ALBUMIN : THE ISO-ELECTRIC REACTION 31 



of neutral particles to the total concentration, and is also the 

 relative measure of the undissociated part of the amphoteric 

 electrolyte. The dissociation equation of water (of which the 

 normality then remains constant at 1000/18) can be applied to 

 aqueous solutions when the concentration is not too high : 



C H . COH = K* (IV.) 



Where K^ = 

 we get 



io- 14 at 25, and, substituting -~ for C OH , 



H 



C H K. 



in which p is merely a function of C H and is at a maximum when 

 the expression 



= u 



has a minimum value. Differentiating, we get 

 du K a K b 



whence the condition for a minimum value, 



,, ,. Kft K a 



the equation ^- = ^5. 



KM, L H " 



Substituting from IV., 





= o, leads to 



Thus p has a maximum value at the iso-electric point (see 

 equation III.). 



The undissociated fraction, that is, the ratio of the number of 

 electrically neutral particles to the total concentration, is at a 

 Maximum at the iso-electric point. 



If values of p and P H are plotted on a graph, this relation is 

 clearly seen. The low concentrations of hydrions are plotted 

 in negative powers of ten, i.e., C H = IO~ PH . The number 

 PH (= log 10 C H ) is called the hydrion exponent by Sorensen, 



* For comparisons with the same protein, the above equations are 

 valid both for molecular and for weight percentage concentrations. 



