IX. ELECTROPHORESIS 277 



microscope for following the movement of particles in the electric 

 field. This method is primarily restricted to studies on particles 

 that are large enough and possess a refractive index sufficiently dif- 

 ferent from that of the suspensions medium to render them visibly 

 detectable, under light- or darkfield illumination, with a microscope. 

 In the moving-boundary method the displacement of the particles 

 in an electric field is observed, in the legs of a U tube, as a change in 

 position with time of a boundary existing between a solution contain- 

 ing the particles and another solution of, ideally, the same comi^osi- 

 tion with respect to all components except the particulate component 

 being studied. This method finds its greatest usefulness in the study 

 of the mobilities of substances in solution of such particle size and re- 

 fractive index value as to be undetectable under the microscope, 

 though it may be employed on stable suspensions of larger particles if 

 desired. The two methods may be regarded as supplementary with 

 respect to the size range of particles that may best be studied by each 

 but this is not necessarily the only or even the most important crite- 

 rion upon which a choice of method will be based. 



C. MICROELECTROPHORESIS METHOD 



1. Applications and Relative Advantages 



Measurement of the mobilities of particles such as blood cells, 

 bacteria, oil droplets, and of finely particulate solids, such as quartz, 

 glass, collodion, etc. can best be made with the microelectrophoresis 

 method. The relative advantages of the technique, whenever it can 

 be applied, may be listed as follows: 



(1) It is the only way in whicli the electrokinetic properties of certain 

 biological systems (e.g., microorganisms) can be accurately investigated. 



(2) The environment in which the particle is observed does not change 

 (lurmg the observation. This eliminates a condition of uncertainty associ- 

 ated with the moving-boundary method in which the values of pH, ionic 

 strength, and individual ion concentration may vary considerably across the 

 boundaries, i.e., the very point in the system where mobilities are being fol- 

 lowed in the U tube. 



(S) Mobility measurements may be made in solutions of very low ionic 

 strength, a condition that vastly ampUfies the uncertainties of the moving- 

 boundary method. On the other hand, mobility measurements at ionic, 

 strengths above about 0.10 should not ordinarily be attempted with the mi- 

 croelectrophoresis method because of convection current disturbances in the 



