144 HYDROGEN ION CONCENTRATION 



41. The influence of salt-formation upon the isoelectric point 



Of particular importance in this coimection is the definition of 

 the isoelectric point. Earlier in this book it could well be defined as: 



•=V^" 



I P = [H+] for p max 



But tliis definition is only justified in the absence of true salts of 

 the ampholyte. The logical general definition of the isoelectric 

 point is given as that hydrion concentration at which an equal 

 number of positive and negative amphotyte ions are present. In 

 order to transfer this definition analytically to this particular case 

 it is necessary to introduce the conception of the degree of charge, 

 X. By means of quantitative transference experiments it can be 

 determined how much of the ampholyte is transported in a current 

 of definite intensity, in a unit of time and in the direction o the 

 current, while the amount migrating against the current is calculated 

 as a negative quantity. When the ampholyte is solely in the form 

 of cations, then its transference is maximal. 



In general, only a fraction of this maximal amount is transferred. 

 It is this fraction that we shall designate as the degree of charge X. 

 It is primarily dependent upon the hydrion concentration and also 

 upon true salt formation by the ampholyte. It must be expressed as 



= a — a 



X may be either positive or negative. The isoelectric point can now 

 be defined as that [H+] at which a = a , and therefore, when X = 0. 

 The calculation of X is based upon the following relations (compare 

 with (2) and (1), page 142). 



a = 



a • ki 

 oh' 



. k 



a 



Hence 



^ a • kb a • ka 



