'^°'jun''e?im''°''}    Finely  Divided  Metals  and  Ferric  Salts.  291 
THE  ACTION  OF  FINELY  DIVIDED  METALS  ON  SOLU- 
TIONS OF  FERRIC  SALTS,  AND  A  RAPID  METHOD 
FOR  THE  TITRATION  OF  THE  LATTER. 
By  Douglas  J.  Caknegie,  B.A.,  Demonstrator  in  Chemistry,  Gronville  and 
Caius  College. 
1.  It  is  universally  admitted  in  text- books  that  of  existing  meth- 
ods for  converting  ferric  salts  into  ferrous  salts  prior  to  titration  with 
potassium  permanganate,  the  safest  and  best,  though  by  far  the  slow- 
est, method  is  to  boil  the  acidified  ferric  solution  with  zinc  in  an  inert 
atmosphere.  The  more  rapid  methods  (excluding  the  stannous  chlo- 
ride method,  which  is  not  applicable  if  permanganate  titration  be  em- 
ployed) are  hampered  by  the  facts  that  in  their  employment  it  is  diffi- 
cult to  determine  the  point  of  exact  reduction,  and  excess  of  the  re- 
ducing agent  is  as  fatal  as  defect. 
It  seemed  to  me  that  the  safest  method  might  be  so  modified  as  at 
the  same  time  to  make  it  a  rapid  one,  by  increasing  the  effective  sur- 
face of  the  zinc  through  the  employment  of  zinc-dust  in  the  place  of 
the  usual  granulated  zinc.  While  experimenting  in  this  direction,  I 
found  that  zinc-dust  instantly  reduces  ferric  to  ferrous  salt,  and  this 
even  in  neutral  solutions.  At  the  same  time,  if  the  solution  is  neutral, 
iron  is  precipitated  partly  as  ferrous,  partly  as  ferric  hydroxide.  In 
acid  solutions,  no  iron  is  precipitated,  but  the  reduction  is  less  rapid 
the  more  free  acid  there  is  present.  In  every  case  zinc  goes  into  solu- 
tion. 
2.  Very  rapid  and  accurate  estimations  of  ferric  solutions  are  real- 
ized by  the  following  method  : 
The  bottom  of  a  dry  and  narrow  beaker  is  covered  with  zinc-dust, 
which  has  been  sifted  through  fine  muslin.  A  known  volume  of  fer- 
ric solution,  previously  nearly  neutralized  by  ammonia,  is  now  deliv- 
ered into  the  beaker,  and  shaken  briskly  with  the  zinc-dust.  Finally, 
a  known  volume  of  dilute  sulphuric  acid  is  added,  and  the  contents 
of  the  beaker  are  once  more  shaken.  It  is  essential  for  rapid  reduc- 
tion that  the  above  order  be  observed  ;  the  nearly  neutral  ferric  solu- 
tion must  j^rs^  be  added  to  the  zinc,  then  the  acid.  In  order  to  with- 
draw for  titration  a  definite  volume  of  the  ferrous  solution  free  from 
particles  of  undissolved  zinc,  I  make  use  of  the  ^'  reversed  filter.^'* 
"  A  glass  tube,  one  end  of  which  is  covered,  first  with  muslin,  then  with 
filter  paper,  held  in  position  by  an  india-rubber  ring. 
