336 
Ammonium  Iodide. 
Am.  Jour.  Pharm, 
July,  1887. 
ence  from  analogy  unsustained  by  practical  experience  would  lead  to 
the  expectation  that  free  ammonia  reacting  on  free  iodine  generates 
iodide  and  iodate  of  ammonium.  But  in  practice  nothing  of  the  kind 
is  realized.  The  black  insoluble  compound  therefrom  resulting  is  gen- 
erally a  mixture  of  the  3  or  4  chemically  possible  and  dangerously 
explosive  iod-ammonias.  A  variety  of  more  or  less  practical  and 
efficient  methods  for  preparing  iodide  of  ammonium  are  extant.  One 
of  the  chief  difficulties  encountered,  however,  in  seemingly  all  the  pro- 
cesses, is  the  occurrence  of  free  iodine  in  the  finished  product.  Some 
very  elegant  ammonium  iodide  is  found  in  the  market,  but  much  of  it 
also  is  not  as  white  as  it  should  be,  whilst  some  of  it  soon  becomes  not 
only  yellow  but  positively  brown  from  the  accumulation  of  free  iodine. 
It  is  presumed  that  exposure  to  light  primarily  induces  this  deterio- 
rating effect.  The  real  root  of  the  diffiulty,  however,  is  due  to  the 
access  and  joint  influence  of  free  oxygen  and  moisture.  When  the 
dry  discolored  iodide  is  submitted  to  a  moderate  heat  in  an  open  vessel 
much  of  the  seemingly  free  iodine  is  dissipated.  But  a  very  small 
proportion  is  persistently  retained  at  all  practically  low  temperatures. 
The  pharmacopoeia  states  that  ammonium  iodide  when  heated  on  pla- 
tinum foil  evolves  free  iodine,  and  is  wholly  volatilized  without 
fusing.  This  is  all  correct  enough,  but  such  a  statement  is  mislead- 
ing, in  so  far  that  it  induces  the  belief  that  ammonium  iodide  is  readily 
decomposable  by  heat.  When,  however,  the  pure  salt  is  heated  in  a 
dry  test-tube  a  faint  grayish  film  first  deposits  on  the  walls  of  the 
glass.  Under  an  increased  temperature  the  salt  then  sublimes 
unchanged  in  white  crystals.  But  as  a  portion  of  the  saline  vapor  in 
the  upper  end  of  the  tube  necessarily  mingles  with  the  air  it  is  decom- 
posed by  the  oxygen  of  the  latter  yielding  free  iodine,  free  ammonia 
and  water.  The  free  iodine  first  appears  as  a  violet  vapor,  but  being 
absorbed  by  the  saline  humidity  forms  a  brown  deposit  which  soon  be- 
comes black  as  the  iodine  crystallizes.  During  the  heating  process 
the  residuary  iodide  at  the  bottom  of  the  tube  remains  white  and  crys- 
talline until  all  has  sublimed  and  deposited  farther  up  in  the  tube  as 
equally  white  crystals. 
The  Pharmacopoeia  also  states  that  the  discolored  salt  may  again  be 
whitened  by  washing  it  with  stronger  ether  and  rapidly  drying.  The 
writer  could  not  obtain  the  requisite  whiteness  by  this  method.  Fur- 
thermore the  escaping  ether  seems  to  invite  both  oxygen  and  water  to 
degenerate  the  salt  anew. 
