570  Synthetical  Carbolic  Acid.  {A%Je°c%wbarm' 
off  by  means  of  a  filter  press.  The  clear  liquor  containing  calcium 
benzolsulphonate  is  treated  with  sufficient  sodium  carbonate  to 
precipitate  all  the  calcium  as  carbonate,  which  is  removed  by  filtra- 
tion, and  the  liquor  is  now  evaporated  to  dryness,  leaving  the 
sodium  benzolsulphonate  as  a  white  powder. 
In  a  large  cast-iron  kettle,  heated  by  a  coal  fire,  caustic  soda  is 
melted,  and  small  portions  of  the  dry  sodium  salt,  prepared  as 
above,  are  gradually  added  and  finally  the  whole  is  kept  in  quiet  fusion 
for  some  time.  The  melt  now  contains  sodium  carbolate  and  sul- 
phite together  with  the  large  excess  of  caustic  soda  used — 
C6H5  —  SOsNa  +  2NaOH  =  C6H5ONa  +  Na2SOs  +  H20 
It  is  ladled  from  the  kettle  into  pans  and  allowed  to  cool,  broken 
up,  dissolved  in  water  and  acidified  with  sulphuric  or  hydrochloric 
acid.  The  phenol  thus  liberated  separates  from  the  concentrated 
salt  solution,  and  can  be  collected  and  distilled. 
In  melting  the  sodium  benzolsulphonate  with  caustic  soda  it  is 
necessary,  in  order  to  obtain  a  good  yield,  to  use  a  very  large  excess 
of  the  latter.  A  greater  yield  is  obtained  with  caustic  potash,  and 
if  as  large  a  quantity  as  six  parts  are  used  to  one  of  the  soda  or 
potash  salt  a  nearly  theoretical  yield  can  be  obtained,  but  as  this 
would  greatly  increase  the  cost,  caustic  soda  is  used  instead. 
It  has  been  proposed  to  treat  the  melt  after  dissolving  in  water 
with  carbonic  acid  gas,  which  would  liberate  the  phenol  equally 
well  as  a  stronger  acid,  and  in  addition  would  form  carbonate  of 
soda  or  potash,  which  together  with  the  sulphite  already  present 
could  be  converted  into  the  hydrate  by  treating  with  lime,  concen- 
trated, and  used  for  a  second  operation.  It  does  not  appear,  how- 
ever, that  this  has  been  carried  out  in  practice. 
The  second  method  is  much  simpler.  A  pure  aniline  oil,  prefer- 
ably that  grade  called  aniline  for  blue,  is  dissolved  in  water  in  a 
lead-lined  tank  covered  with  a  hood  and  provided  with  stirrers  and 
leaden  steam  coils.  The  solution  is  acidulated  very  strongly  writh 
sulphuric  acid,  and  to  the  hot  liquid  a  solution  of  commercial  nitrite 
of  soda  is  gradually  added,  phenol  being  at  once  formed. 
In  this  reaction  the  sodium  nitrite,  in  contact  with  the  acid  solu- 
tion, liberates  nitrous  acid,  which  forms  diazobenzol  sulphate  with 
the  aniline  sulphate — 
(C6H5NH2)2H2S04  +  2HN02  =  (C6H5N  =  N)2S04  +  4H20 
