ON  THE  AMOUNT  OF  TANNIN  IN  TANNING  MATERIALS.  427 
QUANTITATIVE  DETERMINATION  OF  THE  AMOUNT  OF  TAN- 
NIN IN  TANNING  MATERIALS. 
By  Gustav  Muller. 
The  precipitant  employed  by  the  author  for  the  determination 
of  the  tannin  in  fluids  is  a  solution  of  gelatin,  which  has  al- 
ready been  made  use  of  for  the  same  purpose  ;  but  the  author 
has  convinced  himself  by  numerous  experiments,  that  an  accu- 
rate determination  of  tannin  by  means  of  solution  of  gelatin 
cannot  be  made  directly,  inasmuch  as  the  yellowish-brown  pre- 
cipitate of  gelatin  containing  tannin,  or  tannate  of  gelatin, 
thus  produced,  is  never  so  completely  separated  from  the  solu- 
tion that  the  supernatant  fluid  remains  perfectly  clear,  which, 
however,  must  be  the  case  if  the  precipitation  of  the  tannin  by 
solution  of  gelatin  be  adopted  for  the  quantitative  determina- 
tion of  the  tannin  in  fluids. 
After  many  trials,  the  author  found  that  the  addition  of  a 
small  quantity  of  alum  to  the  solution  of  gelatin  furnished  a 
suitable  means  to  enable  tannin  to  be  precipitated  without  any 
difficulty  from  any  fluids  containing  it,  so  that  it  might  be  deter- 
mined with  the  greatest  exactitude  ;  the  separation  of  the  pre- 
cipitate (tannate)  takes  place  so  rapidly  and  completely  that  in 
the  course  of  a  few  minutes  the  fluid  over  the  precipitate  appears 
quite  limpid,  and  consequently  may  be  immediately  tested  for 
any  possible  residue  of  tannin.  For  this  purpose,  according 
to  the  author,  the  best  plan  is  to  employ  two  watch-glasses, 
placed  upon  a  black  ground  to  enable  the  reaction  to  be  detected 
with  more  ease ;  a  few  drops  of  the  fluid  standing  over  the  pre- 
cipitate are  taken  up  with  a  little  stick  and  dropped  into  each 
watch-glass,  and  then  into  one  glass  two  drops  of  the  aluminous 
solution  of  gelatin,  and  into  the  other  a  drop  of  solution  of  tan- 
nin or  decoction  of  galls  ;  a  distinct  rod  should  be  used  for  each 
test-fluid  in  order  to  avoid  mistakes.  In  this  way,  by  the  tur- 
bidity produced,  we  may  detect  on  the  one  hand  the  smallest 
quantities  of  tannin,  and  on  the  other,  the  smallest  traces  of 
gelatin,  in  case  the  quantity  necessary  for  precipitation  should 
have  been  exceeded.  To  avoid  the  latter  occurrence,  the  solu- 
tion of  gelatin  is  only  dropped  into  the  solution  of  tannin  un- 
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