192 
Estimation  of  Menthol. 
J  A.m.  Jour.  Pharm. 
t      April,  1897. 
subtracting  the  normal  factor  of  menthol  (01 56)  from  the  normal 
factor  of  menthyl  acetate  (0-198). 
The  two  following  equations  form  the  basis  of  the  process : 
C10H19OH  +  (C2HsO)20  =  C10H19O.C2H3O  +  HC2H302 
Menthol.  Acetic  Anhydride.  Menthyl  Acetate.  Acetic  Acid. 
C10H19O.C2H3O  +  KOH  =  C10HI9OH  +  KC2H302 
Potassium  Potassium 
Menthyl  Acetate.        Etydroxide.  Menthol.  Acetate. 
Being  frequently  requested  to  estimate  the  amount  of  menthol  in 
peppermint  oils  on  short  notice,  the  writer  has  modified  the  above 
procedure  so  that  the  per  cent,  of  this  constituent  can  readily  be 
estimated  in  three  hours,  while  the  original  method  requires  the 
greater  part  of  a  day  for  execution. 
For  Estimating  the  Combined  Menthol. — Place  from  10  to  12 
grammes  (accurately  weighed)  of  the  oil  into  a  suitable  flask,  add 
about  12  c.c.  of  normal  alcoholic  sodium  hydroxide,  connect  the 
flask  with  an  inverted  condenser  and  boil  for  one  hour.  Retitrate 
the  excess  or  alkali  by  means  of  standard  sulphuric  acid,  using  phenol- 
phthalein  as  indicator.  Each  cubic  centimeter  of  standardized  alkali 
consumed  corresponds  to  0-156  gramme  of  menthol  as  esters.  This 
part  of  the  process  embodies  the  well-known  principle  of  Koettstorfer. 
To  Estimate  the  Total  Menthol. — Place  from  1 2  to  1 5  grammes 
(accurately  weighed)  of  the  oil  into  a  suitable  flask  (the  writer  uses 
an  ordinary  Kjeldahl  digesting  flask),  add  an  equal  weight  of  acetic 
anhydride,  2  grammes  of  anhydrous  sodium  acetate,  attach  to  a  reflux 
condenser  and  boil  the  contents  of  the  flask  one  hour.  Allow  the 
mixture  to  cool  somewhat,  transfer  to  a  250  c.c.  separatory  funnel, 
with  successive  portions  of  distilled  water,  using  about  1 50  c.c.  Agi- 
tate the  funnel  and  contents  well,  set  aside  a  few  minutes,  so  that 
the  mixture  will  separate  into  two  layers.  Withdraw  the  aqueous 
layer  and  wash  again  with  150  c.c.  of  water  as  above.  Having  re- 
moved the  second  wash  water,  add  50  c.c.  of  water,  a  few  drops  of 
phenolphthalein  solution,  and  just  enough  of  a  5  per  cent,  aqueous 
sodium  hydroxide  solution  to  render  the  contents  of  the  funnel 
pinkish,  after  thoroughly  agitating  ;  then  add  enough  water  so  that 
the  aqueous  portion  will  amount  to  about  150  c.c;  agitate  well, 
allow  the  mixture  to  separate  and  withdraw  the  alkaline  aqueous 
solution.  Wash  the  oily  layer  again  with  1 50  c.c.  of  water  as  above. 
Remove  the  water  as  completely  as  possible  and  transfer  the  acety- 
