400  Detection  of  Nitrous  and  Nitric  Acids.        f  Am  jour  Pharm. 
\       Aug.,  1885. 
gives  a  reaction  in  six  minutes,  1  in  500  millions  in  an  hour  or  two, 
whilst  even  1  in  1,000  million  can  be  detected. 
Detection  of  Nitrous  and  Nitric  Acids. — The  four  following  tests 
give  a  reaction  with  both  nitrous  and  nitric  acid,  but  the  reaction  of 
the  first  mentioned  is  the  only  one  which  is  characteristic  for  these 
substances,  inasmuch  as  the  other  three  give  the  same  reaction  with  all 
oxidizing  agents.  The  ferrous  sulphate  test  is  equally  sensitive  with 
both  nitrous  and  nitric  nitrogen,  and  is  conducted  most  effectually  in 
the  following  manner.  Place  1  cc.  of  nitrite  or  nitrate  solution  into  a 
tube,  and  pour  down  2  cc.  of  concentrated  sulphuric  acid  so  as  to  form 
a  lower  layer,  mix  suddenly  and  run  in  immediately  3  cc.  of  a  cold 
saturated  ferrous  sulphate  solution,  which  forms  an  upper  layer.  In 
this  way,  Avith  solutions  containing  1  part  of  nitrous  or  nitric  nitrogen 
in  100,000  parts  of  water,  the  colored  band  appears  at  once,  with  1  in 
200,000  there  is  a  faint  reaction  in  1  or  2  minutes,  whilst  with  1  in 
300,000  the  reaction  is  very  slight.  Indigo  test  (according  to  Bous- 
singault). — The  concentrated  substance  is  boiled  with  a  few  drops  of 
indigo,  and  successive  quantities  of  strong  pure  hydrochloric  acid,  the 
disappearance  or  discoloring  of  the  indigo  is  the  indication  of  oxidation. 
One  division  of  Boussingault's  weakest  indigo  =  0'00077  milligram 
of  nitric  nitrogen.  This  test  does  not  answer  in  the  presence  of  many 
organic  substances.  Brucine  test. — To  get  a  distinct  pink  color,  5  cc. 
of  concentrated  sulphuric  acid  are  added  gradually  to  a  cooled  mixture 
of  2  cc.  of  the  liquid  and  one  drop  of  brucine  sulphate.  One  part  of 
nitric  nitrogen  in  10  millions  of  water  soon  gives  a  reaction,  whilst  1 
in  20  millions  causes  a  faint  tinge  after  some  time.  With  nitrous 
nitrogen,  1  part  in  1  million  of  water  gives  a  full  pink,  but  1  in  10 
millions  produces  scarcely  any  reaction.  Diphenylamine  test. — The 
reaction  is  assisted  by  heat,  by  adding  a  large  excess  of  sulphuric  acid, 
also  by  the  presence  of  chlorides,  but  only  when  the  volume  of  sul- 
phuric acid  is  not  greater  than  that  of  the  liquid  taken.  The  color  is 
fairly  permanent,  and  increases  on  standing.  With  2  cc.  of  liquid,  two 
drops  of  diphenylamine  solution  and  5  cc.  concentrated  sulphuric  acid, 
1  part  of  nitric  nitrogen  in  1  million  of  water  produces  a  deep  blue  at 
once,  1  in  10  millions  a  pale  blue  after  a  few  minutes,  whilst  1  in  20 
millions  gives  no  distinct  blue.  (Trans.,  1881,  644,  it  is  stated  that 
solutions  of  this  strength  do  give  reaction ;  it  has,  however,  since 
been  observed  that  the  reactions  obtained  in  those  experiments  were 
due  to  an  added  impurity.)    Nitrites  act  in  a  similar  manner,  but  more 
