4 
BISMUTHI  ET  AMMONITE  CITRAS. 
seven-eighths  of  which  measure  must  he  made  up  with  distilled 
water,  and  the  remainder  with  alcohol. 
For  preparing  the  citrate  of  bismuth,  the  subcarbonate  is 
preferable  to  the  subnitrate,  because  of  its  more  uniform  com- 
position, as  well  as  its  greater  purity,  at  least  in  commercial 
specimens  of  these  salts.  The  metal  itself  is  inferior  to  either, 
as  it  is  almost  invariably  contaminated  with  arsenic. 
The  bismuth  and  potassa  salts  are  employed  in  nearly  atomic 
proportions  for  the  production  of  a  neutral  citrate  of  bismuth  ; 
the  potassa  salt,  however,  is  in  slight  excess  on  account  of  its 
deliquescent  nature.  By  using  solutions  considerably  diluted, 
the  precipitate  is  rendered  less  dense,  and  the  washing,  by  dis- 
placement, is  thereby  facilitated.  This  is  a  tedious  part  of  the 
process,  but  the  best  means  of  accomplishing  the  purpose. 
Washing  by  decantation  is  inapplicable,  for,  before  it  has  been 
carried  to  a  sufficient  extent,  a  portion  of  the  powder  will  re- 
main suspended  in  the  liquid,  and  can  only  be  separated  by 
boiling. 
It  also  entails  a  greater  loss  of  the  citrate  of  bismuth,  which 
is  slightly  soluble  in  water.  The  absence  of  nitric  acid  from 
the  washings  can  be  judged  of  approximately  by  the  taste,  but 
better  by  faintly  tinging  a  portion  of  the  liquid  with  sulphate 
of  indigo,  and  boiling,  in  a  test  tube,  for  a  few  minutes  ;  if  the 
color  be  retained,  no  nitric  acid  is  present. 
In  the  preparation  of  citrate  of  bismuth  and  ammonia,  the 
citrate  of  bismuth  previously  dried  is  employed,  in  order  that  a 
concentrated  solution  may  be  obtained  without  resort  to  evapo- 
ration, as  the  heat  necessary  in  this  operation  occasions  a  pre- 
cipitate, caused,  apparently,  by  a  loss  of  ammonia. 
The  solution  should  be  spread  with  a  brush,  on  panes  of  glass, 
to  dry. 
Citrate  of  bismuth  and  ammonia,  as  thus  prepared,  is  in  the 
form  of  brilliant  white,  semi-translucent  scales,  not  at  all  deli- 
quescent, but  readily  soluble  in  distilled  water. 
An  aqueous  solution  of  this  salt,  if  long  kept,  like  those  of 
the  citrates  in  general,  is  subject  to  spontaneous  change.  The 
addition  of  small  portions  of  alcohol  and  ammonia,  as  indicated 
in  the  formulas  for  a  solution  of  citrate  of  bismuth  and  ammonia, 
will,  it  is  believed,  preserve  it  indefinitely.    The  same  result 
