53 2  Iron  and  its  Constituents.  {^I7.;l^xm' 
air,  is  oxydized  to  hypo-nitric  anhydrid,  N204,  forming  red  fumes. 
Very  dilute  nitric  acid,  finally,  when  acting  on  iron  in  the  cold,  trans- 
forms it  into  ferrous  nitrate,  Fe(N03)2,  forming  at  the  same  time 
ammonic  nitrate  by  the  action  of  nascent  hydrogen  on  nitric  oxide,  NO, 
in  presence  of  nitric  acid.  The  latter  gas  (NO),  if  absorbed  by  the 
ferrous  solution,  produces  a  black  coloration  of  the  same.  The  form- 
ation of  ammonic  nitrate  takes  place  in  accordance  with  the  following 
equation  : 
NO    +    5H  +  HNOs=  H20+  NH4NOs 
Nitric  oxide.       Hydrogen.     Nitric  acid.         Water.       Ammonic  nitrate. 
The  lower  the  temperature  during  this  process,  the  larger  the 
quantity  of  ammonic  nitrate  formed.  The  different  chemical  pro- 
cesses which  take  place  when  iron  is  acted  on  by  very  dilute  nitric 
acid  are  illustrated  by  the  following  scheme  of  equations  : 
a.  Fe  +   2HNO3  =  Fe(NOs)2    +  H2 
Iron.  Nitric  acid.  Ferrous  nitrate.  Hydrogen. 
b.  Fe+  8HNO3  =  3Fe('NO,)2  +  4HaO  +  2NO 
Iron.  Nitric  acid.  Ferrous  nitrate.  Water.  Nitric  oxide. 
These  two  reactions  take  place  at  the  same  time  and  are  followed 
by  the  formation  of  ammonic  nitrate  above  alluded  to.  On  adding 
five  times  the  equation  a  to  in  the  presence  of  an  excess  of  2HNOg, 
we  obtain 
8Fe  +  20HNO3  =  8Fe(N03)2  +  6H20  +  2NH4NOs 
Iron.  Nitric  acid.  Ferrous  nitrate.  Water.  Ammonic  nitrate. 
thus  expressing  the  different  chemical  reactions  in  one  equation. 
The  nature  of  the  reddish-brown  flocculent  body  which  is  formed 
from  the  combined  carbon  is  not  yet  sufficiently  investigated.  Its 
ability  of  producing  lighter  and  darker  colorations  of  the  ferric  solution, 
proportionately  to  its  quantity,  forms  the  basis  of  Eggertz's  colorimetric 
method  for  its  estimation  in  steel.  The  results  agree  to  0*02  per  cent, 
with  those  obtained  by  combustion  analysis. 
On  dissolving  iron  by  means  of  iodine,  the  total  quantity  of  carbon 
remains  in  the  residue,  the  resulting  solution  containing  but  the  iodides 
of  the  metallic  elements,  viz.  :  ferrous  and  manganous  iodide,  the  latter 
in  minute  quantity.  An  excess  of  iodine,  however,  will  also  oxidize 
the  non-metallic  impurities,  as  phosphorus,  sulphur,  etc.,  by  decom- 
posing water  and  with  the  formation  of  hydriodic  acid,  HI.  For  the  pre- 
paration of  pure  ferric  iodide,  Fe2I6,  it  is  therefore  essential  to  filter  the 
solution  of  the  green  ferrous  salt,  Fe2I4,  before  adding  a  further  quantity 
