Am.  Jour.  Pharm. 
April,  1897. 
Estimation  of  Menthol. 
193 
lized  oil  to  a  suitable  flask,  using  a  small  amount  of  alcohol  to 
transfer  the  last  portions.  To  the  oil  in  the  flask  add  from  50  to  60 
c.c.  of  normal  alcoholic  sodium  hydroxide,  connect  the  flask  to  an 
inverted  condenser  and  boil  for  one  hour.  Retitrate  the  excess  of 
the  alkaline  solution  by  means  of  normal  sulphuric  acid.  Each  cubic 
centimeter  of  normal  alkali  combined  corresponds  to  0-156  gramme 
of  menthol. 
On  deducting  the  amount  of  menthol  contained  in  the  oil  in  the 
form  of  esters  from  the  total  menthol  found,  we  have  the  amount  of 
free  menthol. 
The  table  on  next  page  contains  the  results  obtained  by  the 
above  process  for  commercial  menthol  and  a  number  of  samples  of 
oil  of  peppermint  in  connection  with  the  specific  gravities,  boiling 
points  and  residues.  The  boiling  points  were  determined  with 
metallic  bath. 
It  requires  only  a  casual  review  of  the  contents  of  the  table 
to  show  that  oil  of  peppermint  is  a  most  variable  product.  Then 
when  we  call  to  mind  that  normal  Japanese  oil  generally  con- 
tains about  75  per  cent,  of  menthol,  the  variation  is  still  greater. 
But  it  must  also  be  remembered  that  an  oil  containing  a  high  per- 
centage of  menthol  frequently  does  not  possess  the  desired  fine 
aroma  so  valuable  in  essential  oils.  The  quality  of  the  aroma  is 
generally  indicated  by  the  amount  of  menthol  esters.  Yet,  there 
may  be  some  disturbing  elements  present,  such  as  the  sulphur  com- 
pound, recently  discovered,1  which  will  vitiate  an  otherwise  fine 
aroma  very  materially. 
The  writer  has  every  reason  to  think  that  the  oils  examined 
above  are  genuine,  excepting,  of  course,  the  sample  marked  "  Un- 
known." This  was  highly  adulterated  with  turpentine,  as  the  boil- 
ing points  clearly  show. 
In  examining  oil  of  peppermint  it  is  necessary  to  determine:  (1) 
the  specific  gravity,  although  this  is  only  an  indication ;  (2)  the 
boiling  point,  varying  from  a  few  degrees  below  2000  C.  to  about 
2300  C.  (uncorrected),  with  some  residue ;  (3)  the  amount  of  men- 
thol. The  combined  menthol  varies  from  3  to  16  per  cent.  The 
total  menthol  may  vary  from  30  to  80  per  cent.  These  data,  in 
connection  with  the  aroma  and  identity  tests,  will  undoubtedly  show 
the  character  of  any  oil  of  peppermint. 
iiSg6,  C.  Kleber,  Pharm.  Review,  14,  269  ;  Schimmel  &  Co.'s  Semi-annual 
Report,  October,  p.  48. 
