Am.  Jour.  Pharm.  1 
March,  1884.  f 
Tannins  of  Oak  Bark. 
135 
TANNINS  OF  OAK-BARK. 
By  C.  Etti. 
The  tannin  of  oak-bark  exists  in  two  forms,  viz.,  as  a  tannic  acid, 
which  in  the  free  state  has  a  reddish  white  color,  and  as  an  anhydride 
af  that  acid,  called  phlobaphene,  the  color  of  which  is  brown-red.  The 
distinction  between  these  two  bodies  is  familiar  to  tanners,  who  desig- 
nate the  anhydride  simply  as  "coloring  matter,"  and  reject  barks 
containing  a  large  proportion  of  it,  as  it  imparts  too  red  a  color  to 
leather  dyed  with  such  barks. 
The  question  as  to  the  existence  of  a  glucocide  in  oak-bark  is  now 
decided  in  the  negative,  as  tannic  acid  extracted  from  the  bark  by 
ethyl  acetate  does  not  yield  any  such  substance.  The  reactions  which 
were  supposed  to  indicate  the  presence  of  a  glucoside  were  really  due 
to  lsevulin,  which,  on  treating  the  bark  with  dilute  sulphuric  acid,  was 
converted  into  kevulose. 
The  tannic  acid  obtained  by  agitating  an  alcoholic  extract  of  the 
bark  with  ethyl  acetate  may  be  contaminated  with  two  substances,  a 
brownish  green  amorphous  terpene-resin  and  phlobaphene.  The  former 
may  be  separated  by  its  ready  solubility  in  ethyl  acetate,  ethyl  oxide, 
and  benzene.  The  phlobaphene  is  easily  recognized  by  the  brown- 
red  precipitate  which  it  gives  with  lead  acetate. 
Quercitannic  acid  cannot  be  extracted  from  the  bark  in  the  pure 
state  by  ethyl  acetate,  inasmuch  as  it  decomposes  that  compound  into 
alcohol  and  acetic  acid  almost  as  easily  as  sulphuric  or  hydrochloric 
acid,  and  the  acetic  acid  thus  set  free  dehydrates  a  portion  of  the  tannic 
acid,  producing  phlobaphene.  Pure  quercitannic  acid  dissolves  com- 
pletely in  ethyl  acetate,  and  does  not  give  up  any  foreign  bodies  to 
pure  ethyl  oxide  or  to  benzene ;  its  solution  in  very  dilute  alcohol 
gives  with  basic  lead  acetate  a  precipitate  of  pure  yellow  color. 
Phlobaphene  is  nearly  insoluble  in  water  and  in  ether,  but  dissolves 
readily  in  alcohol  of  all  strengths.  As  prepared  from  the  bark,  it  may- 
be contaminated  with  terpene-resin  and  pectin-substances.  The  former 
of  these  bodies  may  be  recognized  and  separated  by  treatment  with 
ether  or  benzene,  which  dissolve  it;  the  pectin-substances  by  their 
insolubility  in  spirit  of  90  per  cent.  The  presence  of  tannic  acid  in 
the  phlobaphene  may  be  recognized  by  the  fact  that  the  latter,  after 
