NITROPRUSSIDES,  THEIR  COMPOSITION  AND  MANUFACTURE.  235 
The  success  of  this  reaction  on  the  small  scale  suggested  it  as 
a  means  of  manufacturing  nitroprusside  on  the  large  scale.  The 
following  is  the  process  Mr.  Hadow  gives  : — 
A  strong  solution  of  caustic  soda  is  prepared,  and  thoroughly 
saturated  with  the  nitrous  acid  vapors  from  starch  and  nitric 
acid  (old  battery  acid  does  well).  The  amount  of  true  nitrite 
of  soda,  NaON03,  in  this  solution  is  determined  by  perman- 
ganate, by  taking  a  small  measured  quantity,  diluting  largely 
with  water,  acidifying  with  sulphuric  acid,  and  observing  the 
amount  of  standard  permanganate  decolorized.  Fe4=NaO,N03, 
in  decolorizing  power.  The  amount  of  real  nitrite  of  soda  in 
the  solution  being  known,  is  recorded  on  the  bottle. 
A  mixture  of  any  bulk  is  then  made  in  the  following  propor- 
tions : — 
J  Ferric! cyanide  of  potassium  332  grs.  \  in  |  pint  of  boil- 
\  Acetic  acid  (Beaufoy's)  .  .    800  grs.  J       ing  water, 
-g  /  Corrosive  sublimate  .  .  .  .  164  grs.  \  make  up  to  J  pint 
[  Solution  of  nitrite  of  soda  =  80  grs.  of  true  NaONOg  /  with  cold  water, 
adding  acetic  acid,  if  necessary,  until  quite  clear.  Pour  the 
cold  solution  B  into  the  hot  solution  A.  The  mixture  becomes 
at  first  turbid,  but  in  a  few  minutes  afterwards  quite  transparent. 
It  should  be  kept  at  a  temperature  of  140°  (at  which  point  little 
nitrous  acid  is  lost)  for  some  hours,  with  addition,  if  necessary, 
of  more  nitrite  of  soda  and  acetic  acid  from  time  to  time,  until 
all  ferridcyanide  has  disappeared.  When  this  is  the  case,  the 
whole  mixture  may  be  boiled  down,  until,  on  cooling,  it  solidifies 
to  a  thick  paste.  The  right  state  of  concentration  has  been  at- 
tained, when,  on  beating  the  paste  up,  and  squeezing  in  linen,  a 
pale  syrup,  chiefly  of  acetate  of  potash,  is  expressed.  The  pearly- 
looking  mass,  freed  from  acetate  of  potash  as  far  as  possible, 
must  be  redissolved  in  such  an  amount  of  boiling  water,  that,  on 
cooling,  a  large  proportion  of  cyanide  of  mercury  separates  in 
white  pearly  scales,  quite  free  from  nitroprusside  crystals.  On 
squeezing  in  linen,  a  deep-red  solution  of  nitroprusside  is  ex- 
pressed, andawhite,  pearlymass  of  cyanide  of  mercury  remains  on 
the  linen.  On  concentrating  the  red  filtrate,  a  large  crop  of  crys- 
tals of  nilroprusside  of  sodium  is  obtained  in  a  mother-liquid 
containing  more  or  less  cyanide  of  mercury,  in  pearly  scales,  easily 
separated  by  throwing  the  whole  on  a  moderately  coarse  hair 
