' 



30 ELECTRIC CHARGE ON NATURAL SOLUBLE 



S. P. L. Sorensen, and in applying them to the iso-electric state 

 a restriction must be observed which has not so far been suffi- 

 ciently emphasised. Two kinds only of ionised protein particles 

 (negative and positive) , which have originated exclusively from 

 a dissociation into either H> or OH' ions, must exist in the 

 solution. The importance of this restriction in theory and 

 practical work will appear later on. 



For a given concentration of protein (or other amphoteric 

 electrolyte) let K and K ft be the acidic and basic dissociation 

 constants, C A - the concentration of anions, C K ' of cations, while 

 C H - and C H' are the concentrations of H' and OH' respectively, 

 and x that of the undissociated portion ; then the equilibrium 

 equations are : 



C^ . C H = K a . x .......... (I.) 



and 



C K .CoH = K 6 .* .... ..... (II.) 



At the iso-electric point C A = C K by definition, and by division 

 I. and II. give for the dissociation equilibrium : 



(in.) 



The behaviour of the undissociated portion at the iso-electric 

 point is of great practical interest. This portion (x) equals 

 the total concentration (n) less the concentrations of the 

 protein ions, thus :-^- 



x = n C A C K ; 

 substituting from I. and II., 



hence 



and 



n K 



I H- 



The undissociated fraction, p, is the ratio of the concentration 



