OF VITAL PHENOMENA 57 



Dissociation of Amphoteric Electrolytes (Ampholytes) 

 Ampholytes dissociate both H and OH ions, having therefore 

 two dissociation constants, K A and Kb. A pure solution of an 

 ampholyte has an acid reaction, if K A is the greater, and an 

 alkaline reaction if K B is the greater. The following table gives 

 the dissociation of some amino acids and dipeptides at 25 . Ac- 

 cording to these figures, mostly from Scudder (19 14), the dipep- 

 tides are dissociated to a greater extent than the amino acids 

 from which they are formed, and the dissociation of H" has in- 

 creased more than the dissociation of OH', making the dipeptides 

 more strongly acid than the amino acids. 



These dissociation constants show the relative dissociation of 

 H and OH ions by an ampholyte when dissolved in a neutral 

 solute. But the addition of an acid to the solution will reduce 

 the dissociation of H ions and the addition of a base will reduce 

 its dissociation of OH ions by the ampholyte, according to the 

 law of mas's action. At a certain reaction of the solution for 

 each ampholyte, the dissociation of H and OH ions will be 

 equal. The "remainder" of the molecule will be therefore neither 

 electronegative nor positive, but neutral or isoelectric. The re- 

 action at which this occurs is called the isoelectric point. Accord- 

 ing to Michaelis the PH of the isoelectric point is one half of 

 ( — logKw) + ( — logK A ) — ( — logKs) and the PH so calculated 

 agrees with the PH found by experiment. Isoelectric points are 

 given in above table, those of proteins in Chapter VI. 



