Dialysis is the removal from the colloidal solutions and solutions of polymers of low-molecular substances that can penetrate through some natural (bull bladder, vegetable parchment) and artificial (cellophane, collodion) membranes. Dialysis devices are called dialyzers. The simplest of them, the Graham dialyzer (Fig. 1), is a glass cylinder, the lower part of which is tightened by a dialysis membrane. The cylinder is filled with a colloidal solution and immersed in a vessel with liquid against which dialysis is performed. Low-molecular substances penetrate through the membrane into the external liquid and when changing it are removed from the colloidal solution. For colloidal particles, dialysis membranes are impenetrable. In the electrodialyzer (Fig. 2), the process of dialysis (electrodialysis) is significantly accelerated, since under the action of an electric field, the ions of low-molecular substances move directionally and leave the dialyzed fluid more quickly.
Fig. 1. Graham dialyzer (arrows indicate the direction of the current of the fluid): 1 - cylinder; 2 - dialysis membrane; 3 - a vessel with a solvent. Fig. 2. Electrodialyzer: 1 - electrodes connected to a constant current source; 2 - cellophane bag; 3 - dialyzed solution; 4 - flow solvent
Dialysis is widely used for the purification of proteins and other biopolymers, for the removal of toxic substances from the blood in the apparatus "artificial kidney" (see Artificial kidney ).