Arn'ji°n7'i9ih5arm'}  Determination  of  Hydrocyanic  Acid.  263 
and  shake  out  the  ether  with  a  small  quantity  of  potassium  hydroxide 
solution,  thus  obtaining  the  hydrocyanic  acid  in  a  more  concentrated 
form. 
The  method  of  Berl  and  Delpy  has  been  improved  by  Lander 
and  .Walden.6  These  workers  concentrate  dilute  alkaline  solutions  of 
hydrocyanic  acid  by  boiling^  the  final  concentrating  being  carried 
out  in  a  test-tube  almost  to  dryness.  The  small  quantity  of  liquid 
left  is  then  cooled  and  ferrous  sulphate  solution  added.  The  mixture 
is  allowed  to  stand  ten  minutes,  with  frequent  shaking,  then  acidified 
with  hydrochloric  acid  and  warmed  gently. 
Vorlander 7  investigated  the  formation  of  Prussian  blue  from 
cyanides.  He  adds  a  solution  of  ferrous  sulphate  to  the  alkaline 
solution  of  the  cyanide  and  boils  for  one  to  two  minutes.  He  then 
filters  and  acidifies  the  filtrate  with  hydrochloric  acid,  after  which 
he  adds  a  cold,  freshly-prepared,  saturated  solution  of  ferrous 
sulphate.  He  allows  the  mixture  to  stand  until  the  maximum  color 
is  obtained. 
Knight 8  adds  to  the  alkaline  distillate  containing  hydrocyanic 
acid,  solid  ferrous  sulphate,  then  a  solution  of  ferric  chloride,  and 
boils  for  one  minute.  Hydrochloric  acid  in  excess  is  then  added  to 
the  hot  solution.  The  precipitate  of  Prussian  blue  is  filtered,  washed 
with  alcohol,  and  dissolved  in  a  sufficient  quantity  of  sodium  hydrox- 
ide solution.  This  solution  is  acidified  with  acetic  and  hydrochloric 
acids,  ferric  chloride  is  then  added,  and  any  brown  color  is  removed 
by  adding  more  hydrochloric  acid.  The  mixture  is  concentrated  to 
about  one-half  the  original  volume  and  the  precipitate  of  Prussian 
blue,  after  filtration,  is  determined  by  weighing. 
In  a  method  involving  the  quantitative  estimation  of  a  substance 
by  means  of  colorimetry,  it  is  obvious  that  a  given  quantity  of  the 
substance  should  always  produce  a  given  density  of  color  in  a  given 
volume  of  solution.  It  is  also  necessary  that  the  shade  of  color 
should  always  be  the  same.  For  instance,  a  blue  solution  cannot 
be  compared  accurately  with  a  blue-green  solution.  Our  aim  in  this 
work  has  been  to  obtain  a  constant  shade  as  well  as  density  of  color. 
We  have  studied  the  following  conditions  which  influence  the  for- 
mation of  Prussian  blue  and  the  shade  of  the  suspension. 
6  The  Analyst,  36,  266  (1911). 
'Ber.,  46,  181  (1913). 
87.  hid.  Eng.  Chem.,  6,  909  (1914). 
