644  ESTIMATION  OF  COMMERCIAL  SALTPETRES. 
in  a  slight  excess  of  standard  solution.  At  this  point  of  the 
process  fill  a  graduated  burette  with  the  solution  N,  and  add  it, 
little  by  little,  until  the  excess  of  barjtic  standard  solution  is 
exactly  precipitated.  This  last  operation  is  somewhat  tedious 
and  diflScult,  because  the  solutions  do  not  readily  become  clear, 
and  it  is  often  necessary  to  filter  small  trial  quantities.  The 
relation  between  the  total  volume  of  N  employed  and  the  vol- 
ume of  barytic  standard  solution,  gives  the  proportion  of  sulphate 
in  a  given  weight  of  nitre. 
Nitric  acid  is  estimated  in  a  very  elegant  manner,  by  using  dry 
bichromate  of  potash,  which  expels  nitric  acid  from  the  alkaline 
nitrates  without  decomposing  the  chlorides.  The  operation  is 
thus  conducted  : — On  a  lump  of  saltpetre,  melted  and  cooled, 
weighing  from  2  to  5  grammes,  and  placed  in  a  small  but  suffi- 
ciently large  platinum  crucible,  place  about  twice  its  weight  of 
previously  melted  and  pulverised  bichromate  of  potash.  Weigh, 
and  then  carefully  heat  the  whole.  At  first  the  reaction  is 
somewhat  energetic,  and  it  frequently  happens  that  considera- 
ble quantities  of  the  mixture  are  carried  off  and  are  condensed 
on  the  inner  surface  of  the  cover,  which  should  be  shaped  like 
a  deep  capsule,  so  that  the  fusible  matter  projected  against  it 
may  fall  in  the  crucible.  In  proportion  as  the  nitrous  vapors 
diminish,  raise  the  temperature  to  a  dull  red  heat,  taking  care  to 
equalise  the  temperature  of  the  cover  by  directing  a  gas  jet  on 
it.  When  the  operation  is  finished,  leave  the  crucible  uncovered 
for  a  few  instants,  that  the  air  in  it  may  be  renewed ;  allow  it 
to  cool,  and  weigh  it.  The  loss  of  the  weight  indicates  the 
quantity  of  nitric  acid  displaced,  from  which  may  be  calculated 
the  corresponding  proportion  of  nitrate  of  potash  or  soda. 
If  the  nitrate  analysed  contains  at  the  same  time  potash  and 
soda,  an  excess  will  be  found  by  converting  the  estimated  nitric 
acid  into  nitrate  of  potash ;  and,  on  the  contrary,  a  deficiency, 
by  converting  the  acid  into  nitrate  of  soda.  This  excess  or  defi- 
ciency allows  of  the  calculation  of  the  relative  proportion  of  the 
two  alkaline  nitrates  in  the  salt  analysed. — Chem.  News,  Lon- 
don, Aug.  3,  1861.  from  Ann.  du  Conservatoire  des  Arts  et 
Metiers. 
