Salt gradient dialysis

In addition to the processes discussed so far there are two more electromembrane separation processes in which the driving force is not an externally applied electrical potential but a concentration gradient. The processes are referred to as diffusion dialysis and Donnan dialysis. Diffusion dialysis is utilizing anion- or cation-exchange membranes only to separate acids and bases from mixtures with salts. Donnan dialysis can be used to exchange ions between to solutions separated by an ion-exchange membrane. Both processes have so far gained only limited practical relevance [4] and will not be discussed in this chapter. 

Electro dialysis Electric field gradient < 5 nm Dissolved salts... 

In electrodialysis, an applied electric field rather than a concentration gradient is used to draw ions across the membrane. Because it is faster than ordinary dialysis, electrodialysis is often used in biochemical analyses for purposes of fractionation, concentration, and desalting. Reverse osmosis (RO) is a process that uses semipermeable membranes to allow water permeation however, the membranes act as a barrier to the passage of dissolved and suspended particles. Typically, RO membranes are used to extract pure water from aqueous solutions of dissolved salts, such as seawater. The particle size cutoff is typically 0.0001 yum with driving pressures of 200 to 800 psi (1.4 to 5.5 MPa).61... 

Donnan dialysis is a membrane separation process that uses ion-selective membranes to prevent the flow of certain ions from one solution to another. A schematic of the process is presented in Figure 29.8. When a salt solution is separated from its corresponding acid by a cation-exchange membrane, the anions are excluded from the membrane, whUe the cations are redistributed across the membrane to attain Donnan equilibrium. By changing the salt solution periodically, it would be possible to shift the equilibrium favorably to effect simultaneous neutralization of acid on one side of the membrane (feed compartment) and acid recovery on the other side (receiver compartment). The driving force for ion migration is the chemical potential gradient for the cation across the membranes. 

As explained before, because acids selectively adsorb on strongly basic anion exchange membranes and show remarkable permeation through the membranes by concentration gradient, acids can be recovered from waste acid solutions by diffusion dialysis using anion exchange membranes (Figure 6.28, Chapter 6.4.1). Table 3.4 shows dialysis coefficients of various acids and salts, and separation... 

The flux of electrolytes through ion exchange membranes based on a difference of chemical potential is low, except for the fluxes of acids and alkalis, compared with the flux in the presence of an electrochemical potential gradient. Therefore, to separate neutral salts by diffusion dialysis is economically limited, except for special cases. 

Dialysis, a concentration gradient-driven separation process on the basis of molecular diffusion through a membrane. Dialysis is a suitable technique to exchange the buffer in which a protein of interest is dissolved or to decrease the salt concentration of a protein solution. 

Dialysis Symmetric microporous membrane, 0.1 to 10 pA pore size Concentration gradient Diffusion in convection-ffee layer Separation of salts and microsolutes from macromolecular solutions... 

Osmosis is a process of diffusion driven by a chemical potential gradient. For example, if pure liquid is separated from a salt solution by a membrane that is permeable to liquid, but not to salt, pure liquid will diffuse into the salt-rich side. This type of transport occurs in general when liquid A and B are separated by a membrane permeable only to B. A dramatic example is shown in Fig. 1 Yoldas [2] prepared a sol of alumina in water, placed it into a bag made from a dialysis membrane, and immersed the bag in alcohol as the water diffused out of the sol into the alcohol bag, the sol contracted and finally gelled. Of course, it is not necessary to have a membrane to have... 

If it is not possible to use a totally volatile mobile phase it may be necessary to de-salt a preparation following HPLC. This can be carried out simply by traditional methods such as dialysis or solvent exchange on a small column of one of the traditional gel permeation materials. However, the sample may also be diluted, adsorbed onto a small column of RP material and eluted by a rapid gradient of propanol in 0.1 % TFA, or by stepwise elution from a suitable RP sample clean-up cartridge. With microbore columns it is possible to obtain the sample in a few /d by this method (16). 

---