PROCESSES FOR THE ESTIMATION OF TANNIN. 
515 
very easy one. The meeting would be glad to hear whether this was the pro¬ 
cess that it was proposed to introduce in the new edition of the Pharma¬ 
copoeia. 
Professor Eedwood answered in the aiSrmative. 
A CRITICAL EXAMINATION OE THE YAEIOUS PUBLISHED 
PBOCESSES EOB THE ESTIMATION OE TANNIN. 
BY MR. JOHN WATTS. 
There is, perhaps, scarcely any substance in the whole range of organic 
chemistry, of really more practical importance to chemists than the old and 
familiar body known as tannic acid, and yet on account of the difficulty of pre¬ 
paring it in a state of purity, on account of its proneness to pass to other states 
of oxidation, and from its want of crystallizability, the chemistry of this body 
still remains in an imperfect state, and one even now hesitates what formula to 
adopt, when it becomes necessary to assign some composition to it. 
Moreover, from the frequency with which tannic acid occurs in the vegetable 
world, it often becomes necessary to ascertain the percentage amount, and 
having had occasion very frequently of late to do this myself, and, at the same 
time, knowing that it is by no means an easy matter to obtain satisfactory 
results, I thought that an examination of the various processes which have, from 
time to time, come before our notice, by publication in the various journals, 
would tend not only to assist others who may be engaged in the same pursuit, 
but also to throw some additional light upon the various reactions and combi¬ 
nations of this substance. Although the tanning of hides, by means of astrin¬ 
gent vegetable infusions, is of very great antiquity, it is only within recent 
years that Seguin first showed that it was owing to the combination of the tannin 
with the gelatine of the skin, and upon this knowledge founded a process for 
estimating the strength of tanning materials, which I think may be considered 
as the earliest known; as however this is an important method, I will first 
notice some others, which, in my hands, have proved less satisfactory, and revert 
to it again subsequently. 
Monier's process .*—In the ‘ Comptes Eendus’ for 1858, a method is intro¬ 
duced by M. Monier, which depends upon the well-knov/n reaction of perman¬ 
ganate of potash with organic bodies, oxygen being given up, while carbonic 
acid is evolved— 
t CjH.Os + 0,„ = 8 HjO + 7 CO,. 
Gallic acid. 
The reaction for tannic acid being nearly the same, save that some other body 
hitherto undetermined, is simultaneously produced. 
To apply the permanganate, it is necessary to prepare first a solution of 
tannin of known strength, say 1 per cent., which serves subsequently to indi¬ 
cate the proportion of tannin present in the sample operated on by means of 
comparison. Taking oak bark as an example, exhaust 10 grammes by boiling 
with water, acidulate the decoction with hydrochloric acid, and after repose or 
filtration to separate any coagulated albumen, take 50 c. c. by means of a pipette, 
of course having previously made the solution up to a known volume. Into a 
beaker of the same size, measure 50 c. c. of the standard tannin solution, acidu¬ 
late it with dilute sulphuric acid, and add from a burette, solution of perman- 
* ‘Comptes Eendus,’ 1858, p. 577. 
t C=12, 0=16. 
