Am.  Jour.  Pharm.  1 
December,  1920.  ) 
Hydrocyanic  Acid. 
91 
Taking  5  Cc.  acid  of  2  per  cent,  strength,  which  should  require 
not  more  than  19  Cc.  AgNOs,  the  amount  of  N/io  soda  required 
would  be  38  Cc.  In  this  case  the  use  of  4  Cc.  of  normal  soda  will 
ensure  accurate  results.  Also,  more  than  10  Cc.  oi  N/i  soda  should 
never  be  used,  since  this  is  sufficient  for  a  titration  in  which  50  Cc.  of 
N/10  AgNOs  are  required. 
From  equations  (a)  and  (6)  it  is  obvious  that  in  the  case  of  HCN 
the  addition  of  alkali  is  necessary  to  allow  the  Liebig  reaction  to  take 
place  by  converting  finally  the  NHO3  which  would  otherwise  be 
liberated  into  KNO3. 
Evidently,  for  this  purpose,  alkaline  carbonates  would  serve  as 
well  as  hydroxides,  and  at  first  sight  it  would  seem  that  this  is  an 
easy  way  of  overcoming  the  difficulty  by  keeping  the  excess  of  alkali 
required  sufficiently  low.  Were  HCN  not  a  volatile  substance,  such 
would  be  the  case,  but  since  HCN  is  a  very  weak  acid,  which  barely 
reddens  litmus,  and  does  not  decompose  carbonates,  it  is  obvious  that, 
at  the  beginning  of  the  titration,  all  the  HCN  present  is  in  the  free 
state,  and  some  free  HCN  will  be  present  up  to  the  end,  and  this  leads 
to  loss  owing  to  the  continual  agitation  of  the  solution  during  titration. 
After  adding  excess  of  alkaline  hydroxide,  however,  this  excess 
can  be  considerably  reduced  by  the  addition  of  a  little  sodium  bi- 
carbonate. 
NaON  +  NaHCOs  =  NasCOa  +  H2O. 
Influence  of  Other  Substances  on  Liebig  s  Process. — Ammonium 
salts  would  not  interfere,  but  free  NH3  does  so,  as  AgCN  is  soluble 
in  ammonia,  and  if  ammonium  salts  are  present  some  free  NH3  will 
be  liberated  by  the  excess  of  alkali.  This  can  be  overcome  by  addition 
of  a  little  carbonic  acid  water ;  a  bottle  or  syphon  of  aerated  water  is 
useful  for  the  purpose.    Addition  of  NaHCOs  is  not  suitable  here. 
Chlorides,  bromides,  and  iodides  do  not  interfere.  Squire  states 
in  last  two  editions  of  the  Companion  that  chlorides  are  also  estimated 
in  lyiebig's  original  process,  but  this  is  an  error. 
If  halogen  salts  are  present  the  end-point  (precipitation)  is  shown 
by  the  precipitation  of  the  silver  halides,  which  are  more  insoluble 
than  AgCN,  and  this  tends  to  make  the  end-point  sharper.  The 
average  error  in  Liebig's  process,  should  not  exceed  2-3  parts  per 
thousand  too  high.  In  Liebig's  process  i  Cc.  N/ 10  AgNOs  =  0.005^,- 
404  Gm.  HCN. 
