LAWS OF ELECTROLYTIC DISSOCIATION 69 



manifests itself in their ability to add Il-ions (to the C00~ group) 

 as well as to split off H-ions (from the NH3+-group). 



The experimentally observed fact described in the text to the effect 

 that the product ka X kb of any of the known ampholytes has never 

 been found to be equal to or greater than kw, has been recently shown 

 by Bjerrum^^* to be a necessary consequence of the above developed 

 conception. 



18. The determination of the isoelectric point 



The isoelectric point of the ordinary ampholytes (the colloidal 

 ampholytes will be dealt with later) may be determined by a variety 

 of methods. 



1. The first method is especially applicable to easily soluble 

 ampholytes and is based on the following principle. When acid is 

 added to a dilute buffer solution, i.e., to a solution of a definite 

 [H+] but of considerable buffer value, the [H+] rises, and on the 

 addition of a base to the same solution the [H+] falls. Let us desig- 

 nate again that [H+] which corresponds to the isoelectric point of 

 an ampholyte by the symbol I. Then the ampholyte in solution 

 will behave as an acid as long as [H+] < I, and as a base when 

 [H+] > I, In the first case, if added in suffi.cient concentration, it 

 will raise the [H+] of the buffer solution, and in the second case it 

 will diminish it. Only at its isoelectric point does the ampholyte 

 remain without any effect upon the buffer, for then it behaves prac- 

 tically as a non-electrolyte; or stated more exactly, because the very 

 few ions yielded by it consist of equal numbers of anions and 

 cations. The number of H-ions cannot, therefore, be increased in 

 the buffer solution on the addition of the isoelectric ampholyte, 

 because there are no corresponding number of negative ions to 

 maintain the electro-neutrality, and for a similar reason the [H+] 

 cannot be diminished. The following experiment-^^ is quoted in 

 corroboration of this theory: A constant amount of an aqueous 

 solution of phenylalanine was added to each of a series of buffer 

 solutions consisting of a constant small amount of sodium acetate 

 and increasing amounts of acetic acid. The pH of these solutions, 

 determined electrometrically, is given in table 13. 



The same could be demonstrated for glycocoll. Its isoelectric 



" L. Michaelis, Biochera. Zeitschr. 47, 250 (1912). 



