374 
Assay  of  Oil  of  Peppermint. 
( Am.  Jour.  Pharm. 
I      August,  1908. 
In  case  this  loss  of  menthol  is  overlooked  and  the  total  menthol 
in  the  remaining  oil  assayed  according  to  the  U.S. P.,  the  percentage 
found  may  be  much  below  the  truth. 
An  oil  actually  containing  9  2  per  cent,  menthyl  ester  (as  acetate) 
and  51-6  per  cent,  total  menthol  showed,  when  assayed  according 
to  the  U.S. P.,  only  19-6  per  cent,  total  menthol.  Hence  32  per  cent 
had  been  washed  out  from  the  saponified  oil.  (A  large  quantity  of 
menthol  crystals  were  actually  found  in  the  wash-water.) 
Such  a  separation  of  menthol  does  not,  however,  occur  as  a  rule, 
it  evidently  being  dependent  upon  the  nature  of  the  oil ;  but  as  it 
can  happen,  the  author  considers  it  safe  to  change  the  assay  process 
in  such  a  way  that  serious  errors  may  be  avoided. 
The  amount  of  menthyl  esters  can  of  course  be  assayed  accord, 
ing  to  the  U.S.P.;  but  instead  of  acetylizing  the  total  menthol  in  the 
saponified  product,  the  oil  should  first  be  acetylized,  after  which  the 
saponification  of  the  product  will  give  the  amount  of  total  menthol 
present. 
The  reason  for  acetylizing  the  saponified  oil  is  evidently  to  have 
all  the  menthol  before  saponification  in  form  of  menthyl  acetate, 
whereas  by  the  acetylization  of  the  original  oil  the  calculation  of 
the  menthol  would  be  based  upon  the  presumption  that  all  the 
combined  menthol  is  present  in  the  form  of  acetate. 
The  U.S.P.  finds,  however,  no  objection  against  such  a  presump- 
tion in  the  first  part  of  the  assay  process,  and  the  author  thinks  it 
would  be  advisable  to  maintain  the  same  also  in  the  second  part, 
as  such  a  change  of  the  process  not  only  would  eliminate  the  possi- 
bility of  large  errors,  due  to  separation  of  menthol,  but  also  con- 
siderably shorten  the  process. 
Chemical  Laboratory  of  Norwich 
Pharmacal  Company. 
