120 
Co /on 'metric  Method  for  Tannin. 
Am.  Jour.  Fharm. 
March.  lsf  O. 
to  the  tumbler  containing  the  infusion  of  bark,  indicates  the  per- 
centage of  tannin  in  the  bark,  i.  e.,  if  it  is  the  one  in  which  seven 
drops  were  placed,  the  tannin  strength  of  the  bark  is  seven  percent. 
It  is  best  to  observe  the  shades  of  color  horizontally,  rather  than 
vertically,  and  to  hold  up  the  infusion  tumbler,  with  the  one  which 
most  nearly  corresponds,  opposite  to  a  white  wall,  with  your  back 
to  the  light. 
The  above  is  written  for  oak  dark,  but  the  same  process  will 
answer  for  any  substance  containing  less  than  ten  per  cent,  of  tannin. 
The  results  are  necessarily  in  terms  for  commercial  gallotannic  acid, 
and  not  in  those  of  pure  tannin  or  of  the  particular  tannin  in  the 
material  assayed. 
For  substances  containing  between  about  10  and  20  per  cent.,  it 
is  best  to  dilute  {he  infusion  with  an  equal  part  of  water  and  pro- 
ceed as  above,  using  fire  drops  of  the  dilute  infusion,  and  for  the 
answer,  double  the  result.  Thus,  if  the  diluted  infusion  of  tea 
required  eight  drops  "  tannin  solution  R  to  correspond,  call  the  per- 
centage sixteen. 
For  substances  containing  less  than  one  or  one  and  a-half  per 
cent.,  exhaust  ^   ->\:»:s  instead  of  o'S  ^ram,  and  take  one-tenth  of 
'  O  <-> 
the  result  for  the  answer.  For  substances  containing  more  than 
twenty  per  cent,  as  galls,  sumach,  catechu,  etc.,  you  may  dilute  the 
infusion  with  two,  three  or  more  times  its  bulk  with  water,  and  cal- 
culate as  above  (as  with  tea),  or  you  may  use  1,  2,  3  or  4  drops  of 
the  undiluted  infusion  in  the  first  glass  and  make  the  calculation 
thus,  i.  e  :  As  the  number  of  drops  of  infusion  used  is  to  the  num- 
ber of  drops  "  tannin  solution  "  used  (to  correspond),  so  is  5  to  the 
answer — thus,  suppose  twa  drops  infusion  were  used  and  the  corre- 
sponding tumbler  contained  fifteen  drops  tannin  solution — 2  :  15  :  :  5, 
answer  57-5  per  cent. 
The  object  in  diluting  the  infusions  is  because  the  infusion  glass 
may  be  of  too  deep  a  blue  shade  It  is  better  that  it  should  just 
produce  a  light  blue. 
The  tumblers  must  be  perfectly  clear  and  clean. 
The  "  iron  mixture,"  "tannin  solution"  arid  infusion  must  be 
freshly  prepared  and  not  exposed  to  the  rays  of  the  sun. 
The  water  used  must  be  free  of  iron  and  tannin. 
