338 
Ammonium  Iodide. 
(Am.  Jour.  Pharm. 
1      July,  1887. 
On  slowly  evaporating  an  aqueous  solution  of  ammonium  iodide  in 
contact  with  the  air  iodine  is  liberated,  whilst  the  liquor  turns  yellow 
and  gradually  brown.  Meanwhile  rather  large  glassy  crystals  of  the 
salt  are  formed.  After  evaporation  to  dryness  and  extinguishment 
of  the  free  iodine  by  some  suitable  method,  the  finished  product  is  not 
desirably  white  owing  to  the  preponderance  of  transparent  crystals. 
When,  however,  the  solution  is  rapidly  boiled  down  until  most  of  the 
salt  has  crystallized  and  the  remaining  water  is  finally  expelled  by 
water-bath  heat,  a  minutely  granular  and  white  salt  results.  But  no 
process  of  condensation  in  presence  of  the  atmosphere  will  yield  a 
product  untainted  by  free  iodine.  So  remarkably  small  a  proportion 
as  one  per  cent,  of  water  will  dampen  the  salt ;  but  a  water-bath  tem- 
perature readily  dispels  it,  so  that  a  perfectly  dry  granular  powder 
destitute  of  adhesive  capacity  is  obtained.  As  already  stated,  a  much 
discolored  salt  can  be  rid  of  most  of  the  contaminating  free  iodine  by 
means  of  careful  heating.  But  as  total  decolorization  is  thus  im- 
possible other  aids  become  necessary.  When  solution  of  sulphurous 
oxide  is  dropped  upon  practically  white  ammonium  iodide  the  mix- 
ture becomes  yellow  from  liberated  iodine.  But  if  a  reddened  salt  is 
treated  with  the  acid  first  neutralized  with  ammonia  the  free  iodine 
is  extinguished  by  the  ammonium  sulphite.  This  last  reaction,  there- 
fore, offers  a  remedy  for  the  restoration  of  the  normal  appearance  or 
a  barrier  against  its  escape.  The  lesser  evil  of  contamination  by 
traces  of  sulphate  and  sulphite  seems  warranted  against  the  greater  by 
vitiation  through  free  iodine.  Hence,  instead  of  attempting  the 
reclamation  by  washing  with  ether,  which  does  not  reclaim,  it  is  better 
to  treat  the  discolored  salt  with  a  mixture  of  sulphurous  acid  and 
ammonia  and  dry  it  on  a  water -bath. 
The  usual  method  for  preparing  iodide  of  ammonium  consists  in 
precipitating  a  solution  of  either  ferrous  or  ferric  iodide  with  ammonia 
and  evaporating  the  filtrate  to  dryness.  By  this  process  it  is  not 
always  possible  to  obtain  a  salt  wholly  free  from  iron,  especially  when 
cast  iron  filings  were  used  in  the  preliminary  preparation  of  the  iron 
salt.  However,  when  pure  calcium  iodide,  prepared  by  whatever 
method,  is  decomposed  with  excess  of  ammonium  bicarbonate  aided 
by  heat,  several  advantages  are  secured  thereby. 
Prominently  among  these  is  the  crystalline  and  compact  form  in 
which  the  calcium  carbonate  precipitates.  From  this  results  the 
further  possibility  of  procuring  the  generated  ammonium  iodide  in  a 
