64 



HYDROGEN ION CONCENTRATION 



on the addition of A1+++ salts; that concentration of the A1+++ 

 salt at which this change occurred was called the isoelectric point. 

 This conception was introduced by Hardy^*^ for proteins, when he 

 showed that they are negatively charged in alkaline solution and 

 positively in acid solution. According to Hardy, although it was 

 not specifically emphasized, the isoelectric point of protein was 



10 



05 



■16 



IqgH 



■10 



*1 



Fig. 9. Dissociation-residue curves for two ampholytes in each of which 

 kaX kb = 10~'*, but curve 1 is for the case where ka = kb = 10~^, while curve 2 

 (dotted) is for the case where ka = 10"^ and kb = 10~^^. 



Fig. 10. The dissociation-residue curve (pa) of an acid whose ka = 10~^ 

 also of a base whose kb = 10~^ likewise. If curve pa is followed to its intersec- 

 tion with pb and then pb is followed downward, a curve is obtained which is 



almost identical with curve 1 in figure 9. 



equal to the reaction of neutrality. The definition of the isoelectric 

 point of an ampholyte as the maximum of the p-curve as given 

 above was developed by Michaelis in 1910. ^*' 



The maximum of the p-curve can also be analytically defined. 



36 W. B. Hardy, Journ. of Physiol. 33, 251 (1905). 



