ISO-EL. 



'ION 33 



same value. The smaller the product K a . K& the higher and 

 the more flattened the crest of the curve. When K a . K 6 = 

 io- 18 , the highest value of the ordinate, p = i, is attained, and 

 amphoteric electrolytes with K . K 6 < io- 18 give no sharp 

 iso-electric point, but a more or less wide iso-electric zone, for 

 no definite movement in the electric field occurs over a con- 

 siderable range of variation of P H . 



When, on the other hand, the value of K a . K 6 increases, the 

 curve rises to a lesser height, and, for a value of K a . K 6 == io- 12 , 

 reaches a value p = o. That is, neutral particles are no longer 

 found, as the amphoteric electrolyte is completely dissociated. 

 Consequently, to obtain a sharply defined iso-electric point, 

 it is necessary to confine the value of the product K a . K 6 

 between the limits io- 18 and 



io 



-12 



The mechanism of the iso- 

 electric reaction on the basis 

 of our deductions and the 

 necessary restrictions can be 

 illustrated as follows. Let x 

 be the neutral particles and 

 A~ and A+ the anions and 

 cations of an ampholyte due 



H^OH 



FIG. 5. Diagram ol the iso-electric 

 reaction in buffer solutions. 



to the H+ and OH~ ions present. In the case of egg albumin 

 A- > A+ and the conditions are represented by diagram I. 

 (Fig. 5). 



On addition of acid, the ionisation represented by A~H+ is 

 suppressed, until at the iso-electric point A~ = A+, resulting in 

 the effect shown by diagram II. The iso-electric condition is 

 thus characterised by a decrease in the negative ionisation of 

 the albumin A~ and an increase in the number of neutral 

 particles x. The considerations which now follow are valid 

 only for this mechanism of reaction, which, as we shall see later, 

 is not the only possible one. 



The determination of the hydrion concentration at the iso- 

 electric point leads directly only to a knowledge of the value 



J 



of ~. The aim of a complete electro-chemical characterisation 



