Am.  Jour.  Pharm.  1 
April,  1884.  J 
Estimation  of  Picric  Acid. 
213 
cyanide.  This  test  cannot  be  applied  for  the  detection  of  picric  acid 
in  beer,  for  1  litre  of  beer  which  contained  5  milligrams  of  picric  acid 
was  not  precipitated  by  a  solution  of  methyl  green.  Solutions  of  picric 
acid  are  precipitated  by  stannous  chloride,  and  if  a  small  quantity  of 
ammonia  is  added,  the  solution  becomes  red.  The  same  reaction  is 
obtained  when  a  solution,  prepared  by  adding  potassium  hydroxide  to 
a  solution  of  stannous  chloride  until  the  precipitate  at  first  formed  is 
redissolved,  is  added  to  a  solution  of  picric  acid.  The  red  color  is  due 
to  the  formation  of  dinitroamidophenol  (picramic  acid) :  hydrogen  and 
ammonium  sulphides  give  a  similar  reaction.  If  a  solution  of  picric 
acid  or  a  picrate  is  acted  on  with  zinc  and  dilute  sulphuric  acid,  a  yel- 
lowish red  turbid  solution  is  obtained,  which,  when  poured  off  and 
mixed  with  alcohol,  develops  a  green  color,  changing  through  blue  to 
a  violet-green.  For  the  detection  of  picric  acid  in  sweetmeats  or  other 
colored  substances  containing  sugar,  the  potassium  cyanide  reaction 
can  be  applied  directly;  or  the  coloring  matter  may  be  extracted  with 
alcohol,  the  residue  from  this  solution,  after  dissolving  in  water,  pre- 
cipitated by  lead  acetate;  and  the  potassium  cyanide  reaction  obtained 
after  decomposing  the  precipitate  in  the  manner  above  described.  For 
the  detection  of  picric  acid  in  wool  or  cellulose,  the  hydrochloric  acid 
solution  maybe  reduced  by  zinc,  and  the  reaction  with  alcohol  obtained. 
The  substance  may  also  be  digested  with  ammonia,  and  the  potassium 
cyanide  reaction  tried  with  this  solution.  The  detection  of  picric  acid 
in  beer  cannot  be  accomplished  by  means  of  lead  acetate,  on  account  of 
the  other  smbstances  in  the  liquid,  which  are  precipitated  by  this 
reagent;  neither  can  the  coloring  matter  be  removed  by  animal  char- 
coal, for  this  also  retains  the  picric  acid.  For  the  detection  of  picric 
acid  in  beer,  the  author  recommends  the  following  method:  200  cc.  of 
the  beer  are  evaporated  to  a  syrupy  consistence  on  the  water-bath,  and 
then  digested  in  a  flask  with  50  cc.  of  alcohol  (99  per  cent.),  the  mix- 
ture being  allowed  to  stand  for  24  hours,  when  it  is  filtered,  and  the 
residue  washed  with  31  cc.  more  alcohol.  The  mixed  filtrates  are 
evaporated  to  the  consistence  of  a  syrup  and  acidified  with  two  or  three 
drops  of  dilute  sulphuric  acid.  The  mixture  is  then  extracted  with 
five  or  six  times  its  volume  of  ether,  the  latter  removed,  the  solution 
again  acidulated  and  extracted  with  ether.  The  ethereal  solutions  are 
spontaneously  evaporated,  and  the  residue  dissolved  in  5  or  10  cc.  of 
water,  the  solution  filtered,  neutralized  with  ammonia,  and  tested  In- 
one  of  the  methods  above  described.    For  the  estimation  of  picric  acid, 
