462  Purity  of  Commercial  Aspirin.      { Am- j]°£r' ™gTm' 
three  sets  of  figures  should  be  identical.  The  details  of  the  pro- 
ning  the  caustic  into  the  acid. 
The  bromate  solution  consists  of  potassium  bromate  (KBr03)  5 
grams  and  potassium  bromide  (KBr)  20  grams,  water  1,000  Cc. 
The  N/10  thiosulphate  is  standardized  against  the  iodine  liber- 
ated from  potassium  iodate  (KIOs)  and  potassium  iodide  (KD  by 
a  given  amount  of  the  N/$  acid  used  above,  this  precaution  being 
taken  so  that  strict  comparison  is  possible,  all  titrations  being  really 
in  terms  of  one  standard  solution — the  original  iV/5  acid, 
soda  free  from  carbonate,  setting  this  with  phenolphthalein  and  run- 
Determining  the  Acid  and  Ester  Value. 
One  gram  of  the  sample  is  accurately  weighed  and  rinsed  into  a 
flask  with  about  15  Cc.  of  absolute  alcohol  (B.P.)  ;  solution  being 
complete,  phenolphthalein  is  added,  and  the  titration  made  with  the 
iV/5  soda.  Fifty  Cc.  of  the  same  soda  solution  is  now  added  from 
a  pipette,  and  another  50  Cc.  taken,  draining  the  pipette  in  exactly 
the  same  manner,  for  a  blank.  Reflex  tubes  are  fitted,  the  flasks 
set  in  a  water-bath  for  about  twenty-five  minutes,  then  cooled  and 
titrated  back  with  the  acid ;  the  difference  between  the  blank  and 
test  titers  being  the  number  of  Cc.  iV/5  equivalent  to  the  ester,  and 
cedure  adopted  in  the  tests  are  as  follows : 
Standard  N/5  acid  is  prepared  and  a  similar  solution  of  caustic 
should  (theoretically)  be  identical  with  the  acid  titration. 
As  it  is  quite  possible  for  slight  hydrolysis  to  occur  while  deter- 
mining the  acidity,  comparative  tests  were  made  by  estimating  the 
difference  in  the  amount  of  free  salicylic  acid  before  and  after  titra- 
tion. Standard  acid,  equivalent  to  the  number  of  Cc.  of  alkali  used 
in  the  test,  was  returned  to  the  titrated  liquid  before  the  comparison 
was  made  by  the  iron  test.  Half  a  gram  of  aspirin  was  used,  and 
it  was  found  that  0.1  to  0.5  milligram  were  the  extreme  limits  due 
to  titration,  with  a  fair  average  of  0.3  milligram — or,  say,  0.02  Cc. 
iV/5 — for  one  gram.  Since  this  is  accompanied  by  its  equivalent  of 
acetic  acid  an  adjustment  is  required  in  the  relative  acid  and  ester 
values.  One  should,  therefore,  deduct  0.02  Cc.  from  the  experi- 
mental acid  figure  and  add  it  to  that  of  the  ester. 
The  Bromine  Value. 
Accurately  weigh  0.5  gram  of  the  acetylsalicylic  acid,  wash  into 
a  flask,  add  15  Cc.  TV/5  soda,  and  set  in  a  water-bath  as  described 
