116 - 



The electrode vessels with their reserve electrolytic solution 

 may sometimes be eliminated by placing the electrodes on the migration 

 medium itself^'^o (See Figure V,i+„) Under these circumstances, the 

 background solution should contain reserve, undissociated electrolyte 

 as, for example, weak acids or weak bases. Even under these circum- 

 stances, regions of low electrolyte concentration and of high resist- 

 ance often develop at the electrodes, and the potential difference per 

 unit length of the migration system decreases?. 



The migration medium is usually the background electrolytic solu- 

 tion stabilized by various porous substances^* °« These stabilizing 

 substances fall into four principal categories, powdered solids, fi- 

 brous solids such as paper, gels, and density gradients formed with 

 nonelectrolyteso (See Table V,I„) Of the various stabilization media, 

 paper has found the most extensive application5» It may be employed 

 as strips-'-^, sheets''--'-, or pads^^, (See Figures V,l+ and V,5o) It is 

 remarkably iiniformi it is of suitable density and thickness} and it 

 permits the detection and recovery of the separated substances-?. 



Powdered solids and density gradients are usually employed as 

 stabilizing media in vertical glass tubes with the electrodes in side 

 tubes^5„ Powdered solids and gels are frequently used in horizontal 

 troughs connecting the electrode vessels^o 



A great variety of background solutions has been employed^* . 

 Virtually all of these have been aqueous solutions of acids, bases, or 

 salts o Many of these solutes serve as complexing and solubilizing 

 agents and as pH buffers as well as electrolytes. The concentration 

 of the background solutions has been varied over a great range but is 



