/ 
9 1 2  Hydrocyanic  Acid.  { ^"^J^^^^J^^^^ 
2.  METHOD  OF  FORDOS  AND  GELIS. 
This  is  also  a  very  old  process.  The  reaction  was  discovered 
by  Serullas  and  Wohler.  HCN  +  I2  -  HI  +  ICN.  Hence  lo  is  equiv- 
alent to  HCN  (or  KCN)  and  i  Cc.  of  N / 10  iodine=-  0.001351  Gm. 
HCN.  Starch  must  not  be  used  as  indicator,  as  its  presence  leads  to 
low  results.  The  solution  must  be  highly  diluted.  In  the  case  of 
alkaline  cyanides,  the  solution,  containing  about  0.05  Gm.  of  cyanide, 
is  diluted  to  about  400  Cc,  a  little  CO2  water  added,  and  iodine 
solution  run  in  till  a  faintly  visible  yellow  appears  in  the  solution.  In 
the  case  of  HCN,  sufficient  NaOH  is  added  to  convert  the  acid  to  cyan- 
ide, excess  of  carbonic  acid  water  added  to  convert  excess  of  NaOH 
into  bicarbonate,  and  the  solution  titrated  in  the  same  way.  The 
results  are  general^  a  little  lower  than  those  obtained  by  Liebig's 
process,  but  are  fairly  close  to  the  truth,  although  a  little  excess  of 
iodine  is  necessary  at  this  high  dilution  to  give  the  solution  the  faint- 
est visible  tint.  The  process  is  somewhat  troublesome  for  hydro- 
cyanic acid.    It  is  a  useful  process  for  determining  CN  in  HgCN. 
3.  volhard's  method. 
Volhard's  method  is  only  useful  in  special  cases.     It  is  highly 
accurate,  but  the  necessary  filtration  of  the  silver  cyanide  before 
the  excess  of  silver  can  be  determined  makes  the  process  tedious. 
All  the  halogen  elements  interfere.    In  this  case,  i  Cc.  N/io  AgNOa  = 
.  0.002702  Gm.  HCN. 
4.  mohr's  method. 
This  resembles  the  usual  titration  of  chlorides,  etc.,  by  AgNOs . 
using  K2Cr04  as  indicator.  The  solution  must  be  neutral  and  in 
the  case  of  HCN  this  is  accomplished  by  using  MgO.  The  reaction 
is  different  from  Liebig's  reaction,  since  the  end-point  shows  the  com- 
plete precipitation  of  cyanide;  equation  (a)  above.  i  Cc.  N/io 
AgNO,i  =  0.002 702  HCN.  The  acid  is  added  to  excess  of  MgO 
which  must  be  chloride  free,  and  titrated  with  silver  as  usual,  with 
continual  stirring.  Though  at  one  time  official  in  U.  S.  P.,  the  pro- 
cess has  no  advantage  over  that  of  Liebig. 
5.  MODIFICATION  of  LIEBIG'S  METHOD.  (dENIGES) 
Deniges  {Journ.  de  Pharm.)  [5]  (XXIX)  suggests  the  use  of  KI 
as  an  indicator  in  solutions  rendered  strongly  alkaline  with  soda  or 
ammonia,  and  states  that  in  this  manner  the  end-point  is  sharply 
