Am,jiiyr"i9ih9arm' ?      Purity  of  Commercial  Aspirin.  465 
have  200  milligrams  aspirin  =  0.4  milligram  salicylic  acid,  or  0.2 
per  cent.  The  limit  that  admits  of  satisfactory  matching  is  about 
0.5  milligram.  It  has  been  found  necessary  to  adopt  this  method 
because  both  aspirin  and  alcohol  have  an  effect  on  the  iron  coloration. 
The  B.P.  test,  if  used  with  a  little  discretion  in  the  interpretation 
of  "  violet  color,"  is  good  enough  for  detecting  inferior  samples,  but 
in  those  cases  where  one  hesitates  as  to  whether  a  sample  can  be 
passed,  it  is  sometimes  a  trifle  misleading.  The  various  factors 
effecting  solubility,  such  as  crystalline  condition  and  surface  effects, 
come  into  play,  and  where  but  slight  traces  are  present,  a  compari- 
son formed  on  the  B.P.  test  may  be  reversed  on  complete  solution, 
as  in  the  test  described.  It  is  preferable  to  adopt  the  latter  test  and 
work  to  a  definite  limit. 
Another  modification  of  the  test  is  described  in  the  British  Phar- 
maceutical Codex,  the  Belgian  Pharmacopoeia,  and  elsewhere — 
namely,  that  0.1  gram  of  aspirin  when  dissolved  in  5  Cc.  of  alcohol 
and  20  Cc.  of  water  added  should  give  no  violet  color  with  ferric 
chloride.  It  should  be  pointed  out  that  in  this  strength  of  alcohol 
the  coloration  is  strongly  depressed,  and  further  so  by  employing 
ferric  chloride  in  preference  to  iron  alum.  It  has  been  found  that 
with  this  test,  and  using  one  drop  of  5-per-cent.  ferric-chloride  solu- 
tion, a  sample  containing  0.15  per  cent,  of  free  salicylic  acid  gives 
only  a  brownish  yellow,  with  0.2  per  cent,  a  shade  of  pink  to  which 
serious  exception  cannot  be  taken,  and  with  0.3  per  cent,  a  very  dis- 
tinct salicylic  reaction.  This  test  is  a  good  guide  for  sorting-out 
purposes  and  in  routine  work,  but  it  is  not  suitable  for  precise  work. 
Increase  in  the  iron  still  further  depresses  the  coloration. 
Deductions. 
The  first  thing  noticeable  is,  that  there  are  two  distinct  varieties 
of  commercial  aspirin :  one,  where  the  acid  is  either  equal  to  or  ex- 
ceeds the  ester ;  the  other,  in  which  the  ester  is  in  distinct  excess  of 
of  the  acid  as  well  as i being  in  excess  of  theory  in  some  cases ;  while 
in  both  groups  the  saponification  value  is  consistent  throughout,  and 
always  less  than  theoretical. 
Acid  and  Ester  Value. — On  examining  the  figures  for  samples 
1  to  10,  it  will  be  seen  that  if  the  determination  by  iron  be  assumed 
to  measure  the  dissociated  salicylic  and  acetic  acids,  in  sample  1  the 
acid  exactly  agrees  with  the  ester  (27.72  and  27.72),  and  also  very 
