360  Estimation  of  Nitroglycerin.  { au^II'^,  lim!'"'' 
is  probably  the  more  used.  For  standardizing  the  stronger  alcoholic 
solutions  and  the  powder-mixtures  (precautions  being  taken  to  ex- 
clude the  carbonates  present  in  the  latter)  it  may  give  good  results 
in  skilled  hands. 
But  the  nitrometer  requires  experience  in  its  use,  and  par- 
ticularly for  this  substance.  Newfield  and  Marx  in  an  article  on 
the  use  of  the  nitrometer-  with  special  reference  tO'  the  examination 
of  nitrocellulose,  a  kindred  body  of  nitroglycerin, — state  that  the 
sulphuric  acid  used  must  not  be  below  94.8%  strength,  the  time  of 
agitation  not  less  than  3  minutes,  and  that  the  presence  of  other 
organic  bodies  may  seriously  afifect  the  results.  They  say  that  "  a 
great  number  of  details,  some  of  them  apparently  trivial,  affect 
the  results  to  a  considerable  extent."  If  we  add  to  these  the  difficulty 
of  weighing  accurately  and  transferring  completely  to  a  nitrometer, 
so  viscid  a  substance  as  nitroglycerin,  without  unduly  diluting  it 
in  the  transferring,  it  is  not  to  be  wondered  at  that  one  unacquainted 
with  all  of  these  necessary  details  should  obtain  results  varying 
from  82%  to  108%  in  half  a  dozen  assays  on  the  same  sample,  and 
become  discouraged  thereby,  although  accustomed  to  the  nitrometer 
in  the  assay  of  ethyl  nitrite.  Hence  the  statement  often  made  that 
"  experience  is  necessary  "  in  the  use  of  the  nitrometer  for  nitro- 
glycerin assays,  cannot  be  too  strongly  emphasized. 
Furthermore  for  the  estimation  in  tablets  there  are  additional 
difficulties. 
The  method  of  estimating  nitroglycerin  by  saponification  with 
alcoholic  potassium  hydroxide  has  been  proposed  by  several  writers. 
Mr.  Hay  states  ^  that  the  decomposition  of  nitroglycerin  by 
alcoholic  potash  is  of  a  complex  nature,  the  products  being  potassium 
acetate,  oxalate  and  formate,  free  ammonia,  etc.  Nevertheless  he 
offers  an  equation  wherein  5  molecules  of  potassium  hydroxide  act 
upon  I  molecule  of  nitroglycerin  to  give  the  above,  and  proposes 
that  equation  as  the  basis  of  assay  by  titration  with  standard  alcoholic 
potash  and  standard  acid. 
In  1895  Dr.  Charles  Rice  recommended  *  the  estimation  of  nitro- 
glycerin by  saponification  with  standard  potassium  hydroxide  and 
standard  acid,  on  the  basis  that  the  reaction  products  are  simply 
potassium  nitrate  and  glycerin.    The  high  position  and  reputation 
^  Jour.  Anier.  Chem.  Soc,  1906,  page  877. 
^  four.  Chem,  Soc,  1885,  page  742. 
*  Amer.  Drug.  &  Pharm.  Rec.  July  10,  1895,  page  6. 
