658  Titration  of  Certain  Alkaloids.        { AmseJptU,r'i92hiarm' 
A  solution  of  exactly  N/10  hydrochloric  acid  was  prepared, 
standardized  to  phenolphthalein  against  N/10  sodium  hydroxide 
(free  from  carbonate),  which  had  been  standardized  against  pure 
potassium  hydrogen  phthalate.  In  one  experiment  0.817  gm-  anhy- 
drous morphine  was  dissolved  in  28.65  cc-  N/10  hydrochloric  acid, 
and  made  up  to  107.5  cc-  witn  neutral  distilled  water  (giving  a  r 
per  cent,  solution  of  morphine  hydrochloride).  The  hydrogen  ion 
concentration  of  this  solution  was  determined  by.  the  colorimetric 
method,  and  found  to  be  Ph  =  3.65. 
The  mean  of  several  experiments  gave  the  Ph  of  a  i  per  cent, 
solution  of  morphine  hydrochloride  as  3.65.  Now,  this  figure  is 
within  the  range  of  the  color  change  of  methyl  orange,  but  is  on 
the  acid  side  of  the  neutral  tint,  that  is  to  say,  a  1  per  cent,  solu- 
tion of  morphine  hydrochloride  will  give  a  decidedly  pink  color 
with  methyl  orange.  In  order  to  titrate  morphine  to  methyl  orange, 
therefore,  we  must  finish  up  with  a  decidedly  pink  color.  As  the 
usual  procedure  in  morphine  titrations  is  to  add  excess  of  acid 
and  titrate  back  with  alkali,  it  is  much  more  likely  that  the  end- 
point  taken  will  be  on  the  yellow  side  rather  than  pink,  and  the 
result  will  be  low.  But  methyl  orange  is  not  the  best  indicator  for 
morphine  titrations.  If  we  use  brom-phenol  blue,  an  indicator  which 
is  yellow  in  acid  solutions  and  blue  in  alkaline  solutions,  we  find 
that  a  hydrogen  ion  concentration  of  Ph  —  3.65  corresponds  to  the 
first  appearance  of  a  distinct  blue  color  when  we  are  passing  from 
acid  to  alkaline.  If,  therefore,  we  dissolve  morphine  in  excess  of 
standard  acid  and  titrate  back  with  standard  alkali  to  brom-phenol 
blue  until  a  distinct  blue  color  appears  we  get  a  more  accurate  re- 
sult. 
With  cochineal,  which  has  a  range  of  hydrogen  ion  concentra- 
tion of  Ph  =  5  to  7,  it  is  obvious  that  an  accurate  result  cannot 
be  obtained.  The  results  of  three  titrations  with  the  three  indica- 
tors may  be  of  interest.  In  each  case  the  standard  solutions  were 
standardized  to  the  indicators  used.  Methyl  orange  was  taken  to 
its  neutral  orange  tint: 
Per  Cent. 
Indicator.  Morphine  Found. 
Brom-phenol  blue  100.0 
Methyl  orange  99.5 
Cochineal  98.8 
