264  Determination  of  Hydrocyanic  Acid,  j 
Am.  Jour.  Ptaarm. 
June,  1915. 
Influence  of  Concentration  on  the  Formation  of  Prussian 
Blue. 
Bed  and  Delpy  found  that  they  obtained  less  Prussian  blue 
from  given  quantities  of  hydrocyanic  acid  in  a  dilute  than  in  a  con- 
centrated solution.  These  authors,  therefore,  concentrated  dilute 
solutions  of  hydrocyanic  acid  by  shaking  out  with  ether  as  previously 
mentioned.  They  state  that  in  this  manner  nearly  all  of  the  hydro- 
cyanic acid  can  be  obtained.  In  our  attempts  to  concentrate  the 
hydrocyanic  acid  by  means  of  ether,  we  found  that  we  could  not 
obtain  nearly  quantitative  results.  These  low  results  are  due  partly 
to  the  loss  of  hydrocyanic  acid  which  will  occur  by  evaporation  of 
the  ether.  This  evaporation  cannot  be  prevented  at  room  tempera- 
ture, since  the  acid  solution  has  to  be  shaken  out  eight  to  ten  times 
with  ether.  Moreover,  the  volume  of  alkali  needed  to  extract  the 
hydrocyanic  acid  from  the  ether  will  be  so  great  that  the  maximum 
quantity  of  Prussian  blue  can  scarcely  be  expected,  as  shown  below. 
Lander  and  Walden,  finding  also  less  precipitate  of  Prussian 
blue  in  dilute  than  in  concentrated  solutions  of  hydrocyanic  acid, 
boiled  the  alkaline  solution  down  almost  to  dryness.  They  state  that 
this  could  be  done  without  the  loss  of  hydrocyanic  acid. 
The  influence  of  concentration  is  indeed  a  very  important  factor. 
We  made  a  series  of  experiments,  using  1  mg.  of  potassium  cyanide  in 
each  case  and  concentrations  ranging  from  less  than  1  Cc.  to  10  Cc. 
In  a  dilution  of  10  Cc.  less  than  one-half  as  much  Prussian  blue  was 
obtained  as  when  the  volume  was  only  1  Cc.  We  found  that  the 
maximum  color  was  obtained  only  when  the  volume  of  solution  to 
be  tested  was  not  greater  than  1.5  Cc,  while  in  greater  volumes  the 
density  of  the  color  decreased  with  the  increasing  volume.  Where 
the  quantity  of  potassium  cyanide  was  more  than  1  mg.  the  volume  of 
the  solution  to  be  tested  could  be  somewhat  increased  without  loss. 
Hence,  working  with  the  methods  of  Berl  and  Delpy  or  Knight, 
where  larger  volumes  than  those  mentioned  above  are  used,  the  maxi- 
mum quantity  of  Prussian  blue  would  not  be  obtained. 
To  test  the  method  of  concentrating  used  by  Lander  and  Walden, 
we  evaporated  25  Cc.  of  an  alkaline  solution  of  a  cyanide  to  1  Cc. 
in  a  distilling  flask  and  lost  almost  one-third  of  the  cyanide.  When 
the  final  evaporation  was  performed  in  a  test-tube  the  loss  was  less, 
but  still  considerable.  Hence  it  appears  that  the  method  of  Lander 
and  Walden  also  cannot  give  the  maximum  quantity  of  Prussian  blue, 
