170  ON  THE  MANUFACTURE  OF  CITRIC  ACID. 
Hitherto  it  seems  to  have  baffled  the  efforts  of  manufacturers 
to  overcome  this  difficulty,  and  hence 'the  great  cost  at  which  the 
acid  is  prepared  ;  but  the  writer  of  this  paper  has  found  that, 
when  the  concentrated  juice  is  diluted  to  the  same  strength  as  the 
fresh  juice  (which  contains  about  twelve  ounces  of  citric  acid  to 
the  gallon)  a  great  part  of  the  mucilaginous  and  other  im- 
purities will  separate  by  subsidence  in  a  flocculent  form,  and  the 
citrate  of  lime  and  also  the  citric  acid  produced  from  the  juice  so 
purified  will  be  in  a  state  of  comparative  purity. 
Another  very  important  point  for  consideration  of  the  manu- 
facturer is  the  way  in  which  the  solutions  of  these  acids  ar6  evapo- 
rated ;  the  sulphuric  acid,  necessary  to  be  in  slight  excess  at  first, 
so  accumulates  in  percentage  by  the  repeated  evaporation  and 
crystallization  of  the  citric  acid  out  of  the  solutions,  as  to  cause 
a  most  destructive  action  upon  that  remaining  in  the  mother 
liquors.  Various  means  have  been  used  to  prevent  this,  the 
most  successful  being  doubtless  the  substitution  of  water  for 
steam  heat  during  evaporation,  but  as  the  lower  temperature  re- 
quires longer  exposure  to  heat,  the  advantage  thus  gained  is 
almost  counteracted. 
The  most  effectual  means  of  remedying  this  is  by  passing  the 
mother  liquors,  so  soon  as  the}?-  are  found  to  contain  any  dangerous 
amount  of  sulphuric  acid,  through  a  fresh  portion  of  citrate  of 
lime,  which  not  only  removes  at  once  all  free  sulphuric  acid  con- 
tained, but  there  is  also  deposited  a  quantity  of  flocculent  matter 
which  was  held  in  solution  by  it,  and  which  is  no  doubt  the  cause  of 
the  mother  liquors  when  old. crystallizing  only  with  such  difficulty.  ■* 
The  flocculent  precipitate  referred  to  consists  for  the  most  part 
of  sulphate  of  lime,  but  contains  also  some  phosphate  of  iron  and 
alumina,  and  accumulates  more  or  less  according  to  the  care  be- 
stowed in  evaporating  the  solution,  it  being  most  insoluble  when 
the  liquor  contains  about  6  lbs.  of  citric  acid  to  the  gallon, 
marking  1200  to  1250  (according  to  temperature)  on  the  hydro- 
meter, and  if  not  separated  from  the  solutions  at  that  point  it 
partly  dissolves  again  as  they  become  more  concentrated. 
As  a  consequence  of  this,  it  is  found  necessary,  in  the  usual 
working,  to  re-saturate  these  solutions  as  soon  as  they  become 
surcharged  with  these  matters  and  the  sulphuric  acid  above  re- 
