VOLUMETRIC  ESTIMATION  OF  TANNIC  AND  GALLIC  ACIDS,  AC.  317 
obtained  in  three  experiments  after  shaking  for  sixty  seconds, 
show  sufficiently  concordant  results  : — 
gramniss 
±  ill)  11  10  dOlU 
a 
fS  a  1 1 1  s>    fi  r»  i  ^1 
VJTalllO  tlL/ltl 
0-900 
a 
Tannic  acid 
0-070 
it 
Gallic  acid 
0-570 
u 
Tannic  acid 
0-800 
u 
Gallic  acid 
0-200 
n 
Tannic  acid 
0-560 
U 
Gallic  acid 
0-000 
a 
Tannic  acid 
0-700 
a 
Gallic  acid 
rbed  23-4  c.c. 
'      27*5  c.c 
:       44  0  c.c 
61-0  c.c. 
710  c.o. 
A  rather  roundabout  but  quite  conclusive  experiment  may  be 
made  in  the  following  way  :  —  The  mixture  to  be  estimated  is  dis- 
solved in  a  known  quantity  of  water,  and  the  tannic  acid  precipi- 
tated by  gelatine  ;  the  gallic  acid  solution  is  then  transferred 
to  the  absorption  bottle  to  be  estimated.  A  second  experiment 
is  then  made  with  the  mixture,  and  so  all  necessary  data  for 
the  calculation  are  obtained. 
In  making  use  of  the  process  to  determine  the  tannic  acid  in 
leather,  gall  nuts,  sumach,  bark,  &c,  we  proceed  in  the  same 
way  as  with  pure  tannin.  If  the  amount  of  tannic  acid  usual- 
ly present  in  the  substance  is  not  already  known  it  is  advisable 
to  make  a  preliminary  experiment  with  a  small  quantity,  and 
afterwards  proceed  with  the  quantity  calculated  to  absorb  175 
c.c.  By  a  parallel  experiment  made  with  one  gramme  of  tan- 
nin and  shaking  for  120  seconds,  the  amount  of  gallic  acid  will 
be  discovered. 
This  method  of  analysis  is  only  available  for  the  ordinary 
commercial  substances.  Whether  all  the  so-called  tannic  acids 
have  the  same  capacity  for  the  absorption  of  oxygen  as  ordinary 
tannic  acid  is  as  yet  undetermined. 
To  Estimate  Tannin  in  Leather. — From  4  to  7  grammes  of 
the  leather  is  cut  into  the  thinnest  possible  slices,  which  are 
digested  in  about  200  c.c.  of  warm  water ;  after  cooling,  from 
7  to  10  grammes  of  stick  potash  wrapped  in  paper  id  dropped 
into  the  flask,  and  the  shaking  proceeded  with. 
With  sumach  and  oak  bark  the  same  method  is  followed,  or  a 
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