488  Acetates  of  Chrommm,  Iron,  and  Aluminium,  {^"^ocriss^^^"^' 
aluminium  in  tlie  form  of  bastic  acetates,  becomes  utterly  useless,  either 
for  qualitative  or  for  quantitative  analysis. 
The  influence  of  a  chromic  salt  on  the  precipitation  of  ferric  or 
aluminic  salts  varies  to  a  certain  extent  according  as  to  whether  it  is 
in  the  violet  or  the  green  modification.  When  a  green  solution  of 
chromic  acetate  is  mixed  with  a  small  quantity  of  ferric  acetate,  and 
the  mixture  is  boiled,  the  iron  is  separated  in  the  form  of  a  floceulent 
precipitate  ;  but  if  a  solution  of  ferric  acetate  be  added  to  an  originally 
violet  solution  of  chromic  acetate  and  the  mixture  boiled,  then,  what- 
ever may  be  the  quantity  of  ferric  solution  added,  no  precipitation  of 
iron  will  take  place.  Exactly  the  same  is  the  case  with  a  violet  solu- 
tion of  chromic  acetate  obtained  by  boiling  a  green  solution  with 
sodium  acetate;  also  with  a  green  chromic  acetate  solution  boiled 
without  any  addition.  Grreen  chromic  acetate,  therefore,  acts  like 
most  metallic  salts  when  heated,  in  precipitating  a  solution  of  ferric 
acetate ;  whereas  violet  chromic  acetate,  so  far  from  producing  a  pre- 
cipitate, even  renders  a  large  portion  of  the  iron  acetate  as  indifferent 
towards  reagents  as  itself.  Thus,  when  a  violet  solution  or  a  boiled 
green  solution  of  chromic  acetate  is  boiled  Avith  a  moderate  quantity 
of  ferric  acetate  solution,  sodium  acetate  may  then  be  added  and  the 
liquid  boiled,  without  precipitation  of  the  iron.  Neither  is  any  action 
produced  in  such  solutions  for  a  long  time  at  ordinary  temperatures 
by  any  of  the  reagents — such  as  caustic  alkalis,  ammonia,  caustic 
baryta,  alkaline  carbonates  and  phosphates,  barium  carbonate,  or 
potasssium  ferrocyanide — which  precipitate  ferric  salts  unden  normal 
circumstances,  precipitation  not  taking  place  indeed  until  after  several 
hours'  boiling.  The  quickest  action  is  produced  by  ammonium  sul- 
phide; but  even  in  this  case  a  decided  retardation  takes  place,  as 
compared  with  the  rate  of  preci})itation  in  pure  solutions  of  ferric 
acetate.  If  the  quantity  of  ferric  acetate  added  exceeds  a  certain 
limit,  boiling  with  sodium  acetate  or  addition  of  caustic  ammonia,  etc., 
gives  rise  to  partial  separation  of  the  iron. 
The  quantity  of  ferric  acetate  rendered  passive  by  a  given  quantity 
of  chromic  acetate  depends  ujoon  several  circumstances,  the  most 
influential  of  which  are  the  amount  of  chromic  acetate  present,  the 
concentration  of  the  liquid,  and  the  temperature  at  which  the  acetates 
act  on  one  another.  It  increases  up  to  a  certain  limit  with  the  tem- 
perature and  the  dilution  of  the  solutions,  and,  so  long  as  the  heat  is 
kept  below  the  boiling  point,  with   the  duration  of  the  action. 
