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STUDIES ON THE USE OF CELLULOSE TUBING FOR DIALYSIS' ARTHUR W. DEVOR and CHARLES ANDRE The Ohio State University College of Medicine, Columbus
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terms "dialyzable " and "nondialyzable" are used quite commonly in chemistry and biology t o indicate large (colloidal) and w a l l particles in a solution. It seems that too little emphasis is placed on the porosity of the membranes, although it is well known that this is an important factor. The porosity of semipermeable membranes is dependent upon the method and material by which they are prepared; this fact should be emphasized in our chemistry classrooms. During the past four years Visking cellulose tubing has been used in our laboratory for dialysis of large volumes of urine in an attempt to establish the normal value for the nondialyzable constituent of human urine. Two sizes of commonly used cellulose tubing (20/32-in. and 32/32-in.) were tried in order to find the size better suited for this purpose. A surprising observation was made: The 20/32-in. tube consistently yields only about one-half the amount of nondialyzable material as does the larger size. There was little variation in the results when different batches of the same size were used. As a result of these striking observations it was decided that the dialyzing membranes should he checked against a standard material. In an attempt to find a suitable standard, Merk dextrin was dialyzed against distilled water; the results were even more decisive. A second experiment was run on another batch of cellulose tubing; this experiment is described here. The 32/32-in. and 20/32-in. (inflated diameter) Visking cellulose tubings were soaked in distilled water for three days; each day they were washed inside and out and the water was changed a t least once a day. The tubes were then cut in lengths of 100-ml. capacity and they were filled with O.lyo Merk dextrin solution. Both sizes of tubings were treated in the same manner. The dialysis was carried out against distilled water under toluene in the refrigerator. Samples were taken periodically; the dextrin was determined by the recently developed2 presulfonated resorcinol reaction. After dialyzing for five days against an equal volume of distilled water (see table) to allow an equilibrium to be established, the first sample was taken. There was 79.6% of undialyzed dextrin in the large tubing and 59.3% of undialyzed dextrin in the small tubing. These results could indicate that the small tubing is about twice as permeable t o the dextrin as the larger tubing; the analysis on the dialyzate confirmed these findings. However, it could mean that the dialysis Supported in part by a. grant from the Comley fund. ~ D E ~ ARTHUR ~ R , w., CLYDE C , , N ~ E ~ , AND l v A G,L,,, ~ "Archives of Biochemistry and Biophysics," in press. 354
occurs at a more rapid rate. The dialysis approached completion after about 19 days during which the water was changed 9 times; an equal volume of water was need for the first change of water and then 6 liters was added in the next 9 changes after which the volume was increased to 8 liters. Only a small amount of the dextrin dialyzed out during the next 30 days. After 49 days the results (average of three samples) show that 54.5% of the dextrin was not dialyzable through the large tubing whereas 20.8% was undialyzable through the small tubing. Continuation of the experiment for 30 days longer proves that this difference is not merely a difference in rate of dialysis. Apparently some hydrolysis occurs during the dialysis-probably caused by bacteria. At least the particles seemed to be getting smaller upon standing in the refrigerator. The average pore radius for two membranes may be the same, yet one may have a few comparatively large pores so as to make it permeable to a substance which will not diffuse through the other membrane. This property difference could possibly account for the observations described in this report. The Visking cellulose tubing is an inexpensive material which is very satisfactory for dialysis. However, it seems that a check on the permeability should be made when it is used for dialysis of mixed substances which vary considerably in molecular size. Instructors should always stress two important facts when introducing the term "dialyzable": (1) the size of the
particle, (2) the size of the membrane pores. It seems that too little emphasis is placed on the second important fact, and it should he pointed out that average pore size is not necessarily significant. I n addition, one should always mention the t ". v ~ of e membrane when describing a sulktance as being "dialyzable." For example, the Merk dextrin used in the experiment described above is only 45.5% dialyzable through 32/32JOURNAL OF CHEMICAL EDUCATION
in. (inflated diameter) Visking cellulose tubing whereas the same dextrin is 79.2% dialyzable through 20/32-in. Visking cellulose tubing (tubing purchased late in 1956). Even the approximate date of purchase probably should be mentioned because changes in properties may re-
VOLUME 35, NO. 7, JULY, 1958
sult from improved methods of manufacture. It is a simple matter t o keep a standard stock of dextrin on hand and to check each batch of tubing as it is used. One only needs to allow the diffusiont o approach equilibrium and then test the dialysate.
