262  Determination  of  Hydrocyanic  Acid.    {  Am  jiJ"  iST"11' 
until  all  of  the  sulphur  has  been  removed  from  the  nitrate.  Ferric 
chloride  is  then  added  to  the  filtrate  and  the  color  of  ferric  thiocyanate 
is  obtained.  After  an  examination  of  this  method  we  found  that 
it  was  not  accurate  enough  for  our  work.  Our  two  chief  objections 
to  this  method  are  as  follows : 
In  boiling  an  acid  solution  of  a  thiocyanate,  some  free  thiocyanic 
acid  is  lost  because  it  is  volatile,  the  boiling-point  of  the  acid  being 
85  °.  This  loss  was  shown  by  taking  two  equal  quantities  of  potas- 
sium thiocyanate,  diluting  both  portions  with  water,  acidifying 
with  hydrochloric  acid,  and  boiling  one  of  them  for  ten  minutes  in 
an  Erlenmeyer  flask.  The  boiled  solution  was  then  cooled.  Both 
the  boiled  and  unboiled  portions  were  tested  by  adding  equal  quanti- 
ties of  ferric  chloride.  The  portion  that  had  been  boiled  gave  less 
color  than  the  one  not  boiled.  This  test  was  repeated  and  we  found 
that  in  every  case  boiling  resulted  in  a  loss  of  thiocyanic  acid. 
The  second  important  objection  to  the  thiocyanate  method  is 
based  on  the  nature  of  the  reaction  between  a  thiocyanate  and  ferric 
chloride,  FeCl3  +  3KCNS  <=±  Fe(CNS)3  +  3KCI.  The  equi- 
librium of  this  reaction  is  so  easily  disturbed  by  the  addition  of  ferric 
chloride,  intensifying  the  color,  or  the  addition  of  other  salts,  dimin- 
ishing this  color,  that  it  is  difficult  to  adjust  conditions  so  as  to  obtain 
constant  results.  Changes  of  temperature  also  have  a  marked 
influence  on  the  density  of  the  color. 
In  recent  years  several  papers  have  been  published  on  the  forma- 
tion of  Prussian  blue  from  cyanides  and  the  colorimetric  estimation 
of  the  cyanide  by  comparison  with  a  standard  suspension  of  Prussian 
blue.  The  most  careful  attempts  to  arrive  at  a  quantitative  method 
seem  to  have  been  made  by  Berl  and  Delpy  and  by  Lander  and 
Walden. 
Berl  and  Delpy  5  make  the  solution  to  be  tested  alkaline  with 
potassium  hydroxide,  add  a  solution  of  ferrous  sulphate,  allow  the 
mixture  to  stand  at  room  temperature  for  at  least  ten  minutes,  shak- 
ing frequently,  and  finally  boil  for  two  to  fifteen  minutes.  The 
resulting  mixture  is  cooled  and  then  acidified  with  hydrochloric  acid. 
In  the  case  of  very  dilute  solutions  of  hydrocyanic  acid,  Berl  and 
Delpy  shake  out  the  acidified  solution  with  ether  eight  to  ten  times, 
°Ber.,  43,  1430  (1910). 
