428  Determining  Tanning  Materials.     { AMse^Ti, Stf* ; 
C.  Titration. — The  liquid  thus  prepared  is  divided  into  two  parts, 
the  first,  one-third  of  the  entire  volume,  serves  for  direct  determi- 
nation of  the  acetate  of  lead.  Suppose  that  a  gram  of  the  dry 
extract  of  chestnut  has  required — for  10  c.c.  of  the  tannin  liquor — 
in  three  successive  experiments,  16, 17,  and  16  degrees  of  the  burette, 
which  corresponds  to  57  per  cent,  of  tannin.  But  this  figure  57 
represents,  not  only  tannin,  but  every  other  substance  capable  of 
precipitating  acetate  of  lead. 
The  tannin  is  then  absorbed  with  bone-black,  previously  washed 
with  hydrochloric  acid,  and  dried  at  100°  C.  in  the  following  manner  i 
— We  act  with  bone-black  upon  the  tanning  liquor,  and  on  a  solution 
of  pure  tannin  prepared  at  a  standard  somewhat  lower  than  that 
indicated  for  the  extract  by  the  first  direct  titration.  In  the  present 
case  this  solution  of  tannin  should  be  prepared  at  55  per  cent. 
From  one  and  the  same  glass  tube,  about  1  centimetre  in  diameter, 
we  cut  off  two  lengths  of  20  centimetres  each,  and  wre  draw  out  each 
at  one  of  its  ends.  The  two  tubes  are  fixed  perpendicularly,  with 
the  point  downwards,  and  plugged  with  a  little  carded  cotton.  Into 
each  is  put  10  grms.  of  the  bone-black,  pouring  into  one  of  them  the 
second  part  of  the  tanning  liquor  under  examination,  and  into  the 
other  the  same  volume  of  the  pure  solution  of  pure  tannin  at  55  per 
cent. 
We  then  take  of  the  tanning  liquor  (which  has  retained  its  original 
brown  color  in  spite  of  the  bone-black)  20  c.c,  and  after  having 
heated  it  to  60°  C,  we  drop  in  the  standard  lead  liquor  from  the 
burette  as  before.  Two  successive  trials  show  16  degrees,  =8  degrees 
for  10  c.c,  in  place  of  the  16  degrees  found  for  10  c.c  on  direct 
titration.  On  the  other  hand,  20  c.c.  of  the  solution  of  pure  tannin 
require  14  degrees,  or  7  for  10  c.c.  Thus  we  see  that  in  the  tanning 
liquor  (chestnut  extract)  there  is  a  certain  quantity  of  matter  which 
acts  upon  the  standard  lead  solution  like  tannin,  corresponding  to  1 
degree  of  the  lead  liquor,  i.  e.  to  857-thousandths  of  a  centigram 
of  tannin,  28  degrees  therefore  correspond  to  10  centigrammes.  The 
figure  57  obtained  by  direct  titration  is,  therefore,  too  high  by  3*57 
per  cent,  and  the  extract  contains  57-3-57=53-43  per  cent,  of 
tannin. — Chem.  News  {London)  July,  1874,  from  Bull,  de  la  Soc, 
Chim.  de  Paris. 
