164 PHYSIOLOGICAL CHEMISTRY. 



phenol-phthalein. Assuming it in solution along with very weak acids 

 (but stronger than itself of course) it must exist in the practically undis- 

 sociated colorless condition. If we are measuring acids we have the choice 

 of alkalies and may therefore use one of suitable strength, for example 

 sodium hydroxide. This added to the mixture neutralizes the stronger 

 acid first, and the least dissociated or weakest, phenol-phthalein, last. 

 Even the very weak organic acids and carbonic acid take precedence over 

 the indicator in this respect. Finally, as soon as the last trace of other 

 acid is combined and the first sodium-phenol-phthalein molecules formed 

 ionization takes place and color appears. Because of its extreme weak- 

 ness as an acid phenol-phthalein may be used in this way in the titration 

 of a wide range of acid substances. It may be used in titrating the acid 

 combined with very weak bases also, for example the SO* in alum, as the 

 separated base has no effect on the indicator. 



A1 2 (SO4) 3 + 6NaOH 2A1O 3 H 3 



Quite analogous to this in principle is the titration of the protein com- 

 bination of hydrochloric acid, where the separated protein, like the alu- 

 minum hydroxide is without action on the indicator. 



Prot. HC1 + NaOH = Prot. + NaCl + H 2 O. 



This reaction will be studied more fully later. 



Methyl orange and several related bodies are employed because they 

 exhibit the opposite behavior. Methyl orange is represented by the for- 

 mula (CH 3 ) 2 NC 6 HJS[ : N C 6 H 4 SO 3 Na. The free sulphonic acid is known 

 as helianthin and has the formula (CH 3 ) 2 NC 6 HJSr : NCeH^SOaH. Its 

 behavior is the same as that of the methyl orange. Finally, the free base, 

 without the sulphonic acid group, (CH 3 ) 2 NC 6 H 4 N : NC 6 H B , is now largely 

 used. This is the dimethylaminoazobenzene. Its behavior shows that the 

 HSO 3 group has nothing to do with the action of methyl orange, but it 

 is not yet clear on what group the basic property of the indicator depends. 

 Granting, however, its behavior as a weak base we have these considera- 

 tions to notice: 



With weak acids it forms extremely unstable salts and therefore cannot 

 be used in the titration of such acids. Carbonic acid is practically inert 

 with it. But bases, even very weak ones, are able to displace it from its 

 combinations with acids, just as weak acids displace phenol-phthalein. 

 Weak ammonia, for example, which combines imperfectly with phenol- 

 phthalein, is strong enough to react with the acid combinations of the 

 dimethylaminoazobenzene or methyl orange. Protein in the so-called acid 

 albumin combination, in which the protein is really basic in character, is 

 stronger than the indicator and able to displace it from its salts. If we 

 add a weak alkali to a solution of the red salt of methyl orange the color 

 changes immediately on the neutralization of any free acid which may be 

 present. The yellow color of the undissociated base takes the place of 

 the red of the salt or free ion. With the very weak solutions of the 

 indicator used the merest drop of alkali should be sufficient to bring about 



