22 
PREPARATION  OF  IODIDE  OF  AMMONIUM. 
yellow  and  finally  to  brown,  evolving  free  iodine,  which,  of 
course,  made  it  unreliable  by  varying  the  proportion  of  iodine 
contained  in  it,  and  hence  unfitting  it  for  accurate  preparations, 
particularly  in  photography,  in  which,  of  late,  it  has  had  such 
an  extensive  use.  Trials  in  various  ways  failed  to  remove  com- 
pletely the  last  traces  of  the  sulphur  compound  alluded  to  above, 
and  the  process  was  finally  laid  aside  for  others,  in  which  the 
preparation  of  hydriodic  acid  and  its  subsequent  neutralization 
with  carbonate  of  ammonia  were  necessary. 
Different  methods  for  preparing  the  acid  were  accordingly 
made  use  of,  the  first  being  that  of  the  U.  S.  Pharmacopoeia  of 
1860.  It  will  be  remembered  that  this  is  based  upon  the  action 
of  hydrosulphuric  acid  upon  iodine.  A  quantity  of  hydriodic 
acid  was  accordingly  prepared,  and  after  separation  of  sulphur, 
carefully  neutralized  with  carbonate  of  ammonia  and  the  whole 
evaporated. 
It  was  found,  however,  that  the  iodide  of  ammonium  produced 
by  this  method  decomposed  in  precisely  the  same  manner  as 
that  prepared  by  hydrosulphuret  of  ammonia,  and  that  traces  of 
sulphur  (which  probably  exists  in  combination  with  iodine)  con- 
tinued to  be  present,  and  ultimately  determined  the  separation 
of  iodine  from  its  combination. 
It  will  be  noticed  that  this  involves  the  interesting  subject  of 
the  purity  of  the  hydriodic  acid,  as  prepared  by  the  process  of 
the  pharmacopoeia,  and  also  the  question,  whether  the  tendency 
of  hydriodic  acid  so  produced  to  change  color,  is  not  partially 
due  to  traces  of  sulphur  introduced  during  its  preparation. 
Granulated  lead  and  iodine,  according  to  Gmelin,  (vol.  ii.  p. 
267),  shaken  together  with  water  until  the  solution  was  colorless, 
and  the  lead  then  precipitated  by  hydrosulphuric  acid,  was  Un- 
satisfactory, it  being  found  that  wherever  sulphuretted  hydrogen 
or  alkaline  sulphides  were  used  in  the  preparation  of  iodide  of 
ammonium,  the  salt  produced  was  always  liable  to  decomposition, 
even  in  the  dark. 
One  part  of  phosphorus  was  melted  in  forty  parts  of  water, 
and  to  the  mixture  twenty-four  parts  of  iodine  gradually  added, 
with  constant  stirring,  gave  a  solution  of  hydriodic  and  phos- 
phoric acids.    This  was  neutralized  by  carbonate  of  baryta, 
