4 1 8  Methods  of  Estimating  Tannins.      { Kmj{™ ^7h7arm< 
The  next  step  is  to  ascertain  the  proportion  of  gallic  acid  and  impu- 
rities in  our  sample.  To  this  end  we  mix  100  cc.  with  50  cc.  of  our 
salted  gelatin  solution,  and  then,  after  well  stirring,  add  100  cc.  of  the 
salt  and  acid  solution,  and  leave  the  mixture  standing  for  some  hours 
or  all  night,  and  then  filter  it  through  paper.  The  filtrate  should  be 
completely  clear. 
If  we  now  test,  say  50  cc.  of  this  filtrate  with  permanganate  and 
indigo  as  before,  we  shall  obtain  the  amount  of  permanganate  required 
for  the  gallic  acid  and  impurities  alone,  since  the  tannin  has  been  entirely 
precipitated,  and  the  gelatin  has  so  trifling  an  action  on  the  permanga- 
nate that  it  may  be  safely  neglected.  To  make  the  working  clearer, 
we  will  take  an  example  from  Dr.  Lowenthal's  paper : 
10  grams  of  sumach  were  boiled  in  three-quarters  liter  of  water,  and 
after  cooling  were  made  up  to  1  liter. 
(1)  10  cc.  sumach  infusion  }  ,    ,  , 
x  '  •  .    .       V  consumed  i6'6  cc.  permanganate. 
25  cc.  indigo  solution  j  r  s 
Do.  repeated,  .  .     16*5  " 
33-1 
50  cc.  indigo  alone,  .  13-2  " 
Total  permanganate  for  20  cc.  sumach,  19-9  " 
(2)  50  cc.  filtrate  from  the  gelatin  )  , 
v  1  J        •    ,•        ,    •       b  >  consumed  11*2  cc.  permanganate. 
25  cc.  indigo  solution  )  r  & 
Do.  repeated,  .  .  .    iri  " 
22*3  " 
50  cc.  indigo  alone,  .  .  13*2  " 
Gallic  acid  and  impurities,      .  .      9*1  " 
Now,  deducting  9*1  cc.  from  19*9  cc,  we  have  io*8  cc.  as  the  per- 
manganate equivalent  to  the  tannin  of  20  cc.  of  sumach  infusion,  or  0*2 
gram  of  dry  sumach.  If  it  be  desired  to  compare  two  sumachs,  these 
proportional  numbers  are  all  that  is  necessary,  and  indeed  it  will  be  quite 
safe  to  use  them  for  comparing  sumach  with  galls  or  pure  tannin.  In 
the  same  way  bark  may  be  compared  with  bark,  and  valonia  with  valo- 
nia,  but  it  will  not  be  safe  to  attempt  by  this  means  to  compare  bark 
with  sumach  or  with  valonia,  because  the  different  species  of  tannin  con- 
sume different  proportions  of  permanganate.  Oser  states  that  1 J  gram 
of  oak-bark  tannin  consumes  only  the  same  quantity  as  I  gram  of  gall- 
nut  tannin. 
I  may  remark  that  where  many  analyses  have  to  be  performed,  the 
