106 L. C. CRAIG, W. KÖNIGSBERG, A. STRACHER, T. P. KING 

 of solute through a membrane is directly proportional to the cross sectional 

 area of the membrane. It is also proportional to the concentration gradient 

 and, therefore, for a given weight of solute, inversely proportional to the 

 volume inside the sac. The most rapid net rate of transfer of solute, there- 

 fore, would be given when the solution to be dialysed is spread evenly as 

 a thin film over the entire inside surface of the dialysis sac. The concentra- 

 tion outside should be kept as low as possible. This can be accomplished 

 very easily by the simple apparatus^ shown schematically in Fig. 1. 



In this cell all the solution inside is held in a thin film of the order of 

 0- 1-0-2 mm. in thickness. No stirring is required. The rate of escape of a 

 solute is measured by periodic removal of the solution outside for analysis 

 and replacement with fresh solvent. At these times the film inside can be 

 redistributed by moving the syringe plunger at the top of the cell. Actual 

 experiment showed that with this arrangement the rate of dialysis for a 

 given membrane was of the order of 10-100 fold that afforded by the usual 

 dialysis procedures. 



If the experimental arrangement is satisfactory and the solute behaves 

 ideally its rate of escape should be proportional only to the concentration 

 gradient. The rate should thus follow first order reaction kinetics, and when 

 the logarithm of the decrease in concentration is plotted against time, a 

 straight line should be obtained. 



100 



400 



200 300 

 Minutes 

 Fig. 2. Escape rate curves of bacitracin and subtilin. 



This has now been accomplished with many different solutes. Fig. 2 is 

 an example with two different peptides — bacitracin (M.W. 1,422) and sub- 

 tilin (M.W. 3,300) using 18/32 Visking cellophane. If a mixture of equal 

 parts of the two should be taken the dialysis could be interrupted after 

 50 minutes when 80% of the bacitracin would be out, but only 20% of 



