The dialysis (from the Greek dialysis = resolution) is a concentration-driven membrane process (see membrane ), by which one can remove very small particles (typically ions or small molecules) from solutions.
The dialysis is a method of work in the chemical laboratory. It is used to exchange small molecules and ions , such as salts or sugars, while large molecules, such as proteins or nucleic acids , are retained.
In a dialysis method that is frequently used, a thin-walled tube made of acetyl cellulose swollen in water is used, which is vaguely reminiscent of a sausage skin. One end of a piece of tubing is tied and then filled with a solution that contains proteins or nucleic acids, for example . Then the other end of the hose is also knotted so that the solution is enclosed. The hose is now gently swiveled in a much larger amount of a certain salt or buffer solution and left there.
After one night, approximately the same salt concentration has established itself inside the hose as was present in the external salt solution into which the hose was previously introduced. The proteins or nucleic acids have remained in the tube, since only molecules with a molar mass below the pore diameter (this can be between approx. 3000 and 50,000 Daltons for ultrafiltration membranes, depending on the intended use ) can pass through the wall of the tube (see osmosis ). Such a membrane is therefore called selectively permeable (only permeable to certain particles, in this case a certain size). Therefore, the proteins or nucleic acids could not leave the tube, the salt concentrations of both solutions could equalize due to the smaller size of the ions. Since the volume of the solution outside the tube was considerably larger, its salt concentration is also set inside the tube. In this way, the concentration of the undesired salts originally present can be reduced to a negligible level.
- → see main article electrodialysis
If one uses electrical voltages as an aid to separate electrically charged species in a targeted manner, one speaks of electrodialysis .
In an electrodialysis separator, the space between two electrodes is separated by a stack of alternating anion and cation exchange membranes. Each pair of ion exchange membranes forms a separate “cell”. In technical systems, these stacks consist of more than two hundred membrane pairs. If an electrical direct voltage is applied to the electrodes, the anions migrate to the anode. The anions can simply pass the positively charged anion exchange membrane, but they are each stopped at the nearest negatively charged cation exchange membrane.
- E. Korngold, K. Kock, H. Starthmann, Desalination 24, 129-139 (1978)
- H. Strathmann, B. Bauer, HJ Rapp, CHEMTECH 1993, 17-24.