296 DAVID R. BRIGGS 



region originally occupied by the overlying liquid) and by the descend- 

 ing boundary (where the colloid component is moving into a region 

 originally occupied by this component and leaves behind a region, 

 devoid of this component, originally occupied by the colloid solution) 

 were often of quite different magnitudes has long been recognized. 

 The question as to which, if either, constitutes the true mobility has 

 been the subject of considerable discussion. It was not, however, 

 until the refractive index method was used for characterizing fully 

 the concentration gradients that develop in the boundary region that 

 these gradients could be well enough characterized so that a possible 

 quantitative treatment of the phenomenon could be given. At the 

 descending boundary, where the colloid component is moving into a 

 region of identical composition, these boundary uncertainties appear 

 to be of less importance than at the rising boundary. It is a common 

 procedure, therefore, to calculate mobilities from the boundary dis- 

 placement observed in the descending boundary, using the value of 

 X obtained on the colloid-containing solution in the calculation of E. 



The question as to what should be the proper or most suitable 

 overlying solution for making contact with the colloid-containing 

 solution at the boundary is still somewhat a matter of conjecture. 

 Burton early suggested the use of an overlying solution adjusted to 

 the same specific conductivity as that of the sol. This procedure 

 eliminates, initiall.y, the potential gradient that would otherwise occur 

 at the boundary but it may prove only a momentary advantage due 

 to diffusion and other adjustments that must subsequently take place. 

 Kruyt and van der Willigen advised the use of an ultrafiltrate of the 

 sol as the overlying liquid. This or its approximate equivalent, the 

 equilibrium dialyzate of the sol against its buffer, is the solution that 

 seems preferably employed in that it approximates most closely the 

 desired minimum of environmental change for the sol component as 

 it moves from the original boundary regions. In general, the higher 

 the ionic strength of the buffer used, the lower will be the extent of 

 occurrence of the anomalies rising from this boundary dilemma. Longs- 

 worth and Maclnnes (36,37), Dole (38), and Svensson (39) have dis- 

 cussed the behavior of some typical systems with respect to these 

 boundary uncertainties. 



When mixtures of substances are being anah^zed by the moving- 

 boundary method it is sometimes found that the relative areas under 

 the various peaks (denoting electrophoretically distinguishable com- 

 ponents) may vary with conditions of the experiment such as pH, ionic 



