316  VOLUMETRIC  ESTIMATION  OF  TANNIC  AND  GALLIC  ACIDS,  AC. 
and  proceed  as  above  described.  The  absorption  is  at  first 
rapid,  and  in  the  case  of  gallic  acid  is  soon  complete ;  with  tan- 
nic acid  a  longer  time  is  required. 
One  gramme  of  tannic  acid  will  absorb  the  same  amount  of 
oxygen  as  0*7  gramme  of  gallic  acid,  namely,  175  c.c.  at 
20°C.  In  twelve  experiments  the  greatest  variation  in  the  case 
of  tannic  acid  amounted  to  only  2  c.c,  and  with  gallic  acid  only 
1-2  c.c.  From  six  to  ten  experiments  can  be  carried  on  simul- 
taneously with  ease,  and  indeed  with  advantage. 
The  best  alkaline  solution  to  use  is  a  soda  solution  containing 
2  or  3  per  cent,  of  caustic  soda,  or  a  potash  solution  with  from 
3  to  5  per  cent,  of  the  caustic  alkali.  Experiments  have 
proved  strong  solutions  to  be  useless  ;  for  example,  one  gramme 
of  tannic  acid  in  a  potash  solution  containing  35  per  cent,  of 
alkali  only  absorbed  22  c.c  of  oxygen.  Further  experiments 
confirmed  this  unexpected  result. 
It  has  been  already  remarked  that  gallic  acid  absorbs  oxygen 
with  much  greater  readiness  than  tannic.  One  gramme  of  tan- 
nic acid,  after  shaking  for  60  seconds,  only  absorbed  23  4  c.c: 
while  0»7  gramme  of  gallic  acid,  after  shaking  for  the  same 
time,  absorbed  71  c.c.  of  oxygen,  the  same  proportion  of  each 
acid  taking  up  when  the  absorption  is  complete  one  and  the 
same  volume  (175  c.c).  We  have  here  an  indication  of  a 
means  of  estimating  the  two  acids  in  the  presence  of  each  other. 
Suppose,  for  example,  we  have  a  substance,  2  grammes  of  which 
absorb  140  c.c,  this  will  answer  to  jM—  0-800  grammes  of  tan- 
nic acid,  or  to  i|§.0,7=  0-560  of  gallic  acid.  For  a  second 
and  decisive  experiment,  we  now  weigh  as  much  of  the  substance 
as  should  combine  with  175  c.c,  in  this  case  2.|Jg  =  2-500 
grammes,  and  shake  this  strongly  with  the  lye,  for  60  seconds. 
Suppose,  after  this,  we  have  20  c.c  absorbed,  wo  have  then  a 
decisive  proof  ,  of  the  presence  of  both  acids,  and  we  can  draw 
a  conclusion  as  to  their  respective  proportions. 
It  must  be  observed  that  in  these  experiments  an  equal  weight 
of  the  substance,  equally  strong  alkaline  solutions,  and  a  flask 
of  equal  size,  and  of  similar  form  (about  14  litre  capacity), 
are  indispensable  conditions. 
The  following  table,  which  gives  the  means  of  the  numbers 
