ELECTROKINETICS 



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coating on milk globules is protein, for the isoelectric point of 

 casein is 4.6. 



Isoelectric Point. — The isoelectric point was defined by 

 WilHam Hardy as the point of no migration in an electric field; 

 it became also that point on the acid-alkaline scale at which a 

 solution of albumin flocculates. Hardy found that an acid 



4r 



5 6 7 8 9 



Fig. 162. — Change in rate and direction of migration of milk globules with 

 change in pH: ordinates are calculated comparative rates of migration; abscissae 

 are pH values: the pH of fresh milk is 6.6, its isoelectric point is 4.55. 



solution of albumin migrates to the negative pole and is, there- 

 fore, positively charged, while an alkaline solution migrates to 

 the positive pole and is therefore negatively charged. He also 

 found that on both the acid and the alkaline side, a solution of 

 albumin is stable ; but that at an intermediate point, the suspen- 

 sion settles, or flocculates. Hardy stated that a colloidal solu- 

 tion is stable only when the dispersed particles possess a charge, 

 and the point of zero potential, or "point of least density of 

 electric charge," he called the isoelectric point. The particles 

 are decharged because of some change in their environment. 

 As this change is often one of acidity, that is to say, acidity is a 

 measure of it, it is convenient to express the isoelectric point in 



