LAWS OF ELECTROLYTIC DISSOCIATION 



63 



of ka X kb the maximum rises less and less, and the elevation zone 

 becomes narrower. Thus for ka X kb = 10~^^ = kw the maximum 

 value is around only 1/3. However, such ampholytes and those 

 with a still greater ka X kb product are not known to exist. 



The p-curves of all electrolytes having the same ka X kb value 

 are parallel and are but horizontally shifted, as it were, on a common 

 abscissa, as shown in figure 9. In each of such p-curves an ascend- 

 ing and a descending arm are differentiated. The former is almost 

 exactly the p-curve of an acid whose dissociation constant is ka, 



the latter is an a-curve of an acid whose constant is — i.e., a base 



kb 



whose dissociation constant is kb. This relation is practically true 



whenever ka X kb is very small. It follows then that the p-curve 



1.3 



0.5 



■is 



j£9fL 



-10 



-1 0*1 



Fig. 8. Dissociation-residue curves for various amphoteric electrolytes of 

 diflferent ka X kb values. Each curve is marked with its appropriate ka X kb 

 value. It is assumed throughout that ka = kb. 



of an ampholyte may be constructed by considering it in the first 

 place as an acid with the constant ka and secondly as a base with 

 the constant kb and then by uniting the two respective p-curves. 

 If, however, ka X kb is larger, then this method is not exact, as 

 may be seen from the comparison of the curves in figures 9 and 10. 

 The deviation is greatest in the vicinity of the maximum, but the 

 abscissa value corresponding to the maximum remains at least 

 unchanged. 



The hydrion concentration corresponding to the maxunum of 

 the dissociation-residue curve is designated as the isoelectric point 

 of the ampholyte. This term was originally used to designate the 

 point at which the sign of the charge of a colloid changed. For 

 example, the negatively charged gold sol becomes positively charged 



