By E. JOHANSON.
The precipitation of tannin by a solution of gelatin is effected
more completely and in a better condition for filtration if, besides
ammonium chloride, as proposed by Schulze and Lehmann, there is also
added a small quantity of chromium sulphate or of chrome-alum. The
author proceeds in the same manner as Lehmann, but he adds to 100 c.c.
of the solution containing sal-ammoniac from 5 to 8 drops of a solution
containing 1 part chromium sulphate in 25 parts of water. In order to
ascertain the end of the reaction, he filters small quantities into two
test glasses of equal width, adds to the one a few drops of a solution
of gelatin, observing if the two liquids, when held up against a sheet
of black glazed paper, appear opaque or transparent. As long as a
precipitate is formed, these portions and the washings of the little
filters are poured back to the main quantity. If acetic or tartaric
acid is present, the liquid should be neutralized before proceeding to
the determination. Johanson points out that, though this method gives
good results with the tannin of galls and of oak-bark, an extract
of coffee gives no precipitate with solution of gelatin, so that
caffeo-tannic acid cannot be determined in this manner. This shows that
only quantities of tannin of one and the same kind can be compared with
each other.