It  is  first  necessan'  to  have  a  standard  solution  of  iron  in  the  ferric  state,
which  can  be  made  by  dissolving  1004  gm.  of  iron  wire  in  nitro-hydrochloric
acid,  precipitating  with  ammonia,  washing  and  re-dissolving  the  ferric  oxide
in  a  little  hydrochloric  acid,  then  diluting  to  1  liter.  I  c.c.  of  this  solution
contains  I  milligram  of  pure  iron  in  the  form  of  ferric  chloride.  It  may  be
further  diluted,  when  required,  so  as  to  contain  ^\^  milligram  in  a  c.c,  as  in
the  case  of  ammonia  solution  for  Nesslerizing.*  The  solution  used  for
striking  the  colour  is  potassic  ferrocyanide  dissolved  in  water  (1  :  20).
Example :  The  material  containing  a  minute  unknown  quantity  of  iron,
say  a  water  residue,  is  dissolved  in  hydrochloric  acid,  and  diluted  to  100  c.c,
or  any  other  convenient  measure.  10  c.c  are  placed  into  a  white  glass  cylinder
marked  at  100  c.c,  1  c.c.  of  concentrated  nitric  acid  added  (the  presence  of
free  acid  is  always  necessary  in  this  process),  then  diluted  to  the  mark  with
distilled  water,  and  well  stirred.
1  c.c  of  ferrocyanide  solution  is  then  added,  well  mixed,  and  allowed  to
stand  at  rest  a  few  minutes  to  develop  the  colour.
A  similar  cylinder  is  then  filled  with  a  mixture  of,  say,  1  c.c  of  standard
iron  solution,  1  c.c.  nitric  acid  and  distilled  water,  and  1  c.c.  ferrocyanide

*  A  solution  of  the  same  strengtli  can  also  be  made  by  weighing  007  gm.  of  pure
ammomo  ferrous  sulphate  (§  31.2a),  dissolving  in  water,  acidifying  with  sulphuric  acid,
adding  sufficient  permanganate  solution  to  convert  the  iron  exactly  into  ferric  salt,  then
dilating  to  1  liter.
§61.
VIJLU.MKT111C   ANAT.YSIH.
