PREPARATION  OF  IODIDE  OF  AMMONIUM. 
23 
forming  insoluble  phosphate  of  baryta,  and  soluble  iodide  of 
barium.  This,  by  double  decomposition  with  sulphate  or  car- 
bonate of  ammonia,  gave  iodide  of  ammonium  free  from  objec- 
tions due  to  the  presence  of  sulphur. 
Iodide  of  zinc  and  iodide  of  iron,  by  decomposition  with  car- 
bonate of  ammonia,  gave  equally  satisfactory  results. 
These  processes,  however,  were  liable  to  the  objection  that, 
in  the  precipitation  of  carbonate  of  iron  or  zinc,  or  in  the  separa- 
tion of  phosphate  of  baryta,  and  afterwards  of  the  carbonate  or 
sulphate  of  baryta,  the  necessary  amount  of  washing  of  these 
bulky  precipitates,  to  avoid  loss,  involved  so  much  time  and  con- 
sequent exposure,  that  it  was  almost  impossible  to  obtain  a  salt 
which  did  not  contain  free  iodine.  The  large  amount  of  water 
necessary  to  wash  out  the  last  portions  of  soluble  iodide  from  the 
precipitates  required  considerable  time  in  boiling  to  the  crystal- 
lizing point,  and  the  iodide  of  ammonium,  an  unstable  salt  at 
best,  always  suffered.  It  was  desirable,  therefore,  to  make  use 
of  a  process  involving  as  little  delay  in  the  manipulation  as  pos- 
sible, and  a  method  was  finally  adopted,  which  was  described  in 
the  London  Chemical  News,  April  9,  1864,  and  also  in  the  Ame- 
rican Journal  of  Pharmacy,  May,  1864,  p.  245.  It  is  undoubt- 
edly the  best  at  present  in  use,  being  capable,  with  slight  modi- 
fications, of  producing  a  salt  absolutely  pure  and  free  from  the 
objections  to  which  the  previous  processes  I  have  mentioned  are 
liable. 
It  consists  in  the  double  decomposition  of  pure  iodide  of 
potassium  and  pure  sulphate  of  ammonia,  both  of  which  are 
easily  procured.  Iodide  of  ammonium  and  sulphate  of  potassa 
are  formed,  and  the  latter  separated  by  the  addition  of  15  or  20 
per  cent,  of  alcohol,  and  the  whole  evaporated  to  the  crystalliz- 
ing point.  I  have  found  the  following  proportions  to  give  satis- 
factory results : — 
Iodide  of  potassium,  5  parts  by  weight. 
Sulphate  of  ammonia,  2     "  " 
Water,  4     "  " 
Alcohol,  95  percent.,  1     "  " 
The  salts  are  dissolved  by  heat  in  the  water,  which,  on  cool- 
ing, deposits  a  large  proportion  of  the  sulphate  of  potassa,  and 
