AmAprn;f898arm-}       Assay  of  Belladonna  Plasters.  187 
due.  This  usually  remains  on  the  bottom  of  the  flask  as  a  semi- 
fluid layer,  which  may  be  brought  in  thorough  contact  with  the  acid 
by  inclining  the  flask  so  that  different  surfaces  of  the  residue  are 
always  exposed  to  the  action  of  the  acid.  When  the  residue  no 
longer  visibly  diminishes  in  size,  agitate  about  5  minutes  longer,  to 
insure  perfect  solution  of  all  alkaloid.  Solution  should  be  effected 
in  the  cold,  as  heating  with  the  excess  of  acid  might  partially  saponify 
the  alkaloid.  Add  50  c.c.  of  neutral  distilled  water,  then  0*5  c.c.  of 
a  I  per  cent,  solution  of  hematoxylin  in  neutral  alcohol,  and  add 
N 
—  alkali  until  the  color  of  the  solution  changes  from  yellow  to  a 
dull  red.    Subtract  the  number  of  cubic  centimetres  required  from 
N 
4,  multiply  the  remainder  by  0-0145  (the  —  factor  for  atropine  and 
its  isomers)  and  by  100,  and  divide  the  product  by  the  net  weight, 
in  grammes,  of  the  plaster  mass  taken  for  assay.  To  illustrate,  a 
plaster  weighing  12  25  grammes,  with  cloth,  and  the  cloth  weighing 
N 
3-55  grammes,  requires  1-58  c.c.  of  — alkali  to  neutralize  the  excess 
of  acid,  then  : 
(4 —  1-58)  X  0-0145  X  100  c       ,  .  , 
±±  3  '   ^°    ==  0-404  per  cent,  of  alkaloid. 
12-25  —  3-55 
The  separated  alkaloid  is  always  too  impure  to  admit  of  gravi- 
metric estimation,  and  further  purification  entails  too  much  loss  for 
quantitative  purposes.  The  impurities  may,  under  certain  condi- 
tions, also  interfere  in  the  volumetric  estimation.  If  the  alkaloid  is 
dissolved  with  the  aid  of  alcohol  in  the  standard  acid,  the  impurities 
will  become  diffused  throughout  the  liquid,  making  a  very  turbid 
solution.  Under  these  conditions  the  endreaction  is  not  sharp.  The 
change  of  color  is  not  simply  masked,  as  the  impurities  are  nearly 
always  of  a  light  color,  but,  in  some  way  not  understood,  they  pre- 
vent a  decided  change  of  color  until  a  considerable  excess  of  alkali 
has  been  added.  A  rapid  and  convenient  mode  of  dissolving  the 
alkaloid  for  titration  is  the  addition  of  about  0-5  c.c.  of  chloroform 
at  the  time  of  adding  the  standard  acid.  The  chloroform  dissolves 
a  portion  of  the  impurities  and  admits  of  a  rapid  solution  of  the 
alkaloid  in  the  acid,  at  the  same  time  leaving  the  acid  solution  prac- 
tically clear.    The  endreaction  is  then  quite  sharp,  but  the  red  color 
