5H 
Phenol-Phtalein. 
(  Am.  Jour.  Pharm. 
\      Nov.,  1878. 
The  mere  fact  of  the  transition  being  from  a  colorless  solution  to  one 
of  a  high  color,  gives  it  a  great  advantage  over  any  of  the  test  solutions 
in  common  use,  and  to  this  we  may  add,  that  for  delicacy,  tenuity  and 
quickness  of  response  to  infinitesimal  amounts  of  acid  or  alkali,  it  is 
immeasurably  superior  to  them  all. 
The  substance  is  best  employed  in  solution  in  alcohol  of  20  to  25 
per  cent.,  and  of  a  1  per  cent,  solution  4  or  5  drops  are  ample  for  50  to 
100  cc.  of  the  solution  to  be  titrated.  However,  the  strength  of  the 
solution  is  immaterial,  and  the  proper  amount  to  be  added  can  readily 
be  found  by  one  or  two  trials.  In  accurate  work  it  will  always  be 
necessary  to  add  a  few  drops  of  alkali,  to  render  the  solution  exactly 
neutral  as  the  Phenol- Phtalt in,  when  dissolved,  has  an  acid  reaction. 
In  order  to  illustrate  the  delicacy,  I  will  give,  briefly,  the  results  of 
some  few  experiments  upon  it  :  0*05  gram  of  Phenol-Phtalein  dissolved 
in  25  cc.  of  95  per  cent,  alcohol,  the  acidity  counteracted  by  the  addi- 
tion of  i*75  cc.  of  decinormal  solution  of  sodium  hydrate,  when  made 
up  to  200  cc.  with  distilled  water,  contained  of  Phenol-Phtalein  in 
each  cubic  centimetre  0*00025  grarn» 
0*5  cc.  of  this  solution  =  '000125  gram  of  Phenol-Phtalein  was 
made  up  to  250  cc.  with  water,  and  '5  cc.  of  decinormal  solution  of 
sodium  hydrate,  =  *002  HNaO,  was  added  ;  a  very  distinct  pink 
color  was  given  to  the  solution,  although  it  represented  only  1  part  of 
Phenol-Phtalein  in  2,000,000  parts  of  solution. 
An  increase  in  the  amount  of  test  solution  decreased  the  amount  of 
alkali  required  to  develop  the  same  shade  of  color  as  well  as  the 
amount  of  acid  necessary  to  discharge  it.  5*  cc.  of  the  test  solution 
=  '00125  gram  of  Phenol-Phtalein,  when  made  up  to  250  cc.  with 
water  was  colored  a  brilliant  carmine  by  0*2  cc.  of  decinormal  odium 
hydrate  =  '0008  gram  of  HNaO,  or  0*02  cc.  of  normal  solution 
of  Sodium  Hydrate.  This  color  was  completely  discharged  by  the 
addition  of  0*2  cc.  of  decinormal  oxalic  acid  =  -00126  gram  of 
C2H204,  or  0*02  cc.  of  normal  solution  of  oxalic  acid.  This  pro- 
portion represented  1  part  of  the  substance  under  examination  in 
200,000  parts  of  solution. 
It  is,  of  course,  impracticable  to  work  with  such  minute  amounts, 
yet  the  experiments  serve  to  demonstrate  that  the  delicacy  of  the  indi- 
cator is  much  beyond  appreciation  by  ordinary  apparatus  and  with 
solutions  of  the  strengths  commonly  employed. 
