5 7$  Potassium  Ferrocyanide  and  Ferricyanide.  { Am,5g£; f8^rm' 
presence  of  ferric  oxide,  and  that  no  precipitates  are  formed  in 
presence  of  mercuric,  lead,  manganous,  uranic  and  stannic  salts. 
These  different  properties  of  the  two  double  iron  cyanides  render 
it  possible  to  estimate  some  metals  volumetrically  in  presence  of 
one  another,  or  to  estimate  them  gravimetrically,  as  most  ferri- 
cyanides  may  be  readily  filtered  off.  .  Zinc,  for  instance,  may  be 
accurately  estimated  either  volumetrically  or  gravimetrically  by 
means  of  potassium  ferricyanide  in  its  acetic  or  nitric  acid  solution, 
even  in  presence  of  lead,  which  may  then  be  titrated  in  the  filtrate 
with  potassium  ferrocyanide.  Tin  rnay  be  titrated  by  means  of 
potassium  ferrocyanide,  even  in  presence  of  arsenic  and  antimonic 
acids,  after  the  solution  has  been  evaporated  with  oxalic  acid  and 
then  mixed  with  a  little  dilute  sulphuric  acid. 
The  ferricyanide  solution  should  give  no  coloration  with  a 
uranium  solution,  and  no  precipitate  with  a  lead  salt.  If  it  should 
do  so,  it  must  be  mixed  with  a  little  chlorine-water,  and  the  salt 
recrystallized. 
When  titrating  with  ferrocyanide  or  ferricyanide,  it  is  not  possible 
to  add  the  indicator  straight  to  the  liquid  under  examination,  but 
use  must  be  made  of  test  papers.  The  indicator  used  must  show 
either  the  disappearance  of  the  last  trace  of  the  metal  or  else  the 
slightest  excess  of  the  precipitant. 
The  author  prepares  his  test  papers  as  follows :  A  moderately 
thick  but  dense  and  smooth  kind  of  filter  paper  is  cut  into  strips  of 
30  cm.  in  length  and  1  5  cm.  in  width.  Across  the  narrow  part,  at 
a  distance  of  about  4  cm.  from  each  other,  stripes  are  made  with 
the  solution  of  the  indicator,  which  consists  of  cupric  acetate  or 
ferric  chloride,  if  ferrocyanide  is  used  ;  or  cobaltous  or  ferrous  sul- 
phate, when  a  ferricyanide  is  employed  in  the  titration.  When 
both  are  used  in  succession,  a  mixture  of  ferrous  ammonium  sul- 
phate and  ferric  chloride  is  used. 
When  apparently  enough  of  the  ferrocyanide  or  ferricyanide  solu- 
tion has  been  added  to  the  solution  to  be  tested,  a  little  drop  is 
taken  out  by  means  of  a  thin  pencil  and  put  at  a  distance  of  about 
5  mm.  from  one  of  the  stripes,  when  the  reaction  will  make  its 
appearance  if  the  least  excess  is  present. 
The  author  recommends  using  for  the  titration  not  more  than  30 
cc.  of  liquid  containing  about  015  gram  of  metal.  The  process 
may  also  be  performed  by  adding  an  excess  of  the  reagent  and  titrat- 
