266  Determination  of  Hydrocyanic  Acid,  j 
Am.  Jour.  Pharm. 
June,  1915. 
blue  produces  the  green  shade.  To  avoid  the  formation  of  the  green 
shade  we  tried  the  effect  of  acids  other  than  hydrochloric  acid.  We 
obtained  good  results  with  sulphuric  and  nitric  acids  in  different 
concentrations.  Hence  these  acids  are  recommended  instead  of 
hydrochloric  acid. 
Vorlander  and  Knight  both  filter  off  the  iron  hydroxides  from  the 
alkaline  solution  of  sodium  ferrocyanide  and  then  acidify  before 
adding  the  iron  salt.  This  procedure  may  lead  to  a  loss  of  hydro- 
cyanic acid,  since  ferrocyanic  acid  oxidizes  rapidly  according  to  the 
following  equation : 
/HiFe  ( CN )  e  +  02  =  24H  CN  +  Fe,  [  Fe  ( CN )  a]  3  +  2FLO 
Influence  of  Ferric  Iron  on  the  Formation  of  Prussian  Blue 
and  on  the  Color  of  the  Suspension. 
It  is  to  be  noticed  that  Berl  and  Delpy,  Lander  and  Walden,  as 
well  as  Vorlander,  used  only  ferrous  sulphate  for  the  formation  of 
small  quantities  of  Prussian  blue.  In  the  ordinary  laboratory  test 
for  a  cyanide,  ferric  chloride  is  also  added,  which  changes  the  color 
to  a  greenish  shade  and  makes  accurate  colorimetric  determinations 
impossible.  The  addition  of  a  ferric  salt  is  not  necessary,  since 
enough  of  the  ferrous  salt  is  oxidized  during  the  operation  to 
furnish  the  ferric  ions  needed.  According  to  our  experience  it  is 
very  important  that  there  should  not  be  a  large  excess  of  ferric  salts 
present.  Therefore,  to  prevent  the  oxidation  of  too  much  of  the 
ferrous  hydroxide  to  ferric,  we  remove  most  of  the  air  from  the 
solutions  of  the  cyanide  by  means  of  a  water  vacuum  pump. 
In  this  connection  we  may  quote  Vorlander's  statement.10 
"  Die  Meinung  ist  verbreitet,  dass  zum  Zustandekommen  des  Berlinerblaus 
aus  Ferrocyankalium  die  Gegenwart  von  Ferro-  und  Ferrisalz  niitzlich  sei. 
Ich  habe  hierfiir  nicht  das  geringste  Zeichen  finden  konnen." 
Here  one  might  obtain  the  misleading  impression  that  Prussian 
blue  is  formed  in  the  utter  absence  of  a  ferric  salt.  Vorlander  prob- 
ably means  to  say  that  the  addition  of  ferric  salt  is  unnecessary,  as 
enough  of  the  ferrous  salt  is  oxidized  by  the  action  of  atmospheric 
oxygen  to  furnish  the  ferric  ions  needed. 
Influence  of  Heat  on  the  Formation  of  Prussian  Blue. 
It  is  interesting  to  note  that  other  workers  apply  heat  in  the 
test  for  a  cyanide.    Sometimes  the  alkaline  mixture  is  heated,  and 
"Ber.,  46,  188  (1913)- 
