Permselectivity of ions

Secondly, selectivity is not always achievable. For example, permselectivity of ion-exchanging polymer films fails at high electrolyte concentration. We have shown that even if permselectivity is not thermodynamically found, measurements on appropriate time scales in transient experiments can lead to kinetic permselectivity. To rationalise this behaviour we recall that the thermodynamic restraint, electrochemical potential, can be split into two components the electrical and chemical terms. These conditions may be satisfied on different time scales. Dependent on the relative transfer rates of ions and net neutral species, transient responses may be under electroneutrality or activity control. 

The permselectivity of ion exchange membranes can be exploited as a preconcentration technique. Thus suppose two solutions (1 and 2) are separated by a cation exchange membrane which is totally impermeable to anions. Initially solution 1 contains a ions and solution 2 contains B4" ions. It can be shown (27, 28) that at equilibrium the following relationship holds... 

Because the ratio is also proportional to the concentration ratio of ions A and B in the desalting side solution, if the properties of both ions in the solution are the same, the equivalent ratio of fluxes is divided by the equivalent ratio of both ions in the desalting side solution. The permselectivity of ion A to ion B is defined as follows,... 

Membrane Efficiency The permselectivity of an ion-exchange membrane is the ratio of the transport of electric charge through the membrane by specific ions to the total transport of electrons. Membranes are not strictly semipermeable, for coions are not completely excluded, particularly at higher feed concentrations. For example, the Donnan eqmlibrium for a univalent salt in dilute solution is ... 

In porous separators the pore radii are large compared to the size of molecules. Hence, interaction between the electrolyte and the pore walls has practically no qualitative effects on the ionic current through the separator the transport numbers of the individuaf ions have the same vafues in the pores as in the bulk electrolyte, hi swollen membranes the specific interaction between individuaf ions and macromofecufes is very pronounced. Hence, these membranes often exhibit sefectivity in the sense that different ions are affected differentfy in their migration. As a resuft, the transport numbers of the ions in the membrane differ from those in the efectrofyte outside the membrane. In the limiting case, certain types of ion are arrested completely, and the membrane is called permselective (see Chapter 5). 

Consider the same example, of determining the concentration of ions bnt now while using a membrane having ideal permselectivity (i.e., a membrane that is... 

J. Wang and T. Golden, Permselectivity and ion-exchange properties of Eastman-AQ polymers on glassy carbon electrades. Anal. Chem 61, 1397-1400 (1989). 

The extent of ion permselectivity displayed by a membrane can be expressed quantitatively by the transference numbers [88] for cations (t+) and anions (t ) within the membrane. Transference numbers can be determined potentiometrically by using a concentration cell [88], in which the membrane to be evaluated separates two electrolyte solutions that contain the same salt but at different concentrations. For a 1 1 salt, the membrane potential (E ,) is given by... 

The question hits the big unknown in explaining the cation permselectivity of the neutral carrier systems mentioned in my report. In the solvent polymeric membranes we studied, the contribution of anions to the electrical current (anion transport number) is usually negligible if hydrophilic anions (e.g., Cl ) are involved. In the presence of lipophilic anions (e.g., SCN ) there exists some contribution of anions to the total ion flux across the membrane [see Anal. Chem., 48, 1031 (1976)]. The reasons for such a behavior may be ... 

Equations (1.52), (1.53) are the quantitative expressions of the previous statements regarding permselectivity of an ion-exchange membrane. Of course, the treatment that led to (1.52), (1.53) is admissible only in the vicinity of equilibrium. Away from equilibrium treatment of the full nonlinear formulation of type (1.37)-(1.42) is required. Examples of such treatments with and without the LEN approximation will be presented in Chapters 4 and 5. 

The higher the permselectivity of a membrane, the closer is its membrane potential to the ideal equilibrium value corresponding to a complete impermeability of the membrane for co-ions. This equilibrium value... 

For the sake of simplicity, we shall assume again an ideal permselectivity of the ion-exchange membrane. As mentioned before, this assumption is practically justified, since the co-ion transport numbers in modern monopolar ion-exchange membranes, employed, e.g., in electrodialysis, are limited to a few percent. 

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... 

The most desired properties of ion-exchange membranes are high permselectivity, low electrical resistance, good mechanical and form stability, and high chemical and thermal stability. In addition to these properties bipolar membranes should have high catalytic water dissociation rates. 

The membrane permselectivity is an important parameter for determining the performance of a membrane in a certain ion-exchange membrane process. It describes the degree to which a membrane passes an ion of one charge and retains an ion of the opposite charge. The permselectivity of cation- and anion-exchange membranes can be defined by the following relations [4] ... 

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. 

Equation 5.14 indicates that the co ion concentration in the membrane and with that the permselectivity of the membrane is decreasing with salt concentration in the solution and will vanish when the salt concentration in the solution is identical to the fixed ion concentration of the membrane. 

The permselectivity of an ion-exchange membrane for different counterions is determined by the concentration and the mobility of the different ions in the membrane as indicated earlier. The concentration of the different counterions in... 

The permselectivity is the product of ion-exchange selectivity and mobility selectivity. The mobility of different ions is determined mainly by steric effects, that is, the size of the ions and the cross-linking density of the membrane [4],... 

---