276 DAVID R. BRIGGS 



and the ionic strength is high enough so that the thickness of the 

 double layer is small compared to the radius of curvature of the par- 

 ticle. Under these conditions shape of the particle will not affect 

 its mobility. 



It is understandable that, in order to calculate the value of the 

 net charge (or the net charge per unit of interface i.e., charge density) 

 carried by a particle and the value of the electrokinetic potential 

 existing in the region of the interface from mobility measurements, 

 considerable knowledge must be available as to size and shape of the 

 particle and as to the theoretical relationship existing between the 

 thickness of the double layer and the ionic strength of the solution. 

 Such relationships are complex. Fortunately, electrophoretic mo- 

 bihty measurements can yield much valuable information without 

 the necessity of obtaining values of net charge or of the electrokinetic 

 potential therefrom. In solutions of variable environment (variable 

 pH, ionic strength, etc.), for example, particles that migrate always 

 with equal mobilities may safely be considered to possess equal 

 charge density and zeta-potential characteristics and, therefore, can 

 to a high degree of probabihty be considered as identical in shape, 

 size, and surface characteristics, generally. Particles that show dif- 

 ferent mobilities in contact with a solution of common environmental 

 characteristics must, in contrast, possess different surface properties 

 and cannot be of identical surface compositions It is in this regard 

 that electrophoresis experiments have proved most useful in bio- 

 physics ; this method constitutes one means of characterizing a given 

 biochemical preparation as to homogeneity or heterogeneity. Thus, 

 from comparisons of the electrophoretic mobility alone, as obtained 

 imder standardized experimental conditions, considerable informa- 

 tion can be obtained as to the physical nature of the charged particu- 

 late material being studied. As a corollary, electrophoresis can serve 

 as a means of separating, in solution, materials of different surface 

 properties and can thus serve as a preparative method for substances 

 that might be difficult to sejoarate from impurities by other means. 



There are two methods in general use for the determination of the 

 electrophoretic mobility of charged particles dispersed in a liquid, 

 namely, the microelectrophoresis method and the macroelectrophore- 

 sis, U-tube, or moving-boundary method. Each method has certain 

 advantages over the other and also certain limiting conditions under 

 which it may be most successfully employed. 



As the name implies, the microelectrophoresis method employs a 



