Dialysis

DIALYSIS, the liberation of colloids from crystalloid impurities by means of the diffusion of the latter through membranes impermeable to colloids. One of the most important differences between colloids and crystalloids, which led Graham to distinguish between these two groups, is their unequal behavior both in free diffusion and especially in diffusion through membranes. Crystalloids pass freely through various membranes of animal or vegetable origin (e.g., parchment, pigs' bladder, fish swim bladders, etc.), whereas colloids are retained by them. Based on this, Graham proposed a convenient method for separating colloids and crystalloids. He called this method dialysis and the apparatus used for it a dialyzer. Dialyzers. Graham's dialyzer is a cylindrical ring on the lower side of which a membrane made of parchment or similar material is stretched and tightly tied. The ring, with the liquid for dialysis poured into it, floats on the surface of a vessel with distilled water (Fig. 1). If, for example, an aqueous solution of sugar and gum arabic is placed in the ring, the latter will be retained by the parchment, while the sugar will diffuse into the water. If the water is changed frequently enough, practically all the sugar can be extracted from the mixture by prolonged dialysis. Nowadays, very diverse designs of dialyzers are used. Instead of natural membranes, artificial membranes are often used for them. Membranes prepared from collodion present very great conveniences. Firstly, depending on the shape of the vessel on whose walls the collodion is deposited, they can be given any desired shape. On the other hand, depending on the method of preparation, collodion membranes can be made more or less permeable. To prepare a membrane, collodion dissolved in a mixture of absolute alcohol and ether is usually used. The prepared membrane is dried in the air for a short time, then immersed in water. Longer drying makes the collodion membrane less permeable; the use of collodion solutions containing a higher percentage of ether and a lower percentage of alcohol leads to the same result. The initial concept drew a sharp line between colloids and crystalloids; at present, successive transitions between these two states of matter are known.

Figure 2.

Therefore, by carefully grading the permeability of the membrane using the indicated or other methods, it is possible not only to separate crystalloids from colloids, but in some cases even to fractionate the latter. In addition to the material (which is most often collodion or parchment), the shape of the dialyzers is also subject to various modifications. To accelerate dialysis, it is necessary that the membrane have (for a given amount of liquid) the largest possible surface area and be washed as well as possible by fresh portions of pure water. In Kühne's dialyzer, it is a long U-shaped parchment tube immersed in running distilled water (Fig. 2). Zsigmondy's "star dialyzer" works especially quickly and economically. A cylindrical ring with a membrane (Fig. 3, B) stands in it on a support having radial partitions (A). Pure water enters through a tube in the center of the support, moves along it radially to the periphery, washing the membrane in a thin layer, and flows off along the edge of the dialyzer.

Figure 3. Star dialyzer: A - support; B - ring with membrane. Above - top view of the support.

Figure 3 (cont.).

Electrodialysis. The speed of dialysis, like that of ordinary diffusion, depends on the random molecular movements of dissolved substances. It can be increased many times over by the application of electrical forces. To do this, two electrodes are placed on both sides of the semipermeable membranes surrounding the colloidal solution, and a significant potential difference is applied to them (Fig. 4; Pauli electrodialyzer). Ions leave the middle part of the apparatus, being attracted to opposite poles, and are washed out of the near-electrode spaces by running water. This method, representing a combination of dialysis and electrolysis, was introduced into science mainly by the work of Dhéré, and then Pauli. In many cases, it gives very good and rapid results, proving to be significantly more effective than ordinary dialysis. Of course, electrodialysis serves to purify the colloid only from ions, and not from non-electrolytes. A simple homemade device for electrodialysis is described by Baer.

Compensatory dialysis. Dialysis finds application not only for purifying colloids from crystalloid impurities, but also for more precise investigation of the latter. Based solely on the chemical analysis of a colloidal solution, nothing can be said about the state in which the crystalloid substances contained in it are located. For example, the calcium contained in the blood may be chemically (or adsorptionally) bound to serum proteins, may be in a freely diffusing state in the form of some non-dissociated organic compound, or may consist of free ions. Meanwhile, the state of an electrolyte in solution (in particular, the relative content of colloidally bound, freely diffusing, and ionized fractions) is incomparably more important than its total content. It cannot be established using ordinary dialysis, nor by chemical analysis. Indeed, crystalloids, in particular electrolytes, usually form very unstable, easily decomposing compounds with colloids. During the dialysis of blood serum, not only freely diffusing calcium will pass into the dialysate: its removal, disrupting the established equilibrium, will cause further decomposition of the colloidally bound calcium, which can gradually completely diffuse through the membrane. To resolve this question, Michaelis and Rona developed the method of compensatory dialysis. Its essence lies in the fact that instead of distilled water, the colloidal solution is washed in the dialyzer by various concentrations of the substance under study. For example, to resolve the question of whether calcium is bound in the blood by serum proteins (and to what extent), blood serum is dialyzed against an isotonic salt solution to which various amounts of calcium salt have been added. The calcium content during dialysis will remain unchanged only if the external solution has the same concentration of free calcium salt as the serum. Only under this condition will the tendency of calcium to pass into the dialysate be osmotically compensated by its concentration in the latter. In this way, it was found that in horse blood serum about 35% of the calcium is bound by colloids. In the same way, the state of sugar and other substances in the blood was investigated. However, later studies forced a review of both the very principle underlying compensatory dialysis and the results obtained with its help. At first it seemed certain and obvious that upon diffusion equilibrium, each diffusing substance must be in completely identical concentrations on both sides of the membrane. However, studies by Donnan showed that the presence of colloids (more precisely, colloidal electrolytes) affects the distribution of crystalloid ions on both sides of the membrane permeable to them. Some ions are retained in the colloidal solution, while others are pushed out in excess into the dialysate. Therefore, the concentration of the studied ion in the dialysate is not always equal to the concentration of the same ion in the colloidal solution, and the figures obtained using compensatory dialysis therefore require recalculation (which is not always easy to perform). The distribution of ions on both sides of the membrane (between the colloidal solution and the dialysate) was named membrane equilibrium or Donnan equilibrium.

Viviffusion. In the interesting method of viviffusion (Fig. 5), developed by Abel and his coworkers (Rowntree, Turner), dialysis was successfully applied for the isolation and quantitative study of crystalloids directly from the circulating blood of a living animal.

Figure 5. A - severed vessel; B - collodion tubes; B - inflow and outflow of washing liquid; Γ - thermometer.

of colloids from the blood of a living organism. Blood (to which a little hirudin has been added to prevent clotting) is drawn from an artery by a cannula, passed through a series of collodion tubes immersed in a warm Ringer's solution, and returned to the bloodstream by a second cannula. Thus, while maintaining the normal course of blood circulation, blood can be passed through a collodion dialyzer for extended periods. Substances dissolved in the blood diffuse into the external Ringer's solution until equilibrium is reached (and in the case of crystalloid non-electrolytes, complete equilibrium) between their concentrations on both sides of the collodion wall. By comparing the dialyzate of blood from vessels entering and leaving an organ, one can judge the transformation of the substance under study in that organ. The same method has recently been applied for "blood washing" in humans (Haas; 1928). Blood, whose clotting has been prevented by heparin, passing through a collodion tube, is freed by dialysis of the toxic products contained in it and is then returned to the bloodstream.

D. Rubinstein.