524  Alkaloids  and  Alkaloidal  Drugs.  {A^™^£%f; 
the  balance  evaporated  from  a  shallow  vessel.  The  residue  in  the 
evaporating  dish  and  that  left  in  the  flask  are  then  taken  up  with 
acid  (standardized  if  an  alkalimetric  assay  is  intended)  and  the 
operation  finished  as  above.  If  only  an  iodometric  assay  is  desired, 
the  acid  solution  is  made  up  to  a  given  volume,,  shaken  with  a  small 
quantity  of  calcium  hydrate,  and  the  filtered  half  of  the  solution 
treated  with  iodine  as  above.  In  this  case  only  I  gramme  of  opium 
need  be  used. 
COMPARATIVE  MERITS  OF  IODOMETRIC  ESTIMATION. 
Of  the  various  devices  which  have  been  resorted  to  in  the  estu 
mation  of  alkaloids,  two  general  methods  are  especially  worthy  of 
regard  because  of  their  directness  and  simplicity,  namely,  the  gravi- 
metric  method  and  the  alkalimetric  method. 
The  gravimetric  method,  using  chiefly  solvents  to  separate 
the  alkaloid  and  weigh  it  uncombined,  has  the  advantage  of  ex- 
treme simplicity  of  principle,  but  the  disadvantage  of  depending 
upon  solvents  for  separation  from  non-alkaloidal  matters,  a  separa- 
tion lacking  in  exactness  and  requiring  repetitions  which  take  up 
much  time.  The  result  is  apt  to  be  a  compromise  between  the  loss 
of  alkaloid  left  behind  in  the  solutions  and  the  gain  in  weight  by 
impurity  in  the  final  product. 
The  alkalimetric  method  is  certainly  based  upon  the  best  of  prin- 
ciples, and  so  far  as  it  can  be  readily  executed  with  exactness,  it 
should  have  the  preference.  Unfortunately,  the  combination  of 
alkaloids  with  acids  is  not  a  reaction  quickly  and  sharply  defined. 
The  presence  of  ammonia  must  be  excluded.  The  end-reaction 
depends  so  much  upon  the  choice  and  quality  of  the  indicator,  the 
personal  equation  of  the  operator  and  the  light  where  the  observa- 
tion is  made,  that  results  are  often  in  doubt,  or  ought  to  be. 
The  iodometric  method,  in  the  instances  of  alkaloids  which  form 
distinct  higher  periodides  insoluble  in  excess  of  the  reagent,  has 
claims  as  follows:  It  is  based  upon  fixed  chemical  proportions  de- 
duced from  analyses  of  the  products  of  the  reaction  ;  the  end-reaction 
is  very  sharp  indeed  ;  the  actual  exactness  of  the  estimation  has 
been  verified,  and  the  volumetric  reagents  required  are  among 
those  most  commonly  in  use  and  of  earliest  introduction  into  the 
pharmacopoeias.  The  estimation  is  done  in  an  acid  solution,  and 
ammonia  or  other  alkali  does  not  interfere. 
