4o  Manufacture  of  Anthraquinone.  {A%luYry^9\r£?' 
3,000  to  4,000  liters  of  water.  The  temperature  is  raised  to  75 0  G, 
and,  with  constant  stirring,  2,000  liters  of  a  chromic  solution,  con- 
taining 100  grammes  of  chromic  acid  and  250  to  300  grammes  of 
sulphuric  acid  at  66°  Be.  per  liter,  are  added  gradually.  The  2,000 
liters,  therefore,  contain  300  kilos  of  bichromate*  of  soda  and  600 
kilos  of  sulphuric  acid.  During  the  operation  the  temperature 
gradually  reaches  95 0  C. 
The  oxidation  is  carried  on  for  twelve  to  sixteen  hours,  and 
does  not  present  any  particular  difficulties.  The  chromic  acid  must 
not  be  added  too  quickly,  and  the  solution  must  not  be  too  concen- 
trated. Otherwise  a  persistent  and  troublesome  foam  is  produced. 
The  reaction  is  controlled,  from  time  to  time,  by  testing  samples.  A 
sample  is  filtered,  washed,  then  sublimed  over  the  naked  flame  in  a 
small  test-tube,  and,  with  a  little  practice,  it  is  easy  to  see  if  the 
anthraquinone  is  pure  or  whether,  on  the  other  hand,  there  is  still 
anthracene  present.  The  anthraquinone  sublimes  in  fine  needles, 
whereas  anthracene  forms  silvery  scales,  easily  recognizable. 
It  will  be  seen  by  the  proportions  given  that  the  bichromate  is 
employed  in  excess;  200  kilos  of  bichromate,  theoretically,  should 
be  able  to  oxidize  175  kilos  of  95  per  cent,  anthracene. 
When  the  oxidation  is  terminated  the  contents  of  the  vat  are 
forced  by  a  pump  through  a  filter  press,  in  which  the  anthraquinone 
is  thoroughly  washed.  If  the  anthracene  used  is  sufficiently  pure, 
the  yield  is  about  no  per  cent,  of  anthraquinone. 
The  anthraquinone  thus  obtained  is  sublimed  by  steam  in  an 
apparatus  similar  to  that  used  for  anthracene.  On  the  sublimation 
the  weight  is  reduced;  the  theoretical  117  per  cent,  falls  to  about 
80  to  85  per  cent. 
Formerly,  when  non-purified  anthracene  was  oxidized,  the 
anthraquinone  obtained  was  very  impure,  and  had  to  be  treated 
with  two  or  three  times  its  weight  of  sulphuric  acid  (66°  Be.)  at 
ioo°  C,  until  a  sample  poured  into  water  gave  an  almost  white  pre- 
cipitate. By  this  treatment  the  impurities  became  soluble  in  water, 
whereas  the  anthraquinone,  being  insoluble,  could  be  separated  out. 
Since  purified  anthracenes  have  been  obtainable,  this  operation  has 
disappeared,  sublimation  alone  being  generally  sufficient  to  bring 
the  anthraquinone  to  a  sufficiently  pure  state  for  ordinary  pur- 
poses. The  sulphuric  acid  treatment  is  still  resorted  to,  however,  in 
the  rare  cases  when  an  extremely  pure  anthraquinone  is  required. 
