LAWS OF ELECTROLYTIC DISSOCIATION 37 



the differences between the constitution of their ions and the un- 

 dissociated molecules, and the dissociation constants of all indicators 

 are of the "apparent" type. 



Those molecular species which are not acid by virtue of their 

 constitution, but which may be transformed by tautomeric change 

 into acids were designated by Hantzsch^^^ as pseudo-acids. 



The proof for this conception can be demonstrated in a way 

 analogous to that used above for the case of carbonic acid. When 

 acid is added to a red alkaline solution of phenolphthalein the first 

 change ensuing is to the red undissociated acid of form II, and 

 then secondarily the change to the colorless form I. With phenol- 

 phthalein, however, these steps occur so rapidly that they cannot 

 be distinguished. But Hantzsch succeeded in demonstrating the 

 phenomenon with suitable dyes, such as crystal violet and many 

 others. When alkali is added to a solution of one of these dyes and 

 the conductivity is determined at the same time, it is found that it 

 decreases with time, showing the disappearance in the solution of a 

 strong electrolyte. 



A simple experunent with a slowly changing indicator may be 

 performed as follows. To 100 cc. of a weakly alkaline solution, 

 such as conductivity water or a buffer solution of phosphates, is 

 added one cc. of acid fuchsin or of water blue (1:1000). The solu- 

 tion becomes gradually paler until the color reaches a definite 

 intensity depending upon the pH of the solution, at which intensity 

 it becomes stationary. At room temperature this process may take 

 one half to one hour, while at 50° but a few minutes. Acid fuchsin 

 and water blue are indicators in which the two steps described above 

 are distinctly separated. The establishment of ionic equilibrium, 

 like all other ionic reactions, must occur with great rapidity, whereas 

 the tautomeric transformation of the colored form of an indicator 

 into the colorless is a slower process. 



11. The dissociation of bases 



The same holds true for the dissociation of bases as for acids, if 

 [0H-] is substituted wherever [H+] occurs. If the molecule of a 

 base be designated as BOH, then its dissociation occurs as 



BOH ^ B+ + OH- 



28a 



See footnote, p. 97. 



