AbstractMembrane potential differences of a polymethacrylate–polyacrylamide snake‐cage membrane were measured at 25°C. The principal theories of membrane potential have been tested. The fixed‐charge theory is only valid in a range of high membrane permselectivity. The relation developed by the kinetic approach and the thermodynamics of irreversible processes verify the experimental results, but the calculation of some parameters, often improperly assumed to be constant, is not unquestionable. Quasithermodynamics (Scatchard's relation) probably give the best estimates of the membrane potential on the basis of the independent values of transport numbers of counterion and water.

Electrolytic water-transport and ionic transport numbers

This paper gives the results of determinations of the electrolytic water-transport per faraday in the case of copper sulphate solutions of concentrations ranging from normal to 1.8-normal. The parchment diaphragm method has been employed; and difficulties experienced owing to the effect of diaphragm-polarization have been surmounted by adopting a special experimental procedure. The electrolytic water-transport per faraday remains constant with increasing concentration of copper sulphate up to normal, as the author has shown in a former paper, but it decreases regularly with further increase in concentration. The probable factors contributing to this decrease are considered. The sequence in the values of the electrolytic water-transport per faraday corresponds with that in the values of the slope of the curve connecting the Hittorf transport number with the electrolyte concentration, but the corresponding values are not in numerical agreement as required by an expression proposed by Riesenfeld and Reinhold. This discrepancy is investigated by means of a more accurate expression derived by the author. The determinations of the electrolytic water-transport are employed to correct the values of the experimental Hittorf transport number.