AmAp°rn!'if89arm  }     Tests  for  Tannic  and  Gallic  Acids.  181 
ON  SOME  NEW  TESTS  FOR  TANNIC  AND  GALLIC 
ACIDS. 
By  S.  G.  Rawson,  B.  Sc. 
The  ordinary  reagents  which  are  used  in  detecting  these  acids  are 
ferric  chloride,  which  gives  its  well-known  reaction  with  both,  and  a 
solution  of  gelatin  which  reacts  only  with  tannic  acid.  In  the  case, 
however,  of  a  dilute  solution  of  tannic  acid  no  precipitate  is  obtained 
with  gelatin  ;  and  hence  considerable  difficulty  arises  in  distinguishing 
between  the  two  acids.  Young's  test  for  these  acids,  viz.,  potassic 
cyanide,  I  found  to  work  well,  giving  a  reddish  color  with  gallic  acid 
and  none  with  tannic  acid. 
If  a  solution  of  tannic  acid  be  treated  with  ammonium  chloride 
alone  a  precipitate  falls,  but  only  with  extreme  slowness ;  whereas  on 
the  addition  of  ammonia  a  beautiful  white  precipitate  instantly 
appears,  but  this,  probably  by  oxidation,  becomes  rapidly  of  a  red- 
dish brown  color.  With  gallic  acid  no  precipitate  falls  in  either  a 
strong  or  a  weak  solution,  but  the  liquid  becomes  of  a  red  color.  In 
a  solution  of  tannic  acid  containing  one  part  of  tannic  acid  in  5000  of 
water  a  precipitate  falls,  but  slowly,  and  with  more  dilute  solutions, 
therefore,  it  is  better  to  drop  the  mixture  of  ammonic  hydrate  and 
chloride  very  cautiously  on  to  the  top  of  the  tannic  acid  solution. 
Where  the  two  liquids  come  in  contact  with  one  another  the  white 
precipitate  makes  its  appearance  at  once  in  a  well-marked  line.  This 
white  line  is  distinctly  visible  in  a  solution  of  tannic  acid  containing 
1  part  in  20,000  of  water.  If,  however,  a  piece  of  black  paper  be 
held  behind  the  tube  the  delicacy  of  the  test  is  increased  and  1  part  in 
50,000  parts  of  water  may  be  now  detected.  There  is  no  advantage 
in  substituting  other  ammonium  salts  such  as  the  phosphate,  carbon- 
ate or  oxalate  for  ammonium  chloride. 
With  gallic  acid,  as  previously  mentioned,  no  precipitate  is  formed ; 
but  a  ring,  usually  of  a  greenish  color  on  its  lower  surface,  is  pro- 
duced, this  being  recognizable  in  solutions  containing  1  part  of  gallic 
acid  in  100,000  of  water. 
Some  few  experiments  were  tried,  in  the  case  of  tannic  acid,  with  a 
view  to  using  the  method  as  a  quantitative  one ;  but  owing  to  the 
unstable  nature  of  the  precipitate  in  alkaline  solution,  this  attempt 
was,  as  I  anticipated,  a  failure. 
Another  delicate  test  for  both  gallic  and  tannic  acids  is  to  add  to 
