DETECTION AND ESTIMATION OF TANNINS. 49 
2. Indigo Solution .—A solution of 30 grammes of 
sodium sulphindigotate (“ Carminum coerul. opt., pure 
Indigotin I. of Gehe and Co.,” Dresden) in 3 litres of 
dilute sulphuric acid (1 volume of the acid to 3 volumes 
of water). This is agitated for some time, filtered, and 
sufficient water added through the filter to make 6 
litres. The value of this solution should be such that 
20 cc. in 750 cc. water should require about 10.7 cc. 
of the above permauganate solution to decolorize it. 
3. Hide Powder .—This must be white and woolly 
in character, and not yield to cold distilled water any 
substance capable of reducing permanganate. Such a 
powder is prepared by Dr. Roth, of Berlin, and by 
the Vienna Research Station, and can usually be had 
of importers and dealers in chemicals throughout the 
world. 
4. Standardizing the Permanganate Solution .—The 
purest obtainable tannin (that of Schering was recom¬ 
mended by the Commission) is used for the standard. 
For this purpose a portion is dried at 100° to constant 
weight, then such a quantity of the air-dried substance 
as to represent 2 grammes of it in the dried condition 
is taken and dissolved in one litre of water. 
10 cc. of this solution and 20 cc. of the indigo solu¬ 
tion are added to 750 cc. of water, and the permanganate 
solution is run in from a graduated burette with constant 
stirring until the liquid assumes a greenish color, and 
then more cautiously, a drop or two at a time, until a 
pure yellow color with a pinkish rim is reached, which is 
most apparent on the shaded side. 50 cc. of the tannin 
solution are then macerated with 3 grammes of hide 
powder (which has been previously well moistened and 
5 
