PBEPARATION  OF  IODIDE  OF  AMMONIUM.  *  391 
gas,  which  would  no  doubt  deter  the  generality  of  pharmaceu- 
tists from  attempting  it. 
In  the  method  here  proposed,  the  above  objections  are  avoided, 
and  by  following  the  directions  a  pure  salt  is  easily  obtained. 
The  formula  is  based  upon  the  fact  of  the  insolubility  of  bi- 
tartrate  of  potassa,  which  is  thrown  down  when  iodide  of  potas- 
sium and  bi-tartrate  of  ammonia  in  concentrated  solutions  are 
mixed  together. 
Thus,  KI+NH3,204H206+H0  =  NH4I+KO,2C4H205. 
The  first  step  of  the  process  is  the  formation  of  the  bi-tar- 
trate of  ammonia ;  the  second,  its  double  decomposition  by 
means  of  iodide  of  potassium,  and  finally  the  separation  of  all 
traces  of  bi-tartrate  of  potassa  from  the  resulting  salt. 
The  detail  of  the  whole  process  is  as  follows  :  Dissolve  300 
grains  of  tartaric  acid  in  3  ounces  of  water,  and  saturate  ac- 
curately with  water  of  ammonia  (about  14  fluidrachms.)  The 
bi-tartrate  of  ammonia  copiously  thrown  down  at  first,  redis- 
solves  as  the  solution  becomes  neutral  to  a  test  solution  of  lit- 
mus ;  300  grains  more  of  tartaric  acid  is  now  to  be  added  and 
the  mixture  raised  to  boiling.  A  solution  of  662  grains  of 
iodide  of  potassium  in  an  ounce  and  a  half  of  water  is  now  to 
be  stirred  in,  and  the  boiling  continued  for  a  few  minutes 
longer ;  then  set  aside  for  several  hours,  filter,  and  when  all  the 
liquid  has  passed  through,  pour  on  the  filter  2  ounces  or  more 
of  cold  water,  until  the  filtrate  has  but  little  saline  taste.  Now 
evaporate  all  the  filtered  liquid  until  it  begins  to  thicken,  and 
while  yet  warm,  pour  on  a  mixture  of  2  ounces  of  alcohol  and 
J  ounce  water ;  this  alcoholic  liquid  readily  dissolves  all  the  hy- 
driodate  of  ammonia,  but  leaves  a  small  quantity  of  cream  of 
tartar.*  After  being  allowed  to  settle,  the  clear  solution  is  to 
be  decanted  off,  the  residue  washed  with  a  little  more  alcohol  and 
water,  and  finally  the  whole  evaporated  at  a  gentle  heat  to  per- 
fect dryness. 
The  resulting  salt,  which  while  yet  moist  appears  like  brown 
sugar,  becomes  at  last  nearly  white.     It  is  inodorous,  very  de- 
*In  the  present  case  the  weight  of  cream  of  tartar  thus  separated 
amounted  to  13  grains. 
