Am.  Jour  Pharm.  ) 
December,  1920.  ) 
Hydrocyanic  Acid. 
909 
2.  Method  of  Fordos  and  Gelis:  Little  used,  but  useful  in  special 
cases,  e.g.,  for  mercuric  cyanide. 
3.  Volhard's  method:  Complete  precipitation  as  silver  cyanide 
by  adding  excess  of  standard  silver  solution,  filtering,  and  then  de- 
termining the  excess  of  silver  in  the  filtrate  by  standard  thiocyanate. 
4.  Titration  of  cyanide  by  Mohr's  method,  using  potassium 
chromate  indicator  of  the  complete  precipitation. 
5.  Deniges'  modification  of  Xiebig's  process,  on  which  the  pro- 
cess of  the  B.  P.  1 9 14  is  based. 
i.ie:big's  original  process. 
Properly  applied,  this  method  is  quite  satisfactory,  but  when 
free  hydrocyanic  acid  is  in  question  there  are  several  possible  sources 
of  error. 
The  acid  must  be  saturated  with  but  a  slight  excess  of  NaOH  to 
form  the  cyanide,  and  any  great  excess  of  NaOH  delays  the  end- 
point,  hence  leading  to  somewhat  higher  results.  But  a  deficiency 
of  NaOH  causes  a  much  more  serious  error,  the  results  being  then  too 
low,  and  the  greater  the  deficiency  of  NaOH  the  lower  are  the  results. 
Results  may  be  obtained  showing  only  a  mere  fraction  of  the  true 
strength  in  such  cases:  a  serious  matter  in  the  testing  of  such  a 
poisonous  medicinal  agent  as  hydrocyanic  acid. 
In  the  older  analytical  textbooks  the  directions  are  given  to 
make  the  solution  of  HCN  strongly  alkaline,  but  since  NaCN  and 
KCN  are  strongly  alkaline  to  litmus,  this  is  no  safe  guide.  A  strongly 
alkaline  reaction  to  litmus  paper  is  obtained  when  only  lo  per  cent, 
of  the  free  acid  present  is  saturated.  In  such  cases  the  process  of 
Liebig  gives  the  end-point  when  the  NaCN  present  is  converted  com- 
pletely into  AgCN,  NaCN,  and  so  determines  at  this  point  the  NaCN 
present.  If,  now,  the  free  HCN  remaining  in  solution  is  saturated 
with  NaOH,  the  titration  can  be  continued.  The  extra  silver  noN 
used  is  equivalent  to  the  free  hydrocyanic  acid  remaining  unsaturated 
at  the  first  addition  of  the  soda.  When  a  solution  contains  alkaline 
cyanide  and  also  free  HCN  this  last  process  of  titration  allows  of  the 
determination  of  both  in  the  same  lot  of  solution. 
These  points  were  communicated  to  the  meeting  of  the  B.P.C. 
in  1874  in  an  excellent  paper  by  vSiebold.  He  stated  that,  for  every 
10  Cc.  of  normal  soda  present  in  excess  of  that  required  to  form 
NaCN,  an  additional  o.i  Cc.  of  N/io  AgNOs  was  required.  The 
error  is  small  in  this  case,  and  even  if  this  excess  has  been  used,  the 
