598  Detection  of  Hydrogen  Cyanide.    {Ar%^^  f9hi8r.m' 
DETECTION  AND  ESTIMATION  OF  SMALL  QUAN- 
TITIES OF  HYDROGEN  CYANIDE.1 
By  P.  Lavialle  and  L.  Varenne. 
The  authors  recommend  the  following  method  as  more  sensitive 
and  exact  than  any  previous  modification  of  the  ferric  thiocyanate 
method:  The  solution  of  alkali  cyanide  contained  in  a  very  small 
beaker  is  treated  with  a  solution  of  calcium  polysulphide,  drop  by 
drop,  until  it  is  distinctly  yellow.  The  reagent  is  made  by  washing 
20  grms.  of  lime  until  free  from  chlorides,  mixing  with  100  Cc. 
water,  and  passing  a  brisk  stream  of  hydrogen  sulphide  through  the 
milk  of  lime  fifteen  minutes ;  the  liquor  is  filtered  from  the  excess  of 
lime,  5  grms.  of  washed,  powdered,  roll  sulphur  added,  and  the  mix- 
ture heated  on  the  water-bath  for  fifteen  minutes.  After  treatment 
in  the  cold  with  this  reagent  for  fifteen  minutes  the  beaker  contain- 
ing the  suspected  liquid  is  placed  on  a  boiling-water  bath  and  evap- 
orated to  dryness,  adding  a  drop  of  the  polysulphide  reagent  from 
time  to  time  if  the  color  shows  signs  of  disappearing.  There  can 
then  be  no  loss  of  cyanogen  at  this  stage.  The  residue  is  taken  up 
in  5  Cc.  cold  water  and  acidified  with  5  drops  of  dilute  (1:5)  sul- 
phuric acid.  Sulphur  and  calcium  sulphate  separate.  Calcium  car- 
bonate is  added  gradually  as  long  as  there  is  effervescence,  and  then 
in  excess  to  facilitate  separation  of  sulphur  by  filtration.  The  fil- 
tered liquid  is  evaporated  to  dryness  and  taken  up  in  1,  0.5,  or  0.25 
Cc.  water,  and  the  solution  acidified  with  4,  2,  or  1  drop  of  dilute 
(1:5)  sulphuric  acid.  The  proportion  of  acid  is  important:  the 
concentration  must  be  sufficient  to  prevent  the  formation  of  red 
ferric  sulphite,  which  would  interfere  with  the  test,  and  yet  insuf- 
ficient to  hinder  the  production  of  ferric  thiocyanate.  The  use  of 
smaller  quantities  of  water  than  1  Cc.  makes  it  possible  to  detect 
smaller  quantities  of  hydrogen  cyanide.  Ferric  sulphate  solution  (5 
per  cent.)  is  now  added,  drop  by  drop,  until  there  is  no  longer  any 
increase  in  color  intensity.  The  method  will  detect  0.01  mgrm.  or 
even  less  of  hydrogen  cyanide. 
The  quantitative  modification  of  the  test  requires  all  the  reagents 
to  be  halogen  free.    It  depends  on  the  addition  of  very  dilute  stand- 
1J.  Pkarm.  Chim.,  1918  (VII),  17,  97-102.  From  the  Analyst  for  April, 
1918. 
