Am.  Jour.  Pharm.  ) 
December,  1920.  ) 
Hydrocyanic  Acid. 
913 
and  quickly  determined.  (See  Y.  B.  P.,  1894.)  TJiis  causes  at  the 
end-point  the  precipitation  of  Agl,  which  is  the  most  insoluble  of  all 
silver  salts,  and  thus  gives  the  most  accurate  results  in  any  modi- 
fication of  lyiebig's  method.  Again,  since  Agl,  unlike  AgCl  and 
AgBr,  is  highly  insoluble  in  ammoniacal  water,  the  substitution  of 
ammonia  for  the  soda  required  in  Liebig's  process  becomes  possible, 
and  a  considerable  excess  of  ammonia  does  not  interfere.  The  use  of 
ammoniacal  KI  solution  has  long  been  employed  as  indicator  in  the 
cyanide  titration  of  nickel.  The  process  of  the  B.  P.,  1914,  is  based 
on  this  method.  I  found  it  highly  accurate  ten  years  ago,  but  the 
''Conference  Research  List"  says  that  it  is  unsatisfactory,  and  I  have 
been  led  to  look  up  the  exact  details  given  in  the  B.  P.  and  to  compare 
them  with  those  I  followed,  and  also  with  those  given  by  the  U.  S.  P. 
and  the  French  Codex. 
The  compilers  of  the  B.  P.,  19 14,  have  apparently  copied  from  the 
U.  S.  P.,  and  have  overlooked  the  fact  that  the  U.  S.  P.  solution 
of  KI  is  a  20  per  cent,  one,  while  the  B.  P.  solution  is  only  10 
per  cent.,  yet  the  amount  to  be  used  is  3  drops  in  each  case. 
On  further  comparison  of  these  processes  it  appears  that  at  the  end 
of  the  titration  of  the  U.  S.  P.  process  the  total  volume  of  solution  will 
be  about  37  Cc,  while  at  the  end  of  the  B.  P.  process  the  volume  will 
be  about  80  Cc.  Hence,  in  the  B.  P.  process  the  mass  concentration 
of  KI  in  the  solution  at  the  end-point  is  only  about  a  fourth  as  strong 
as  in  the  U.  S.  P.  process.  It  would  seem,  therefore,  that  the  B.  P. 
prescribes  too  little  KI  for  the  process  to  give  a  sharp  end-point. 
May  not  this  be  the  reason  that  the  B.  P.  process  has  been  found 
unsatisfactory? 
The  French  Codex  orders  10  drops  of  solution  of  KI  (20  per  cent, 
wt.  in  wt.)  to  be  used  with  10  Gms.  of  acid  and  15  Cc.  of  NH,3  solution 
and  dilution  to  200  Cc.  Here  the  mass  concentration  of  KI  in  the 
final  volume  is  practically  three  times  that  of  the  B.  P.  I  used  o.i 
to  0.2  Gm.  of  KI  in  working  on  this  process. 
Another  suggestion  to  improve  Liebigs'  process  is  due  to  Guerin 
in  1906,  who  recommended  the  use  of  a  3  per  cent,  solution  of  borax 
as  the  alkaline  agent  in  place  of  NaOH.  This  is  excellent.  I  found 
the  amount  of  silver  used  in  comparative  titrations  agreed  exactly 
with  that  required  by  the  original  Liebig  method  when  the  excess 
of  alkah  was  kept  as  low  as  possible  in  the  latter  method.  Any  excess 
of  borax  is  immaterial.  Guerin  states  that  ammonium  salts  must  l)e 
absent,  but  that  this  difficulty  can  be  avoided  by  adding  10  Cc.  of 
