On the Preparation of Tannic Acid.—On testing the method pre-
scribed in the Prussian Pharmacopoeia, for the preparation of tannic
acid, Sandrock finds that it does not fulfil the desired object. In directing
that water should be added to the ether employed, the authors of the
Pharmacopoeia would appear to have aimed at an approximation to the
method originally adopted by Pelouze, in which crude ether was used;
and to have assumed that when watery ether is used, the lower layer of
the percolated liquid is a solution of tannic acid in water. However,
Mohr found that this is not the case, but that the lower layer is a solu-
tion of tannic acid in ether; and Sandrock has obtained the same result
on repeating his experiments. The addition of water to the ether is,
therefore, useless, and moreover injurious, for the solution of tannic acid
in ether is so thick that the percolation goes on very slowly, and some-
times stops altogether. The use of pure ether is open to the same
objection.
The extraction of the tannic acid from galls may on the contrary be
effected with ease by crude ether, and on account of the small quantity
of alcohol which it contains. The alcohol facilitates the percolation by
rendering the solution of tannic acid less viscid.
Instead of crude ether a mixture of sixteen parts ether and one part
alcohol may be used with equally satisfactory results. The percolated
liquid separates into two layers. The lower one containing the tannic
acid may easily be separated, and yields a perfectly pure product on
evaporation. The upper layer contains the gallic acid, coloring matter,
and some tannic acid.
When a mixture of eight parts ether and one part alcohol is employed,
the percolate still separates into two layers, but the lower one is smaller
than when the proportion of alcohol is less, and the upper layer contains
a considerably larger quantity of tannic acid.
Finally, when a mixture of four parts ether and one part alcohol is
employed, the percolate does not separate into two layers, and it is diffi-
cult to separate the tannic acid from the impurities with which it is
mixed.
By means of the above process a much larger product of tannic acid
may be obtained than with either pure or watery ether. The tannic
acid remaining in the upper layer may likewise be obtained by evaporating
the liquid to dryness, treating the residue with pure ether, until the
lower of the two layers into which the liquid separates no longer pre-
sents a green color. It is then separated, filtered, if necessary a little
alcohol added, and evaporated.
The process recommended by Mohr, of treating the galls with a mix-
ture of alcohol and ether in equal volumes, then evaporating the perco-
late which does not separate into layers, and regarding the residue as
tannic acid, is altogether inadmissible, inasmuch as it gives a very im-
pure product.—Pharm. Journ. from JTrchiv. de Pharmacie.