AmXJu°gU,r'I87h7!riia' }      Methods  of  Estimating  Tannins.  4 1 7 
will  give  results  strictly  comparative  for  any  single  tanning  material. 
As  it  is  likely  to  be  of  great  practical  importance,  I  venture  to  give 
working  details,  referring  for  further  particulars  to  Lowenthal's  paper 
in  the  "  Zeitschrift  fiir  Analytische  Chemie  "  (1877,  p.  33),  and  to 
an  excellent  paper  by  Neubauer  abstracted  in  the  "  C.  S.  Journal " 
(ix>  595)- 
Of  solutions  the  following  are  required  : 
1.  — 4  grams  pure  permanganate  of  potash  in  3  liters  of  distilled  water  (or  \  deci- 
normal  answers  well  and  saves  calculation). 
2.  — 5  grams  of  pure  "precipitated  indigo"  in  1  liter  of  water. 
3.  — Dilute  sulphuric  acid  (1  to  3  of  water). 
4.  — 25  grams  of  good  transparent  glue,  well  swollen  in  cold  water,  and  then  dis- 
solved by  the  aid  of  heat.  The  solution  is  made  up  to  a  liter,  and  saturated 
with  pure  salt  (table  salt). 
5.  — A  saturated  solution  of  pure  salt,  containing  25  cc.  of  sulphuric  or  50  cc.  of 
hydrochloric  acid  per  liter. 
To  make  an  analysis,  10  grams  of  sumach  or  valonia,  or  20-25  of 
bark  are  exhausted  by  repeated  boiling  with  portions  of  water,  and  the 
infusion,  when  cold,  made  up  to  1  liter. 
Of  this  infusion,  10  cc.  are  mixed  with,  say  three-quarter  liter  of 
good  drinking  water,  25  cc.  of  the  indigo  solution,  and  10  cc.  of  the 
dilute  sulphuric  acid  are  added,  and  then  the  permanganate  solution  is 
run  in  drop  by  drop  from  the  burette,  with  constant  stirring,  till  the 
deep  blue  of  the  indigo  changes  to  a  clear  yellow,  and  the  moment  this 
takes  place  we  note  the  quantity  of  permanganate  used.  We  will  call 
this  quantity  a. 
Next  we  repeat  exactly  the  same  process  with  the  indigo  and  sul- 
phuric acid  alone,  and  will  call  the  quantity  b.  Then,  subtracting  B 
from  a,  we  obtain  the  amount  of  permanganate  consumed  by  the  total 
astringents  of  10  cc.  of  our  tannin  infusion.  The  permanganate  acts, 
of  course,  as  an  oxidizing  agent,  oxidizing  and  consuming  both  the 
tannin  and  the  indigo  ;  but  as  the  tannin  is  the  most  readily  oxidized  of 
the  two,  it  is  consumed  first,  and  when  the  indigo  is  all  bleached  we 
may  be  sure  that  the  tannin  is  destroyed  also.  In  order,  however,  to 
obtain  this  satisfactorily,  the  proportion  of  indigo  should  be  such  as  to 
require  about  twice  the  quantity  of  permanganate  which  would  be  con- 
sumed by  the  tannin  alone.  Thus,  if  the  indigo  alone  requires  10  cc. 
of  permanganate  to  decolorize  it,  the  indigo  and  tannin  infusion  together 
must  not  take  more  than  about  15  cc,  and  if  it  does  so,  the  tannin 
infusion  must  be  diluted  accordingly,  or  a  less  quantity  employed. 
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