THE  AMERICAN 
JOURNAL  OF  PHARMACY. 
NOVEMBER,  1878. 
NOTE  on  PHENOL-PHTALEIN  as  an  INDICATOR  in 
TITRATIONS. 
By  Charles  W.  Drew,  Ph.B. 
The  want  of  a  delicate  and  reliable  indicator  for  use  in  volumetric 
determinations  of  acid  and  alkali  has  long  been  felt  by  analytical 
chemists  and  practical  pharmacists.  To  those  indicators,  such  as 
litmus,  turmeric  and  cochineal,  which  have  been  most  extensively 
employed,  there  have  been  numerous  and  valid  objections  based  upon 
the  facts  that  they  are  colored  in  both  acid  and  alkaline  solutions,  and 
that  the  transition  from  one  color  to  the  other  was  appreciably  gradual, 
and  hence  introduced  a  source  of  error  into  analytical  operations, 
which  could  be  avoided,  and  then  only  partially,  by  continued  practice 
with  uniform  solutions. 
Within  the  past  few  years  several  substances,  as  fluorescein,  rosolic 
acid,  neutral  salicylate  of  sodium  and  iron,  anthocyan,  etc.,  have  been 
proposed  as  substitutes,  but  all  have  met  with  objections  either  from 
their  rarity,  and  consequent  high  price  ;  from  the  fact  that  the  changes 
in  color  were  not  marked  at  exactly  the  point  of  saturation,  or  else  on 
the  ground  that  the  reaction  was  not  characteristic  of  acids  and  alkalies 
alone — other  substances  producing  similar  changes  in  color. 
My  attention  was  lately  directed  to  a  substance,  Phenol-Pktale'in, 
which  was  discovered  a  few  years  ago  by  Baeyer  and  whose  general 
properties  were  briefly  described  by  Dr.  E.  Luck  (<c  Ber.  d.  Deutsch. 
Chem.  Ges.,"  4,  658).  This  substance  seems  to  the  author  to,  in  a 
great  degree,  meet  the  wants  of  analysts  who  desire  an  extremely 
delicate  and  reliable  indicator. 
The  neutral  and  acid  solutions  of  this  substance  are  entirely  colorless 
and  by  the  faintest  excess  of  alkali,  an  intense  carmine  color  is  developed. 
This  color  is  discharged  with  equal  readiness  by  the  addition  of  suffi- 
cient acid  to  render  the  solution  neutral  in  reaction. 
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