618         borne  Sodium  and  Manganese  Compounds.  {Am'De°eU^9arm' 
settle  for  an  hour  or  so,  and  afterwards  run  into  shallow  iron  crystal- 
lizing pans,  where,  after  four  or  five  days,  a  copious  crop  of  yellow  or 
brown  crystals  is  formed  having  the  formula  Na2S4-  9H20.  The 
mother-liquor  is  siphoned  off  and  again  boiled  down  to  31  °B.  for 
further  crystallization.  The  large  crystals  formed  are  broken  up, 
allowed  to  drain,  and  ultimately  jigged  in  a  centrifugal .  machine, 
whereby  they  obtain  a  lighter  appearance.  Dark  crystals  are  washed 
with  light  concentrated  liquor,  which  improves  their  appearance. 
The  salts  obtained  during  the  concentration  of  the  liquors,  owing  to 
their  oxidation,  are  employed  in  the  furnace  in  the  place  of  sodium 
sulphate. 
MANUFACTURE  OF  SODIUM  NITRITE. 
Sodium  nitrite  is  one  of  the  most  important  chemicals  used  in  the 
manufacture  of  azo  dyes.  There  exist  several  methods  for  its  manu- 
facture, all  of  which  are  based  on  the  reduction  of  sodium  nitrate  hj 
means  of  lead.  For  this  purpose  the  latter  is  fused  and  poured  on 
iron  plates  in  thin  layers.  After  cooling,  the  lead  plates  are  rolled 
up,  and  weighed  out  in  parcels  for  use.  The  lead  should  be  free 
from  zinc  and  antimony.  The  sodium  nitrate  is  then  fused  in  shallow 
cast-iron  pans  fitted  with  a  mechanical  agitator,  and  when  the  mass  is 
thoroughly  fused,  two  and  a  half  parts  of  lead  are  added  at  short 
intervals  for  each  part  of  sodium  nitrate  (95-96  per  cent.  NaN03)* 
The  reduction  ensues  immediately.  The  thin  leaves  of  lead  fuse 
easily,  and  are  oxidized  to  yellow  lead  oxide  (litharge).  After 
some  time  a  further  amount  of  lead  is  added,  as  an  excess  of  lead  is 
necessary,  and  the  agitation  of  the  fused  mass  is  continued  for  at  least 
three-quarters  of  an  hour,  in  order  to  obtain  a  product  of  high 
strength.  The  mass  has  a  tendency  to  stick  to  the  sides,  and  great 
care  must  be  exercised  to  remove  these  crusts  with  an  iron  spatula,  as 
the  slightest  inattention  causes  the  burning  through  of  the  pan.  If 
the  crusts  turn  reddish,  it  is  a  certain  sign  that  the  pan  is  in  great 
danger  of  being  destroyed.  These  crusts  must  at  once  be  covered  by 
a  fresh  quantity  of  nitrate,  and  the  fire  withdrawn  if  necessary.  The 
whole  process  of  fusion  is  finished  in  about  3J  hours.  A  sample 
drawn  should  test  at  least  90  per  cent,  of  sodium  nitrite.  The  mass 
after  cooling  is  lixiviated  in  wrought-iron  tanks  with  water  or  weak 
liquor.  As  soon  as  the  solution  stands  at  36°  to  38°  B.  it  is  siphoned 
off  and  neutralized  with  nitric  acid  of  1'029  specific  gravity.  The 
liquor  is  then  concentrated  to  42°  or  43°  B.,  allowed  to  clarify  for 
