Ideal cation exchange membrane

These equations mean that the flux of electrolyte through the ion exchange membrane is governed by the diffusion coefficient of the co-ion, not the counterion. For example, the diffusion flux of hydrochloric acid should be equal to that of sodium chloride in an ideal cation exchange membrane. 

An ideal cation exchange membrane allows sodium cation to pass, but no hydroxy and chlor anions. The cell liquor contains only a small amount of sodium chloride so that it can be used as product caustic soda without any further treatment. In some cases the cell liquor is further concentrated according to the consumer s requirements. The quality of caustic soda produced from the membrane process is similar to that from the mercury process. 

Figure 3.4.9. Concentration profiles in electrodialysis (ED) with an ideal cation exchange membrane (profiles around the anion exchange membrane have not been shown).

The cation exchange membrane prefers cations as the counterion. In the limit of an ideal cation exchange membrane, all current in the membrane is carried by cations, i.e. here = 1. Therefore <2m = 0- Since neither ti nor <2s is zero in the solution, one observes in Figure 3.4.9 that, at steady state, the rate at which Na ions leave the solution from the left-hand side of the membrane into the membrane is given by (Ihm - Due to electroneutrality,... 

Another prototype of an ideally cation-permselective interface would be a cation-exchange membrane (C-membrane). Most practically employed C-membranes are extremely permselective, so that their polarization curves would be expected to coincide with those at electrodes (given the same... 

Formulation. Consider two unity thick diffusion layers of a mixture of 1, 1-, and 1, z-valent3 electrolytes with a common anion, adjacent to a planar ideally permselective cation-exchange membrane. Direct the axis x normally to the membrane and let x = 0 coincide with the outer boundary of the diffusion layer. The diffusion layers will thus be located at 0 < x < 1 and 1 + Aelectro-diffusional transfer of ions across the membrane and the diffusion layers is... 

An ideal permselective cation-exchange membrane would transmit positively charged ions only, that is, for a transport number of a counterion in a cation-exchange membrane is T m = 1 and the permselectivity of the membrane is xFcm = 1. The permselectivity approaches zero when the transport number within the membrane is identical to that in the electrolyte solution, that is, for T = Tc is xFcm = 0. For the anion-exchange membrane the corresponding relation holds. 

To restore electroneutrality, sodium ions are transported selectively in the electrochemical field gradient across the cation-exchange membrane from the anode to the cathode chamber. Ideally the membrane should be 100% cation permselective, therefore excluding any hydroxyl ion transport but in practice this is not the case and current efficiencies are always less than 100%. This current inefficiency is represented by the reaction of hydroxyl ions with chlorine. Patent applications for this method of chlor-alkali production appeared as early as 1949 (2). 

S STATIC BOUNDARY LAYERS C - CATION-EXCHANGE MEMBRANE A - ANION-EXCHANGE MEMBRANE FIGURE 21.2-5 Idealized representation of concentration gradients in electrodialysis. 

Because the ion exchange membrane shows ideal permselectivity at sufficiently low solution concentration, compared with the fixed ion concentration of the membrane, the difiusional flux is expressed as follows, for a cation exchange membrane... 

When a cation exchange membrane is ideal, the caustic current efficiency of the electrolyzer, oh> should be 100%, whereas the gaseous chlorine current efficiency, Cl2> is expected to be less than 100% because of (1) chlorine losses arising from the finite solubility of chlorine in the brine and the several reactions involving the dissolved chlorine ... 

In an ideal cation exchanger only the positive ion will pass through. If brine, purified previously by ion exchange to contain less than 0.1 ppm of Ca " and Mg " " ions, is used in the cell only the Na ions should pass through the membrane to give the following reactions ... 

Figure 9.10 (a) Membrane potential versus ln(ci/C2) for dense regenerated cellulose RgC (A), RgC(H3P04) (V) and RgC(NaOH) (x) with NaCI solutions, and RgC with BaCl2 solutions ( ) porous regenerated cellulose PRgC sample (0). Ideal cation exchanger... 

The ideal chlor-alkali process is one that is energy-efficient and does not use mercury. A type of cell offering these advantages is the membrane cell, in which the porous diaphragm of Figme 19-24 is replaced by a cation-exchange membrane, normally made of a fluorocarbon polymer. The membrane permits hydrated cations (Na" " and H3O ) to pass between the anode and cathode compartments but severely restricts the backflow of Cl and OH ions. As a result, the sodium hydroxide solution produced contains less than 50 ppm chloride ion contaminant. 

Equation (4.4.1b) expresses impermeability of the ideally cation-permselective interface under consideration for anions j is the unknown cationic flux (electric current density). Furthermore, (4.4.1d) asserts continuity of the electrochemical potential of cations at the interface, whereas (4.4. lg) states electro-neutrality of the interior of the interface, impenetrable for anions. Here N is a known positive constant, e.g., concentration of the fixed charges in an ion-exchanger (membrane), concentration of metal in an electrode, etc. E in (4.4.1h) is the equilibrium potential jump from the solution to the interior of the interface, given by the expression ... 

The basic structure of the battery is the same as the electrodialyzer a plurality of a pair of cation and anion exchange membranes is alternately installed to form the concentrated and dilute compartment between electrodes at both ends. Then the concentrated and dilute solutions flow into each compartment and electric power based on the membrane potential is taken out from the electrodes. Various ion exchange membranes have been examined to calculate the energy conversion efficiency.284 A maximum power would be 0.33 Wm-2/pair when 0.57moll-1 solution (concentrated stream) and 0.026 mol l-1 solution (the dilute stream) are fed into a electrodialyzer with 30 pairs of cation and anion exchange membrane (effective membrane area 232 cm2).283 Also, it is calculated to be 0.6 W m-2/pair of electric power in an ideal scale-up based on experimental data when 30 gl-1 and 3 gl-1 solutions flow into the concentrated and dilute compartments.285... 

The most widely used sensor for chloride ions in clinical analyzers is based on an ion-exchanger, a quaternary alkylammonium chloride, dispersed in a plastic membrane. It is not an ideal sensor due to the interference of lipophilic anions (e.g., salicylates, bromides) and lip-ophylic cations (e.g., bacteriostatic agents, anesthetics) and a relatively poor selectivity towards hydrogen carbonates (bicarbonates). However, compared to charged anion- and neutral carrier-based membranes that have been tested, it is still the best-suited for automated analyzers. 

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