464  Purity  of  Commercial  Aspirin.      { Am-j{SynIg^rm" 
above.  After  the  twenty-five  minutes  this  solution  is  cooled  to 
room  temperature,  diluted  and  transferred  to  a  250-Cc.  flask,  and 
carefully  adjusted  to  the  volume  and  well  shaken.  One  should  be 
sure  either  that  the  instruments  are  exact  or  else  take  the  precaution 
of  graduating  the  flask  by  ten  deliveries,  each  made  in  exactly  the 
same  manner,  from  the  25-Cc.  pipette  about  to  be  used.  Twenty- 
five  Cc.  is  withdrawn  by  a  pipette  and  delivered  into  an  accurately 
stoppered  16-oz.  shop  round ;  add  25  Cc.  of  the  bromate  solution, 
also  very  accurately  delivered,  and  then  250  Cc.  of  wTater.  Fifteen 
Cc.  of  hydrochloric  acid  (B.P.)  is  added,  and  the  bottle  stoppered. 
A  blank  is  prepared  in  precisely  the  same  manner,  and  both  are  set 
aside  for  twenty  minutes  with  occasional  shaking.  Fifteen  Cc.  of 
10-per-cent.  sodium  iodide  solution  is  then  run  in  with  all  necessary 
precautions,  the  solution  well  shaken  for  a  few  minutes  and  then 
titrated  with  N/10  thiosulphate,  using  a  solution  of  soluble  starch 
as  indicator.    Duplicate  tests  agree  to  within  0.05  of  a  Cc. 
To  check  both  solutions  and  the  manipulation  comparison  was 
made  with  two  samples  of  specially  prepared  salicylic  acid.  In  the 
test  in  question  this  showed  99.9  and  100  per  cent,  of  the  acid  taken. 
From  the  difference  between  the.  blank  and  the  test,  the  number  of 
Cc.  AT/5  taken  by  one  gram  of  the  aspirin  is  calculated,  dividing  this 
by  six — since  six  atoms  of  iodine  thus  shown  are  equivalent  to  one 
molecule  of  salicylic  acid — there  is  obtained  a  number  of  Cc.  which 
should  (theoretically)  exactly  agree  with  the  acid  and  ester  figures. 
Determining  Free  Salicylic  Acid. 
Dissolve  one  gram  of  salicylic  acid  in  about  60  Cc.  of  alcohol, 
and  then  adjust  to  100  Cc.  with  water  ;  10  Cc.  of  this  liquid  is  diluted 
to  1,000  Cc,  making  1  Cc.  equal  to  1/10  milligram  of  salicylic  acid. 
Prepare  also  a  i-per-cent.  aqueous  solution  of  iron  alum. 
For  the  test,  dissolve  0.6  gram  of  the  aspirin  in  a  measuring 
cylinder  wTith  9  Cc.  of  alcohol,  dilute  with  water  to  90  Cc,  and  well 
mix.  Take  two  precisely  similar  Nessler  glasses.  Into  one  pour 
60  Cc.  of  the  solution,  into  the  other  30  Cc,  together  with  3  Cc.  of 
alcohol,  and  adjust  to  the  volume  of  the  first ;  this  gives  a  difference 
of  200  milligrams  of  aspirin  in  a  mixture  of  equal  parts  of  alcohol 
and  water.  One  Cc  of  the  iron  solution  is  addeel  from  a  pipette 
to  each,  and  the  color  matched  by  adding  the  standard  salicylic-acid 
solution  from  a  burette.    Supposing  4  Cc  to  be  required,  we  then 
