234  NITROPRUSSIDES,  THEIR  COMPOSITION  AND  MANUFACTURE. 
Fe2Cy6M3— MCy+No  s=Fe2Cy5NO*M2. 
Ferridcyanide.  Nitro-prusside. 
The  question  to  be  solved  was,  What  oxide  of  nitrogen  replaces 
the  MCy  ?  In  the  first  instance,  pure  binoxide  of  nitrogen  was 
passed  through  a  warm  solution  of  ferridcyanide  of  potassium, 
which  was  acidulated  with  sulphuric  acid,  so  that  the  N02  might 
have  the  easier  work  of  replacing  HCy,  if  it  could.  The  liquid 
became  blue  and  muddy,  but  no  trace  of  nitro-prusside  was  formed. 
The  next  experiment  was  to  pass  the  red  nitrous  vapors  from 
starch  and  nitric  acid  through  the  warm  acid  solution  of  ferrid- 
cyanide. The  difference  was  wonderful ;  no  prussian  blue  ap- 
peared, the  color  changed  rapidly  to  the  red  of  nitro-prusside, 
and  much  hydrocyanic  acid  came  off.  The  conversion  to  nitro- 
prusside  was  almost  perfect.  It  thus  became  clear  that  the  re- 
placing oxide  of  nitrogen  is  either  N03  or  NOr  That  such  is 
the  case  is  further  shown  by  a  very  simple  experiment,  the  re- 
verse of  the  last.  On  adding  to  a  solution  of  pure  nitro-prusside 
a  solution  of  potash 'and  some  prussic  acid,  and  warming,  the  red 
of  the  nitro-prusside  changes  for  an  instant  to  deep  yellow,  and 
then  to  pale  yellow.  The  solution  is  found  to  contain  ferrid- 
cyanide, with  plenty  of  nitrite,  but  no  trace  of  nitroprusside. 
Hence  N03  or  N04  has  been  replaced  by  KCy,  and  ferridcyanide 
reproduced. 
To  determine  between  N03  and  IST04  the  author  sought  to  re- 
place the  KCy  by  pure  N03,  obtained  by  the  action  of  acetic 
acid  on  alkaline  nitrite,  whereby  N04  is  absolutely  excluded. 
Ferridcyanide  of  potassium  solution  was  mixed  with  a  solution 
of  corrosive  sublimate  and  acetic  acid,  and  a  nitrite  added,  and 
the  whole  was  left  for  some  hours,  when,  on  examining  the  mix- 
ture, nitro-prusside  was  found  in  abundance. 
The  reaction  was  as  follows  : — 
Fe3  Cy,K,+Hg  Cl+N03=Fe2  Cy5N03K2+Hg  Cy+KCl. 
And  the  true  formula  of  the  sodium  salt  is — 
Fe2  Cy5N03+Na,+  4  110=306. 
Curiously  enough,  this  formula  agrees  more  closly  with  Play  fair's 
analysis  than  any  yet  proposed. 
