68 
Volumetric  Estimation  of  Acetone.  {A 
Solution  of  hydrochloric  acid,  specific  gravity  1-025. 
A  decinormal  solution  of  sodium  thiosulphate. 
A  starch  solution. 
A  dilute  solution  of  acetone  containing  from  1  to  1  y2  per  cent,  of 
acetone  by  weight.  This  is  prepared  from  the  acetone  or  ace- 
tone solution  to  be  examined.  The  writer  prepares  this  by  weigh- 
ing the  acetone  in  a  beaker  containing  water,  transferring  to  a  gradu- 
ated cylinder,  rinsing  the  beaker  well  with  water  and  making  up  to  a 
definite  volume. 
Having  prepared  the  above  solutions,  place  from  25  to  30  c.c.  of 
the  potassium  hydroxide  solution  into  a  suitable  flask,  add  1  or  2 
c.c.  of  the  diluted  acetone  solution,  very  carefully  measured,  or  if 
greater  accuracy  is  desired,  carefully  weigh  the  aqueous  acetone,  mix 
well,  and  run  in  from  a  burette,  while  rotating  the  flask,  from  25  to 
30  c.c.  of  the  iodine  solution  ;  insert  the  stopple  quickly  and  agitate 
vigorously  for  one  minute.  After  shaking,  render  the  mixture  acid 
by  means  of  the  hydrochloric  acid  solution;  add,  while  rotating,  an 
excess  of  the  sodium  thiosulphate  solution.  Allow  the  mixture  to 
stand  several  minutes,  add  the  starch  indicator,  and  re-titrate  the 
excess  of  the  sodium  thiosulphate  with  the  iodine  solution.  From 
the  above  data  the  per  cent,  of  acetone  can  readily  be  calcu- 
lated ;  thus  1  molecule  of  acetone  (58)  requires  3  molecules  of  iodine 
(762)  to  form  I  molecule  of  iodoform.  Expressing  it  in  the  form 
of  a  proportion,  letting  y  equal  the  amount  of  combined  iodine,  and 
x  equal  the  amount  of  acetone,  we  have  ; 
Before  leaving  the  process,  it  may  be  well  to  direct  attention  to 
several  important  points.  After  adding  the  iodine  solution,  agita- 
tion must  not  be  delayed  if  concordant  results  are  desired,  since 
the  active  agent  KIO  is  rapidly  converted  into  KI  and  KI03. 
Experiments  have  proven  that  it  becomes  inactive  in  one-half  an 
hour.  It  is  essential  to  allow  the  mixture  to  stand  a  few  minutes 
after  adding  the  sodium  thiosulphate  solution,  in  that  the  reaction  is 
not  immediate.  It  is  necessary  to  add  an  excess  of  the  iodine  and 
sodium  thiosulphate  solution,  respectively,  at  the  time  of  adding 
them,  in  order  to  secure  completed  reactions. 
y62:$8::y:x  orx  =  y. 
762 
or  x  =  y.  0-07612. 
