Am jJner'i9i5arm" }  Determination  of  Hydrocyanic  Acid.  265 
since  these  authors  evaporate  over  a  free  flame.  It  may  be  men- 
tioned in  this  connection  that  evaporation  in  an  open  dish  on  a  steam 
bath  resulted  in  still  greater  losses  than  those  mentioned  above. 
After  numerous  failures  we  found  that  alkaline  solutions  of 
hydrocyanic  acid  can  be  concentrated  without  appreciable  loss  by 
distilling  under  diminished  pressure,  as  described  below. 
Influence  of  Salts  on  the  Formation  and  Precipitation  of 
Prussian  Blue. 
Vorlander  states  that  the  presence  of  salts  delays  or  prevents 
the  formation  of  Prussian  blue.  We  tried  the  addition  of  salts, 
because  in  our  early  experiments  we  often  obtained  what  seemed  to 
be  a  colloidal  form  of  Prussian  blue.  Since  salts  are  known  to  pre- 
cipitate colloids  we  tried  the  effect  of  adding  sodium  chloride  and 
found  that  a  clearer  blue  color  was  obtained  than  without  the  addi- 
tion of  this  salt.  We  also  tried  the  effect  of  several  other  halogen 
salts.  Finally,  we  found  that  the  presence  of  potassium  fluoride 
had  a  remarkable  effect  on  the  formation  and  color  of  the  Prussian 
blue.  On  acidifying  in  the  final  stage  of  the  test  the  color  appears 
at  once  and  is  very  brilliant.  The  absence  of  a  green  shade  makes  it 
particularly  suitable  for  comparison  with  a  standard.  If  the  acid 
is  added  very  gradually  the  iron  hydroxides  dissolve  and  a  colorless 
liquid  with  a  white  precipitate  is  obtained.  On  the  addition  of  more 
acid  the  blue  color  appears.  This  is  explained  by  the  fact  that  ferric 
salts  produce  a  complex  salt  with  potassium  fluoride,9  K.,FeF6. 
When  an  excess  of  acid  is  added  this  complex  salt  is  decomposed  and 
the  ferric  ions  needed  for  the  formation  of  Prussian  blue  are  fur- 
nished. Contrary  to  the  statement  of  Vorlander,  our  experiments 
show  that  certain  salts,  such  as  sodium  chloride  and  particularly 
potassium  fluoride,  hasten  the  formation  of  Prussian  blue. 
Influence  of  Acids  on  the  Formation  of  Prussian  Blue  and 
on  the  Color  of  the  Suspension. 
Previous  workers  all  seem  to  have  used  hydrochloric  acid  in  the 
final  stage  of  the  test  for  a  cyanide  to  precipitate  the  Prussian  blue. 
We  found  that  an  excess  of  this  acid  tends  to  make  the  color  of  the 
suspension  green,  owing  to  the  formation  of  ferric  chloride.  The 
mixture  of  the  yellow  ferric  chloride  solution  with  the  Prussian 
9  Greef ,  Ber.,  46,  251 1  (1913). 
