A METHOD FOR THE QUANTITATIVE ESTIMATION OF 
TANNIN IN PLANT TISSUE 1 
By Paul Menaul 2 
Research Chemist, Oklahoma Agricultural Experiment Station 
A search of the literature on the subject reveals no method by which 
small amounts of tannin may be estimated in plant tissue. Since a 
study of the chemistry of the grain sorghums was being made by the 
author, it was decided to estimate the tannin content of the grains. 
The method employed and the results obtained are given in this article. 
COLOR REAGENT 
The color reagent is made by boiling ioo gm. of pure sodium tungstate, 
30 gm. of pure arsenic acid (As 2 o 5 ) with 300 cc. water and 50 cc. concen¬ 
trated hydrochloric acid for two or three hours under a reflex condenser. 
The solution is then cooled and diluted to 1 liter. This reagent is not 
affected by phenols or proteins nor is it affected by dextrose, and the 
color produced by tannic acid is stable for an hour. Since the color 
reagent used is sensitive to many reducing agents, it must be kept away 
from hydrogen sulphid and other reducing reagents. This color reaction 
is not specific for tannins, but it is evident that tannins are the only 
compounds left in the final solution which will affect the color reagent. 
PROCEDURE 
Grind the sample to pass a No. 40 mesh. Extract 20 gm. of the sample 
in a 300 cc. Erlenmeyer flask with 100 cc. of petroleum ether, stopper 
and shake occasionally, and allow to stand overnight. Then filter 
through a dry filter and wash with 100 cc. of petroleum ether in 20 cc. 
portions. Dry the sample, return it to the flask, and treat with 200 cc. 
of 95 per cent alcohol. Shake from time to time and allow to stand 
about 16 hours. Again filter through a dry filter. This method of 
extraction is described by H. C. Fuller in his “Chemistry and Analysis 
of Drugs and Medicines.” 3 Now take 10 cc. of this filtrate in a urine 
centrifuge tube, add 2 cc. of a 10 per cent solution of lead acetate, place in 
water heated to about 75 0 C., and leave until the precipitate coagulates. 
Centrifuge for three minutes, then pour off the supernatant liquid and 
drain as completely as possible. Add from 5 to 10 drops of 5 per cent 
H 2 S 0 4 to the residue and mix thoroughly. Too much sulphuric acid is 
to be avoided, but enough must be added to dissolve the lead tannate 
and to precipitate the lead completely. Pour in enough water almost to 
fill the tube, stir, and centrifuge for three minutes. Transfer the super¬ 
natant liquid to a 50 cc. or 100 cc. volumetric flask and at the same time 
prepare as a control a similar flask containing 1 mgm. or 2 mgm. of pure 
1 Accepted for publication July 18, 1923. 
2 The possibility of finding a practical colorimetric method for the determination of tannins was sug¬ 
gested to the author by Dr. C. T. Dowell, director of the station and station chemist. 
3 Fuller. Henry C. the chemistry and analysis of drugs and medicines, ix, 1,072 p., illus. New 
York. 192c. 
Journal of Agricultural Research, 
Washington, D. C. 
ahh 
- 257 ’ 
Vol. XXVI, No. 6 
Nov. 10, 1923 
Key No. Okla.-s 
